Archean rifts and triple-junctions revealed by gravity modeling of the southern Superior Craton

Domain: earth_science
Published: 2025-10-06
Editorial outcome: major_revision

Abstract
The nature of Archean tectonics and the associated geodynamic regimes are much debated in modern geoscience, despite decades of research. In this study, we present a geophysical model to show that, by the Neoarchean, convective forces from rising mantle plumes or early forms of plate subduction caused widespread extension, creating linear zones of crustal growth. These regimes can be identified as Archean rifts in the ancient rock record by the topography of the Moho, i.e., a shallowing of the boundary between the crust and the lithospheric mantle. Gravity data collected over the Abitibi greenstone belt, a particularly well-preserved portion of Neoarchean crust located in Canada’s Superior Province, was modeled to produce a topographic map of the Moho. The model shows corridors of shallow Moho surrounding islands of thick, intrusion-filled crust and is interpreted to be a snap-shot of microplate growth and breakup between 2.75 to 2.69 Ga. The connectivity of the interpreted relict rifts is possible evidence for the existence of Neoarchean plate boundaries and triple junctions and supports a model of at least local mobile-lid tectonics during this stage of Earth’s history.

Introduction
During the Archean the Earth’s mantle was hotter than it is today, leading to thicker crust and likely different tectonic processes from those observed now1–3. A proposed lack of mobile-lid tectonics was suggested, in part, from an apparent lack of Archean rifting4. Mantle plumes and convection are thought to have driven much of the crustal extension in the Meso- to early Neoarchean, spanning 3.2–2.5 Ga5–7, with mafic material infilling early microcontinental fragments8–11. However, extension and crustal infilling does not necessarily constitute crustal rifting. The Meso- to Neoarchean is seen by some as a pivotal time in Earth’s history, that was potentially transitional between stagnant-lid and mobile-lid tectonics2,12. The timing of the onset of global mobile-lid tectonics is a contentious issue, with some authors suggesting it was a gradual and spatially heterogeneous process across the Earth’s surface2,12. This study investigates the Abitibi greenstone belt, a Neoarchean terrane where mobile-lid tectonics may have been active but the architecture of the ancient rifts is not well established13.

In mobile-lid tectonics the crust comprises of many rigid or semi-rigid lithospheric plates12,14, bounded by divergent, convergent, and/or transform margins. The plates are widely affected by horizontal displacements and by break-up and growth through rifting12,14. Driving mechanisms for the formation, breakup, and displacement of the plates or microplates are thought to include subduction12 or plume-induced mantle convection6,7,15. Bird (2003) defined modern microplates as small, mostly rigid areas of lithosphere that range in size from 1000s up to 1,000,000 km216. Detailed studies of today’s oceanic microplates show that they behave as independent plates but are continuously deforming structures17,18.

The Abitibi greenstone belt, situated in the southern Superior Province of Canada, is the largest and one of the best preserved Neoarchean greenstone belts in the world19 (Fig. 1). It formed between 2.75 and 2.69 Ga in a series of submarine volcanic events thought to have been centered in Archean rift basins. Most authors agree that the east-west trending, predominantly mafic volcanic units were emplaced along similarly oriented rifts that formed by north-south extension20–23. However, the geometry of the rift network and the number of rifts are uncertain. In particular, the present-day surface expression of the rifts is obscured by folding, thrusting, uplift, and erosion. A deeper image is required to identify the rift architecture below the upper crustal deformation. In this study, we show signs of crustal thinning and rifting preserved as extensional scars at the interface between the crust and lithospheric mantle, i.e., the Moho24,25. Crustal extension and thinning brought dense mantle closer to the Archean Earth’s surface. As a result, the zones of crustal thinning retain an anomalously high gravity signature that can be modeled as the ancient topography of the Moho24–27. Although rifting by itself does not prove a large-scale mobile-lid tectonic regime, the existence of crustal fragments that are encircled by rifts are a strong indication of at least a local microplate regime in the Abitibi.

The original lithostratigraphic assemblages of the Abitibi greenstone belt have remained largely intact since the end of the Archean19. The two main stages of tectonic deformation since 2.69 Ga were: 1) N-S compression followed by 2) E-W shearing28. The N-S shortening thickened the crust by upper crustal imbrication and rotated the near-surface volcanic units that lie above the brittle-ductile transition zone. Seismic profiles have shown that this imbrication does not extend to the Moho, thus preserving the Archean Moho topography in relation to the upper crustal units29. Many features of the upper crustal rocks are widely distributed by syn-tectonic intrusions, but certain elements observed at surface correlate spatially with the inferred rifts. These include the spatial distribution of volcanogenic massive sulfide (VMS) deposits30 and geochemical whole-rock and time-resolved isotope data20,21.

In this work we present a gravity-derived model of the Moho surface beneath the Abitibi greenstone belt that reveals a possible Neoarchean rift network. The proposed geometry of the microplate mosaic, supported by trace element and isotopic data of associated volcanic rocks and the presence of rift-related mineral deposits, shows an empirical relationship between mantle plume activity, microplate formation, and the development of major mineral systems within a local mobile-lid tectonic environment.

Results
Gravity modeling of the Moho
To model the Moho topography in the Abitibi, we inverted Bouguer gravity data compiled by Natural Resources Canada31 (Fig. 2; see “Methods” for details on data processing) to produce a three-dimensional (3D) density model of the Earth’s crust (Fig. 3). The inversion modeling approach we took created a petrophysical model by iteratively solving for a density distribution in the subsurface that created a gravity signal closely resembling the measured gravity data. The Moho interface was then extracted from the 3D density inversion model by isolating the 3070 kg/m3 iso-density surface. Extracting an iso-density layer from a 3D density model has been shown to be an accurate proxy for the Moho when performing minimum-structure inversion modeling25,26,32. Previous studies similarly modeling the Moho have used the 3020 kg/m3 iso-density contour25,26,32, however through further testing we found that the 3070 kg/m3 contour better fit known Moho depths in the Abitibi. In contrast to the minimum-structure style inversion modeling used in this study, a more common approach is to use the surface-based Parker-Oldenburg modeling method to model the Moho33,34. This surface-based method solves for the depth of an interface between an assumed homogeneous-density crust (e.g., 2850 kg/m3 27) and the mantle (e.g., 3330 kg/m3 27). Homogeneity in the crust and mantle is generally assumed after applying wavelength filtering to the gravity data aimed at removing the signal resulting from density variations in the crust and upper mantle, isolating the signal from the Moho35. We opted not to use the surface-based modeling method, instead using the more generalized, albeit more computationally demanding, minimum-structure inversion method (see “Methods” for details), to account for the known heterogeneity in the density of the Abitibi crust36. However, a noted strength of surface-based modeling methods is that they solve for discrete interfaces. The Moho under the Abitibi, as well as in other Archean terranes, is believed to be a sharp boundary37 and should ideally be modeled as such. Therefore, we implemented a two-step modeling procedure that first solved for the 3D density distribution, as described above, from which a representation of the Moho was extracted in the form of an iso-density surface. The next step was to rerun the inversion fixing the modeled surface of the Moho and separately constraining the densities in the crust and upper mantle as to better agree with petrophysical measurements from the Abitibi38 (see “Methods” for more details). The final Abitibi 3D density model then included a discrete density change across the Moho. The smooth nature of the initial 3D inversion model allowed for an uncertainty in Moho depth (approximately ±3.5 km) to be quantified using a ±20 kg/m3 uncertainty in model density25.

A reference model of 0.0106*z kg/m3 was used to both resolve the Moho and a model with increasing density with depth39. The slope of this reference model was determined through iterative testing (see “Methods”).

Comparison with seismic profiles
The results of the gravity inversion are compared to Moho depths from the Lithoprobe reflection seismic experiments40, a Lithoprobe seismic refraction model41,42, those from the CRUST1.0 model43, and a teleseismic model44 (Fig. 3B, C). The reflection seismic sections were depth-migrated using a constant velocity model of 6 km/s, creating an approximate depth model of the Moho and overlying crustal features40. The Lithoprobe seismic refraction line modeled by Winardhi and Mereu42 resolved a Moho line from a 2D velocity model of the crust and upper mantle, providing more accurate depth information on the Moho. The teleseismic measurements were inverted to solve for the velocity structure of the crust, and their radial receiver functions were plotted to reveal the Moho44. The CRUST1.0 model is the coarsest of the Moho models with a spatial resolution of 1 arc-degree (~111 km) and Moho depths derived from a velocity model of generalized Archean crust (upper crust: 6.2 km/s, middle crust: 6.4 km/s, and lower crust: 6.8 km/s)43. As such, the CRUST1.0 model is used as a comparison for the general depth to Moho within the Abitibi, as the model is too coarse to resolve any short-wavelength features in the Moho.

A close fit is observed between our model and previously published Moho depths (Fig. 3). The closest fit is seen between the Moho derived from the teleseismic model and the refraction seismic model’s Moho (Fig. 3C). While resembling the Moho in our gravity model, the Moho derived from the reflection seismic section provided a worse depth fit of the seismic models when used as a comparison (Fig. 3B). In particular, there is a discrepancy between the models within Line 12, where the gravity model shows a shallowing of the Moho, but the reflection model shows a deepening. This offset likely results from the depth information of the reflection profile being calculated by assuming a constant velocity throughout the crust. As this deepening of the Moho is only seen in Line 12 of the reflection profile, and does not exist in the gravity, teleseismic, or refraction models, we conclude that it is likely a distortion due to the assumed homogeneity of crustal velocities.

Whereas earlier models of the Abitibi Moho have been mostly two-dimensional (i.e., the Lithoprobe and teleseismic sections) or low-resolution (i.e., the CRUST1.0 model; 1 arc-min spacing), our model provides both 3D and higher resolution (10 km) information about the Moho topography.

Crustal density structure
Our 3D density model of the Abitibi greenstone belt shows a range of densities, correlating with known volcanic and intrusive domains (Fig. 3). The exposed felsic plutons have a lower density than the surrounding mafic to intermediate volcanic rocks, as they contain more SiO2 (i.e., quartz and feldspar) and less of the denser MgO- and FeO-bearing minerals (i.e., amphiboles, biotite, and Fe-Ti oxides). The calculated depth of the Moho is shown in cross-sections through the density model (Fig. 3B, C). One of the strengths of 3D density modeling is the ability to separate near-surface features in the model (Fig. 3A) from those at depth (e.g., undulations in the Moho; Fig. 4A)45. As such, the visual correlation between the presence of high density mafic volcanics at the surface of the 3D model to the location of the modeled rifts is likely a result of a geologic process whereby these volcanics were created at these rifts and not the result of a modeling artifact. The results show that the density of the mantle below the Abitibi, up to 70 km depth in our model, is strongly layered and increases sharply with depth. Above the Moho, the crust continues to be strongly layered for approximately 10 km, which we interpret as the high-density, seismically reflective lower crust40. The mid to upper crust (30-0 km depth) shows more lateral variation in density. This reflects the contrast between volcano-sedimentary domains and the felsic plutons that have lower densities (Figs. 3B, 4D).

Geometry of the Abitibi Moho
The Moho depth derived from our 3D density model ranges from 32 to 45 km, with a mean depth of 39.7 km. A map of the Moho topography is shown in Fig. 4A. Large felsic intrusive complexes that have been exposed through exhumation are spatially correlated with a deep Moho. These plutons formed by buoyant felsic melt pooling within the crust through both partial crustal melting of the crust and melt fractionation19 (Fig. 4D). Zones of anomalously shallow Moho are located between these domains and correlate with the locations at surface where base metal deposits are abundant (Figs. 4B and 5C: see “Discussion”). The location of rifting also correlates with the contrasting densities of the volcano-sedimentary domains and the felsic plutons (Figs. 3B, 4D), as noted in other Archean terranes45. The abundance of felsic plutons located at the flanks of the rift zones may reflect deeper levels of erosion at the rift flanks compared to the rift valleys, the latter subsiding and being filled with denser mafic volcanics. Subsequent erosion would have led to a greater exposure of the pluton infused Neoarchean crust between the rifts resulting in the pattern we observe today46,47.

The main E-W shallow Moho anomalies are in the north (Northern Volcanic Zone, NVZ, between Detour and Chibougamau), the south (Southern Volcanic Zone, SVZ, between Timmins and Amos), and in the southeast corner of the mapped area (Swayze area) (Fig. 5B). In the NVZ, the main rift is occupied by volcanic rocks having ages of 2.735–2.724 Ga (Deloro assemblage19). Here, three semi-continuous Moho anomalies are aligned east-west. The anomaly passing though Detour is 60 km in length, the Matagami anomaly is 100 km in length, and the Chibougamau anomaly is 120 km in length (see Figs. 1 and 5B for location names). In the SVZ, the main rift is occupied by volcanic rocks of 2.72–2.695 Ga (Kidd-Munro, Tisdale, and Blake River assemblages19). Five anomalies pass through the areas of Timmins, Matheson, Noranda, Amos, and Val d’Or, including the 65 km long Timmins anomaly, the 65 km Matheson anomaly, the 125 km Noranda anomaly, the 60 km Amos anomaly, and the 60 km Val-d’Or anomaly. One oblique SW-NE trending corridor of shallow Moho also extends from Amos to Desmaraisville, following a portion of the 2.72–2.711 Kidd-Munro assemblage (Fig. 5B). This feature is approximately 130 km in length and 50 km wide.

Although regional north-south shortening has likely affected the overall geometry, the lengths and widths of the inferred rift zones (between 30 and 50 km in width between 60 and 130 km in length) are comparable to modern continental (e.g., East African Rift48) and oceanic rifts (e.g., the Lau Basin25). At the same time, we expect the relative topography of the Moho to be largely preserved through the deformation owing to the ductile behavior at depth. The lower Archean crust has been shown to flow under stress49, a condition that leads to the thickening of the lower crust when undergoing post-rifting compression. However, the shallowing of the Moho is still preserved despite the potential relative post-rifting thickening of the lower crust with respect to the upper crust50.

Methods
Our models were constructed by minimum-structure style inversion modeling65, a predictive method used to produce a model of the subsurface that generates a geophysical signal closely resembling that measured through surveying. In our study we modeled gravity data to produce a density model of the subsurface. The model was parameterized by discretizing the sub-surface with tetrahedral cells that can accurately fit the topographic surface while also varying in size to reflect the decreasing model sensitivity with depth. Figure S1 shows the tetrahedral mesh, which was constructed using the program TetGen66 and used to discretize the Abitibi crust and upper mantle. The model contained 642,516 cells.

Objective function structure
Inversion modeling is largely a minimization process, where a term known as an objective function is defined and minimized through an iterative process. The inversion approach used considers an objective function structured as

ϕ = λϕd + ϕm : (1)

This objective function is composed of two primary components: the data misfit term, ϕd, and the model measure term, ϕm. The trade-off parameter λ is present between the data misfit and model measure to allow the data misfit to achieve an appropriate target value during the minimization process65. The data misfit is composed as

ϕd = 1/N ΣN i=1 (d i,pred - d i,obs / σi)2 : (2)

with d i,pred the predicted data (i.e., the forward signal from the inversion model), d i,obs the observed data that is being modeled, σi the Gaussian noise of the observed data, and N the number of data points. This function acts to minimize the difference between the observed and predicted data, which results in an inversion model that produces a forward signal closely resembling the observed data. During the modeling process we wish to avoid fitting the observed data’s noise, which is why σi is present in the denominator, as this allows the expected value of the data misfit to converge to N. Normalizing the misfit by N then leads to the term to minimize to a value of 1. The predicted data, d i,pred, calculated from the model, are calculated using Okabe’s forward calculation algorithm67.

Simply minimizing the data misfit through inverse modeling would lead to a non-unique solution. To fix the non-uniqueness issue a model measure is added to the objective function, which acts to smooth density variations throughout the cell-based model. This model smoothing implies a sort of “Occam’s Razor” approach to the inverse modeling, where when presented with a large number of equally valid model solutions, the simplest model is chosen as the best65. We employ the classic L2 norm smoothing to the model measure,

ϕm = vT f Ws Df m 2 : (3)

Within the above model measure, Ws is a diagonal weighting matrix, vf an array of integration cell volumes, and Df is a general difference matrix applied to the array of model parameters m68. A variation of Li and Oldenburg’s sensitivity weighting was employed to preferentially weight cells with low sensitivities69, as otherwise the inverse modeling would preferentially place all density anomalies near-surface. The sensitivity weighting matrix was composed of elements

wj = [ΣM i=1 (Gij / vi)2] 1/2 , (4)

for the jth cell in the mesh with M cells, whose volume is vj. G denotes the inverse problem’s sensitivity matrix for the gravity problem, with elements Gij.

Often the difference matrix, Df, is left in its general form that calculates the difference in physical properties between each cell and its direct neighbours (i.e., the smoothing term). However, model smoothing can be further controlled along each of the three Cartesian directions by defining the difference matrix as

Df = ωx Dx + ωy Dy + ωz Dz : (5)

with Dx, Dy, Dz representing gradient operators in each Cartesian direction, and ωx, ωy, ωz are scalar weights68. By lowering one of the Cartesian weights, a model can be produced with more discrete changes in its physical property in that direction. ωx, ωy, ωz = 1 represents the scenario where there is equal smoothing in all directions. We used a ωz = 10−5 to promote sharper vertical variations in density, as at large scales the Earth’s subsurface is primarily composed of a generally layered system of increasing density with depth.

Reference model
The Bouguer dataset was taken from the Natural Resources Canada database, which is a compiled dataset of surface-level gravity readings31. Inverting the data as is resulted in negative relative density features at depth (relative to 2670 kg/m3) that should have corresponded to the upper mantle, and no density gradient with depth as would be expected (Fig. S2A). This discrepancy in baseline densities results from the latitude correction performed on the free-air gravity data, which removes an average signal of the assumed elliptical Earth model from the data. After the latitude correction, any anomalous density bodies in the crust will have densities that are relative to that of the crust, but any anomalous bodies in the mantle will have densities relative to the mantle. A reference model of the crust was needed to remove the discrepancy in crustal versus mantle density anomalies. An optimized reference model of relative densities equal to 0.0106*z kg/m3 was used (z being depth). This reference model was derived by iteratively increasing the density gradient of the model, adding the forward signal of the reference model to the Bouguer anomaly data, inverting the data and comparing the geometry of the 3070 kg/m3 iso-density contour to the Moho from modelled seismic sections40,42,44. This resulting forward signal of the reference model was 1092 mGal. Inverting the Bouguer anomaly data with the forward signal of the reference model resulted in the model shown in Fig. S2B.

A priori moho structure constraints
In addition to Archean crust being thicker than modern day seafloor crust, the Abitibi Moho is found to be a sharper boundary in sound velocity and density37. Considering that density distributions change smoothly with depth, as in our minimum-structure type inversion modeling65, the Moho under continental crust can be represented by the approximate 3070 kg/m3 iso-density layer26. After the initial inversion model was produced, the 3070 kg/m3 iso-density layer was extracted and used to improve the model of the Abitibi. It was improved by adding constraints to the density values that the model could resolve in the portion of the model that corresponded to the crust versus the lower portion defined as mantle. This two-layer model was made by including the extracted 3070 kg/m3 iso-density interface into the mesh design for the second model (Fig. S2C). The density constraints of the two layers were: 1) between 2600 and 3000 kg/m3 in the crust38, and 2) between 3070 kg/m3 and infinity for the mantle. This final version of the model then had a discrete change in density across the Moho.

Forward model verification
To verify that the density anomalies in the upper mantle shaped the topography of the modeled Moho, we created a series of forward models of components of our inversion model (Fig. S3). The first forward model calculated the gravity signal due to anomalies above the Moho, i.e., crustal anomalies (Fig. S3E). The second forward model calculated the gravity signal from the Moho, treating it as representing the 3070 kg/m3 interface as it was in the inversion model (Fig. S3C). A third forward model calculated the gravity signal from the component of the inversion model below the Moho, from which the forward signal of the Moho (Fig. S3C) was subtracted. The remaining gravity signal represented density variations in the upper mantle (Fig. S3D). Comparing the forward signals of the crustal anomalies and Moho anomalies to the modeled data (Fig. S3A) it is possible to quantify how much of the modeled data was fit due to density features in the crust versus features in the upper mantle. As expected, most of the data was fit by crustal density anomalies, but a significant, more regional component of the data had to be fit by the deep Moho density feature. The amplitude of the forward signal of the Moho component was ~20–30 mGal (Fig. S3D), much greater than either the Model Data Residual (Fig. S3B) or the data uncertainty that averaged 0.3 mGal31. The amplitudes of the forward signal from the crust were slightly higher, at 20–50 mGal, but occurred over shorter wavelengths. An example section is shown in Fig. S3F, which shows the amplitudes of the modeled data, the forward signal from the crust, Moho, and upper mantle, and the model data residual. All the data along this section have been shifted relative to their minimum value. The section shows that although density variations in the upper mantle did contribute in fitting the modeled data, their amplitude was small and in some cases less than the amplitude of the model’s data residual.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Thank you for submitting this work. The results of the geophsyics were extremely interesting and for the first time directly image Archean rift architecture and systems. However, there are some aspects that need to be improved in my view - see my detailed comments for revisions. Good luck with the revisions.", "reviewer_2": "This is an interesting paper suggesting development of variably oriented rift centers organized into triple junction(s) in the Archean Superior Craton. This suggestion is supported by analyses of gravity data combined with seismic profiles. It is suggested that (multi-directional) extension and subduction were induced by Archean mantle plumes, which mark transition from stagnant to more mobile lid behavior. This new data and hypothesis are potentially important for better understanding of the Archean geodynamics. However, materials in support of this hypothesis are not very well presented. Many geographical and geological features discussed in the text are not indicated in figures. Conceptual sketch representing the geodynamic hypothesis and the proposed geometry of micro-plates, rift centers and triple junction(s) is absent. This makes argumentation discussed in the paper difficult to follow. Specific comments are given below.\n\nTaras Gerya, Zurich, 28.11.2024\n\nSpecific comments\nFig. 4 Caption. “in A) showing a shallowing of the Moho below the Noranda and Matagami VMS districts.” These districts are not indicated on the figure.\nLine 139. “We propose the linear zones of shallow Moho are relict Archean rifts. The interpreted rifts are environments of primarily mafic-intermediate volcanics that overlie a shallow Moho. The pluton-infused regions are characterized by a deeper Moho and could represent early continental crust and/or a basement of older Opatica crust 11.”\nThese the linear zones of shallow Moho need to be clearly indicated in Fig.4A, which is otherwise difficult to interpret. Geometry of the proposed triple rift junction needs to be outlined.\nLine 160. “Abitibi is consistent with primarily E-W trending extensional zones and at least one oblique NE-SW trending zone (Fig. 4A). The main E-W anomalies are in the Northern Volcanic Zone (NVZ; between Detour and Chibougamau), the Southern Volcanic Zone (SVZ; between Timmins and Amos), and in the southeast corner of the mapped area (Swayze area) (Fig. 1). In the NVZ, the main rift is occupied by volcanic rocks of 2.735-2.724 Ga age (Deloro assemblage10). The Detour anomaly is 60 km in length, the Matagami anomaly is 100 km in length, and the Chibougamau anomaly is 120 km in length. In the SVZ, the main rift is occupied by volcanic rocks of 2.72-2.695 Ga (Kidd-Munro, Tisdale, and Blake River assemblages10).”\nAll these names are not in Fig.4A and it is difficult to guess which features are referred.\nLine 207. “Bouguer gravity data collected over the Abitibi Greenstone Belt was modelled to study the architecture of the Neoarchean crust. The model suggests the Abitibi greenstone belt was composed of a number of pluton-infused micro-plates surrounded by Neoarchean rift centers. The model also suggests the existence of triple junctions, a previously unrecognized feature whose absence in the rock record has been called upon to dismiss the presence of Archean rifting 37.”\nThis discussion is not supported by an adequate conceptual figure. This could be a separate Figure 5 containing the interpreted sketch based of Fig. 4A with clearly indicated micro-plates, rift centers and triple junction(s).", "reviewer_3": "Reviewer # 1’s comments\nReviewer 1 found the manuscript to provide important insight into Archean crustal and\ntectonic activity, with a dataset and imagery that were convincing and match other\ndatasets related to the Abitibi crust. The reviewer was not convinced that this paper\nneeded to get into \"the Archean geodynamics controversy\", and we agree. Rather, the\nimpact of the paper is in imaging relict Archean rifts alone, and seemed less concerned\nabout the plate tectonic debate. Nevertheless, we have added a fuller discussion of\nhow alternative tectonic scenarios would influence the interpretation of our results. The\nabstract is re-written, and the paper has been edited to be less ambiguous about the\ngeneral findings.\nMajor points\n1. More figures to backup the arguments and models – at least one schematic\nfigure clearly showing what they consider ‘microplates’\nA new illustration has been included as Figure 5.\n2. A figure showing how they see the temporal evolution of the Abitibi, in\nregard to their model/data, including the late multi-phase compression.\nWe have further discussed the impact of the late compression event and\nshow the age progression of crustal growth in our new synoptic Figure 5B.\n3. More links to the isotopic maps of Mole et al (2021) to reinforce the\narguments.\nWe agree and have discussed the similarities to Mole et al. more in the\nrevised text and in Figure 5.\n4. Are microplates required to explain your data? The Reviewer called for\nmore justification of a model whereby the proposed complex network of\nplates and triple junctions is really a microplate mosaic, citing examples\nfrom Iceland, Lau Basin, Japan.\nDifferent views have been expressed about what constitutes a \"microplate\nmosaic\", ranging from subduction driven fragmentation to plume induced\nfragmentation. To clarify, we have included a straightforward definition\n(i.e., crustal fragments that are encircled by rifts) that do not imply a\nparticular tectonic regime. In particular, proximity to subduction is not\nnecessary for microplate formation. We cite the key literature on this topic\n(Mammerickx & Klitgord, 1982; Mueller et al., 2017) that describes the\ntypical sizes of microplates and their kinematics that closely resemble the\nstructures inferred from our model.\n5. You have 41 refs, but Nat comms allows up to 70, hence I have added a\nnumber of suggestions.\nThe suggested references have been added, and the text and figures\nhave been revised accordingly.\n6. Include more on the geochemical and isotopic mapping that has been\ndone, which supports your arguments.\nWe added more references to the extensive work of Mole et al. to\nstrengthen the interpretations of the geophysical model results.\n7. The reviewer did not see why rift networks and triple junctions could not\noccur in a stagnant lid or \"non-plate\" scenario, citing additional details in\nthe comments below.\nThis is addressed in comment #17 below and in the revised manuscript.\n8. The reviewer was not sure this paper needs to get into the Archean\ngeodynamics controversy – the data on rifts is of high impact enough.\nFrom what I can see, it could be sued to make arguments for multiple\nsettings, and does not support the existence of plate tectonics, perse.\nOur model focuses on physical evidence in the deep crust and mantle for\nthe existence of Archean rifts in the Abitibi, and how they could have been\nconnected. The geometry and potential connectivity of the rifts point to a\nmicroplate model, and we describe how this could apply to a variety of\ntectonic processes from plate-dominated to non-plate dominated,\nincluding a plume setting.\n9. In the end, the reviewer thought it would be good to discuss various\nsettings, AFTER presenting the main finding of the rift architecture,\nincluding what might be the best modern analogy.\nWe added a new section specifically addressing tectonic scenarios for the\nmodelled rift zones, including specific comparisons to the microplate\narchitecture of present-day Iceland as an example. We argue the Moho\ntopography of the Abitibi best fits a horizontal tectonic regime, which\ncontrasts with the coupled shortening and extension of the crust that\nwould be associated with diapirism and a vertical tectonic regime.\n10. Use of geology to support the model (e.g., the occurrence of komatiite).\nMany known occurrences of komatiite align with the rifts, but some don’t.\nThis is partly because the modelled rifts are deep features, and komatiites,\nlike the major faults, are near-surface features that may be displaced\nrelative to the Moho topography due to lateral crustal flow (as see in in the\nYilgarn). This is discussed in the revised text, but a more detailed\ndescription of the surface geology, vis-à-vis the rift model, is being\nprepared for another manuscript.\n11. There is good evidence that these rifts were continental in nature (syn-\nvolcanic zircon from these rifts do not suggest an oceanic origin, as well as\nrare, but constrained Mesoarchean zircon inheritance). How does that\naffect the model?\nSee response to comment #4.\n12. Can you infer plate tectonic scale features based on the scale of features\nyou have mapped – i.e., in a diagram that summarises your findings and\nclarifies the scaling.\nThe new Figure 5 clarifies these relationships and the scales. We\ndescribe in the revised text (and in the response to comment #17) our\ninterpretation of the rift geometry with comparisons to different tectonic\nsettings.\n13. Line 10: Whether plate tectonics was indeed occurring in the late Archean\nis still debated – it should not be presented as fact.\nWe have adjusted the language accordingly.\n14. Line 27: This model has been published by others for the Superior and\nother cratons and should be cited.\nWe have added the references.\n15. Line 28: Weak hot crust would be more likely to flow laterally, which could\npossibly aid rifting.\nWe have clarified this statement, emphasizing the comparison of hot\nArchean crust to modern seafloor spreading centers and subduction-\ndriven rifting.\n16. Line 32: Again, be careful stating debated points as facts.\nWe have changed the sentence to emphasize the general consensus,\nrather than a statement of fact.\n17. Line 36: Rifts do not necessarily = mobile lid.\nSee response to comment #4. We have clarified the distinction between\nhorizontal (or mobile-lid) and vertical tectonics, as objectively as possible.\nIn horizontal tectonics the crust comprises many rigid or semi-rigid\nlithospheric plates bounded by divergent, convergent, and/or transform\nmargins. We believe this is what our model shows and emphasize that\nthis is the extent of our results, without reference to the motion of the\nplates (only the extension and fragmentation). We think most readers\nalready recognize that the broader aspects of subduction are still a matter\nof debate, and we have tried to characterize them as such.\n18. Line 37: Previous work on Abitibi rifting.\nWe have revised the text to more fully describe the previous work on\nAbitibi rifting, based on isotopic mapping, and agree it is great that these\ntwo completely independent datasets are showing such similar features.\n19. Line 50: What is the evidence that the Moho topography have survived 2.6\nGa of modification?\nWe have added a comparison of our model to the time-resolved isotope\ndata from Mole et al. (2022) and also to the distribution of VMS, as two\nindependent pieces of evidence for survival of the Archean Moho\ntopography and the corresponding rifts, as suggested by the reviewer. The\ntime-resolved εHf map, in particular, shows the existence of a rift-like\nfeature at >2695 Ma that corresponds closely to the Moho topography\nshown in Figure 5.\n20. Line 62: Why is the mid-lower crust not affected by the deformation?\nWe explain in the revised manuscript that the N-S shortening thickened\nthe crust by upper crustal imbrication above the brittle-ductile transition\nzone. Seismic profiles have shown that this imbrication does not extend to\nthe Moho, thus the Archean Moho topography is preserved. The mid-lower\ncrust is also not affected because is it largely below the BDZ.\n21. Line 83. Is it possible the intrusive (less dense) and volcanic areas (more\ndense) are creating the appearance of belts of deep/shallow Moho? Dense\nvolcanic rocks are understandably more abundant in rifts, but why would\nintrusives dominate the flanks?\nDeeper features, such as Moho topography, which we interpret as rifts,\nproduce long wavelength, low amplitude anomalies in the gravity data.\nNear-surface features produce shorter wavelength, higher amplitude\nanomalies. The iterative process of the inversion modelling is able to\nresolve these in 3D. We explain in the revised text (see also comment\n#24) that the abundance of felsic plutons at the flanks of the rift zones may\nreflect shallower levels of erosion at the rift flanks compared to the rift\nvalleys, the latter subsiding and being filled with more dense mafic\nvolcanics. Subsequent erosion would also have led to a greater exposure\nof the pluton infused crust between the rifts resulting in the pattern we\nobserve today, acknowledging that some intrusions may be younger than\nthe rift system.\n22. Line 93: Why do you not show any 3D images?\nThe model is 3D, but we find it is more informative to show aspects of the\nmodel in plan view or cross sections (Fig. 3B and 4D) rather than in a 3D\nperspective. We could extract a 3D surface from the model to represent\nthe Moho, but studying it is best done in section and elevation/depth\nmaps.\n23. Line 94: I would argue here that you have added further resolution, and\nconfirmed the architecture, but it is not the first rift architecture map of the\nAbitibi.\nWe have revised to text throughout the manuscript to more accurately\ndescribe the contribution made here relative to the sparse Moho\ninformation available from seismic studies and isotope mapping.\n24. Line 116: If the rifts are dominated by volcanics, perhaps this is because\nthey were localised into grabens, which filled with volcanic material and\nform a lower crustal level. The plutons are represented by the felsic\nintrusion dominated flanks, which are horsts. There are likely felsic\nintrusions under the grabens, they just aren’t exposed yet. This is\nimportant because many people think we cannot image the rifts as has\nbeen done here and isotopically.\nWe have added this discussion to the text and in response to comment\n#21.\n25. Line 119: What about the thickness of volcanic rocks?\nWe have clarified this in the text and in response to comments #21 and\n24.\n26. Line 122: Can we presume these rifts would have been much wider, and\nwhat we are seeing is the inverted, closed versions? Also the ‘depth’ of the\nmoho is presumably a minimum?\nYes, this is correct, and we have emphasized that the modelled features\nare post-tectonic. We have tried to restore the N-S compression and E-W\nshearing but chose to leave that for another paper, given that it is not\ndirectly related to the gravity modelling. We also avoid the term “crustal\nthickness”, as opposed to \"Moho depth\" to avoid any assumptions about\nthe depth of erosion and thickness of the crust at ~2.7Ga. Instead, we\nfocus only on the topography of the Moho.\n27. Line 139: Please clarify what crust you think is being rifted.\nWe note that the rifts are similar in their architecture to continental rifts and\nmay have developed in crust with a thickness similar to that of modern\ncontinental crust. However, we do not discuss compositional of\npetrological implications that would be far beyond the scope of this paper.\n28. Line 152: Lateral ‘flow’ of hot mafic crust could explain how the lower and\nmid-upper crust can become dissociated.\nWe have added this point to the discussion.\n29. Line 158: Rifting versus discrete plates and triple junctions.\nSee response to comment #4 and #17. We have provided the accepted\ndefinition of microplates in the paper and discuss them several times in\nthe context of crustal fragments that are encircled by rifts. We do not\nimply a particular tectonic regime and consider that they can form in either\ncrustal type (oceanic or continental). We acknowledge that rifting can\noccur without discrete plates or triple junctions, but we show here that\ntriple junctions likely existed, requiring a microplate architecture.\n30. Line 160: Why are we only hearing about this oblique zone now?\nWe have moved the description of the SW-NE trending corridor of shallow\nMoho to the results to better introduce this feature in the context of the\nmodelling.\n31. Line 166: How continuous are these rifts?\nWe have described the segmentation of the rifts (and the scale of\nsegmentation) which is similar to modern continental and ocean rifts,\nincluding the boundaries of microplates.\n32. Line 168: It would be good to see the rifts presented as one continuous\nsystem.\nWe have illustrated the potential continuity of the rifts in the new synoptic\nFigure 5.\n33. Line 171: Please reference points that are not original.\nReferences have been provided where indicated.\n34. Line 175: This is the interpretation of Mole et al 2021, 2022.\nReferences have been provided where indicated.\n35. Line 175: Note the occurrence of these kind of transform systems in\nIceland.\nWe have added the comparison.\n36. Line 180: Reference the isotopic mapping which imaged this feature and\nsupports your identification of it.\nWe have added the comparison.\n37. Line 180: You have excellent evidence for a complex rift system, but can\nyou demonstrate the rifts separate ‘plates’? Isotopic and geochronologic\nevidence suggests pre-existing crust, so how does this factor into your\nmodel? Triple junctions are usually between oceanic or mafic plates.\nWe have addressed this question in our definition of microplates, which\nare closed polygons encircled by rifts. Triple junctions are a requirement\nof microplate formation, regardless of the makeup (oceanic or continental)\nor age of the crust. The oblique shallow Moho anomaly in the east creates\nthe possibility of triple junctions in our model. We have tried to re-\nemphasize these points in the revised text.\n38. Line 180: If you are going to make the link to horizontal tectonics, here\nwould be best.\nThis discussion has been moved to another location.\n39. Line 183: The link to ore deposits requires more references, including in\nthe introduction.\nDone.\n40. Line 195: Be more clear about what crust you think is being rifted (oceanic\nversus continental).\nSee responses to comments #4 and #29. We chose not to include a\ndiscussion of the makeup of the crust, but focus instead on the Moho\ntopography as the main control on rifting.\n41. Line 196: You have not discussed to any extend the nature of the crust.\nSee response to comment #40.\n42. Line 204: Wording\nAdjusted.\n43. Line198-204: You do not need to get into the Archean geodynamics\ncontroversy with this paper. The identification of the rift architecture is big\nenough news.\nWe have clarified that the rift model does not imply a particular\ngeodynamic, including subduction. For detailed comments regarding the\ndifferent scenarios see responses to #4 and 17.\n44. Line 207: The end of the paper is not decisive about linking the data to\nplate tectonics and ‘plate boundaries’.\nWe have modified the ending to better match the tone of the paper\nthroughout.\n45. Figure 2: Improvements\nWe have added the outline to the data figure, and the corresponding\ngeographical details are provided in Fig. 1. We have added landmarks and\nlabels to the other figures, including the new Fig. 5.\n46. Figure 3: The red-blue colour profiles used are not generally good for\ncolour-blind people and may not present data clearly. Also be consistent\nacross all figures.\nWe are familiar with Crameri at al. 2020 and use the CET colour ramps\nthe paper discusses. The red-white-blue colour ramp in Fig. 3 is standard\nfor density figures, but the same bounds are not used in Fig. 4. Crust is\nmainly blue and the mantle red. This is explained in the caption.\nVariations in density are the main focus of Fig. 4, and particularly Fig. 4D,\nwhere we removed the mantle component to highlight just the variations in\nthe crust.\n47. We encourage the authors to ensure all raw data are available for others to\nrepeat and test their model.\nWe have added the data and instructions on how to access the modelling\nprogram.\nReviewer # 2’s comments\nRevier #2 found the manuscript to contain important new data and hypotheses\nthat potentially offer better understanding of Archean geodynamics. However,\nimprovements are suggested for better presentation of the materials in support of\nmodel, in particular several details of the figures and a conceptual sketch\nrepresenting the proposed geometry of microplates, rift centers and triple\njunction(s).\nSpecific comments\n1. Fig. 4. The districts are not indicated on the figure.\nWe have added the labels to the figure.\n2. Line 139. The linear zones of shallow Moho need to be clearly indicated in\nFig.4A, which is otherwise difficult to interpret. Geometry of the proposed\ntriple rift junction needs to be outlined.\nWe have created a new Figure 5 to better outline the rifts, and compare\nthem to the Moho depth model. We also introduce the role of oblique\nextension earlier in the paper that highlights the connectivity of the rifts\nand the locations of possible triple junctions that support the microplate\nmodel.\n3. Line 160. All these names are not in Fig.4A and it is difficult to guess which\nfeatures are being referred to.\nWe have added all this information in the new Figure 5.\n4. Line 207. This discussion is not supported by an adequate conceptual\nfigure containing the interpreted sketch based of Fig. 4A, the indicated\nmicroplates, rift centers and triple junction(s).\nThese features are all indicated in the new Figure 5.\nReviewer # 3’s comments\n1. Line 49. There is an assumption in this paper that studies of today’s gravity\nfield can be used to infer crustal structure in billion-year-old cratons. This\nassumption should be stated and justified.\nWe have revised the text to better explain how the Archean Moho\ntopography is preserved through subsequent deformation (see response\nto comment #20 of Reviewer #1). We outline in the introduction how\ngravity inversion modelling can reveal the original Archean Moho and that\nany subsequent deformation has not erased the original topography. We\nhave also discussed how studies have shown the lower crust/ Moho is\nlargely unobscured and can indeed support large stresses on long\ngeological timescales (Benn, 2006).\n2. Lines 68/69. The statement that “Inversion modelling creates a\npetrophysical model” is misleading. Inversion in gravity interpretation is a\nprocedure that creates a body with a certain specified density contrast with\nits surroundings directly from an observed gravity anomaly.\nWe have clarified this statement in the Methods section. There are two\nmain types of inversion modelling parametrizations: geometric and voxel-\nbased. In geometric inversion modelling, the physical properties in the\nmodel are fixed (assigned) and the geometry of the body is solved for. We\nperform minimum-structure inversion modelling, which uses a voxel-based\nparameterization. This type of inversion modelling produces a pixelized\npetrophysical model of the sub-surface.\n3. Line 71/72. Why is it relevant that the vidi inversion code was run on\nCanada’s narval cluster? Is the code limited to certain platforms? Is it\npublicly available?\nWe mention the Narval cluster to document the infrastructure and\nresources used in the modelling. The code is not limited to certain\nplatforms. We have added information on how the program can be\naccessed in the Code Availability section. It was developed at Memorial\nUniversity of Newfoundland, and is now the main tool employed by\nGeotexera Inc., a commercial company. The executables can be shared\nby contacting the senior author.\n4. Line 73. Give the reasons why the “3.07 g/cm3 iso-density surface” was\nselected for the Moho.\nWe elaborate on our choice in the revised manuscript. We used 3.07\ng/cm3 as the iso-density surface, which has been shown to accurately fit\nthe seismic Moho when performing minimum-structure inversion modeling\n19,21,26. This contrasts with the commonly used surface-based Parker-\nOldenburg modeling method 27,28, that solves for the depth of an interface\nbetween an assumed homogeneous-density crust and mantle. Due to the\nknown heterogeneity in density in the Abitibi crust33 we opted not to use\nthe surface-based modeling method, instead using the more\nsophisticated, albeit more computationally demanding, minimum-structure\ninversion method.\n5. Line 78. Explain what the “signal” is that was output from the inversion\ncode. The measured Bouguer anomaly is referred to the geoid and\nassumes in its reduction a certain density for the rock between the\nphysical surface and mean sea level. Was the “signal” similarly referred to\nsea level?\nWe have modified the figure caption to explain the nature of the signal.\nThe inversion code outputs the forward signal of the inversion model. The\ndifference between the inversion model's forward signal and the measured\nBouguer anomaly is the model residual, which is plotted in this figure. The\nforward signal of the inversion model would be similarly referred to sea-\nlevel.\n6. Line 100/101. The Moho derived from the inversion model is compared to\nCRUST1.0, a teleseismic model and five Lithoprobe seismic lines. However,\nthere are issues associated with each of these sources of seismic data.\nThere is [1] no evidence provided in the paper that there is any active\nsource data available within the region of the study area. The telesesismic\ndata is limited to Receiver Function data which is based on an assumption\nof P wave crustal velocity between the physical surface and the Moho.\nThere is [2] no evidence provided in the paper of what velocity was used or\ndiscussion on whether or not this velocity is an appropriate one to use in\nthe study area. Finally, the Lithoprobe data are seismic reflection profiles\nand so are in two-way travel time, not depth. There is [3] no evidence\nprovided in the paper of what velocities were used to convert the reflection\ndata to depth.\n1) We provide evidence in Fig. 3 showing the locations of each of the\nseismic surveys (which were located within the Abitibi), and the citation\nto each of the respective studies.\n2) Each of the published surveys used in the paper provides details on\nthe processing and modelling that were done to resolve a Moho\ninterface. The seismic studies all focused on the Abitibi and used\nvelocity models best tailored to this region’s crust.\n3) All the velocity information is provided in the original studies. The\nreflection profiles are originally in two-way travel time, but they were\nconverted to depth as in other Lithoprobe studies.\n7. Figures 3 and 4. The relationship between the observed Bouguer anomalies\nand the inferred Moho undulations is very difficult to see in the paper. At\nthe very least profiles of the observed Bouguer anomaly should be shown\nin both Figures 3 and 4 so that the reader can see the relationship.\nWe have added the Bouguer data near the Figure 4 profile to better show\nthis relationship. Since this is not a geometric inversion there won’t be a\n1:1 relationship between the Moho topography and the Bouguer anomaly.\nWe show that many density variations in the crust contribute to the overall\nBouguer anomaly signal (see the new Fig. S4). We hope the change to\nFig. 4 and the new Fig. S4 clarify the relationship.\n8. Line 105. Was it not assumed in the model that the sub-surface density\nstructure was layered?\nWe did not assume that the sub-surface density distribution in the Abibiti\nwas layered. As explained in the “Gravity Modelling of the Moho” section,\nour modelling produces a 3D density distribution in the sub-surface, from\nwhich a contour level can be isolated to represent the Moho. But the\nmodel is not itself a collection of layers. To help clarify how we\nparameterized our model we added a figure of the model mesh to the\nMethods section, showing the distribution of “pixels” that made up the\nmodel. We also added further description on the parameterization used to\nproduce the model.\n9. Acknowledgments. Makes no mention of data availability or access to the\ninversion software used in the paper. Web links should be provided that\nlink to the Bouguer anomaly and topography data and to the vidi inversion\ncode.\nWe have added these components to the Data Availability and Code\nAvailability sections.\n10. In summary, the reviewer had many concerns about the validity of our\nresults because of the lack of forward modelling.\nTo address the reviewer's concerns, we performed forward modelling to\nshow that the undulations in the Moho produce a signal much greater than\nthe model residual, and therefore is significant. However, we believe\nthese concerns are related to a misunderstanding of the parameterization\nused in our modelling (see response to comment #2). Because we did not\nperform a geometric inversion, we were not solving for a (or a collection\nof) iso-density layer(s). If we had, indeed forward modelling the signal\nfrom each layer would inform how each layer contributes to the total\nsignal. Instead, we solved for a 3D distribution of density throughout a\nmodel of tetrahedral cells. Isolating the component of the model that lies\nbelow the resolved Moho, and forward modelling its signal shows the\nundulations in the Moho (and density anomalies in the upper mantle). The\nanomalies have amplitudes of 20-30 mGal, well above the model residual,\nwhich has a mean of 0 mGal and standard deviation of 2 mGal (see the\nnew Fig. S4).\nRegarding the undulations observed in the Moho, the many seismic\nstudies performed in the Abitibi have all shown changes in Moho depth, as\nwe showed in Fig. 3B, which we verified our model against. Our model\nwould be wrong if it presented a flat Moho. If forced to have a flat Moho,\nthe model would still produce a density distribution in the crust to fit the\nobserved Bouguer anomaly data. However, we verified our model depths\nto those previously published (and derived from methods independent of\ngravity modelling).\nManuscript NCOMMS-24-59268B - Archean rifts and triple-junctions\nrevealed by gravity modelling of the southern Superior Craton.\nReviewer 1 comments\nIntroduction\nThe revision of this paper by Galley et al represents an improvement on the\nprevious version. The majority of the revisions are excellent – however a few\nfurther comments are provided below. This paper deserves to be published in Nat\nComms, but it needs to be more exact in its points in key places. Parts of the\npaper and its major points on tectonics are still far too vague.\nThis is a good paper, but it could be a great paper with a bit more work and care\nand attention. The are numerous typos and errors, and the prose itself could be\nclearer in places. However, the science is sound and I think this will be a really\nnice contribution after these revisions. I envisage that after this relatively\nstraight-forward revisions the paper will be suitable for acceptance.\nResponse to previous comments\nI do not particularly like the way the authors have summarised the reviewer’s (my)\npoints and then made their comment or rebuttal. Please paste the actual\ncomment, verbatim, into your report, and then address it. This avoids any\nambiguity; the editor and I can see the original comment next to the response.\nApologies. This has been changed.\nIn general, the revisions were thoughtfully performed and comments addressed\nor rebutted well. However, I have further questions regarding the responses\nbelow:\nThank you for adding figure 5. This is excellent and I think is a very powerful link\nbetween the geology, geochemistry, and geophysics. This is something that is,\nunfortunately, quite rare. I suspect this figure will be used a lot.\nThank you.\nI still find the microplate argument ambiguous. Were these pieces of crust\nactually totally separate plates? Or different terranes? I am not sure… but I think\ncalling them microplates will at least generate some interesting discussion. Your\ndefinition is progress, but the rifts surely have to be ‘mantle taping’? i.e., like at\nplate margins? I wonder if the situation in East Africa is a good analogy. Some\ndiscussion on the size and geometry of microplates needs to be in the main text.\nApologies if I missed it.\nWe have expanded the description of microplates in the introduction\n“Bird (2003) defined modern microplates as small, mostly rigid areas of lithosphere that\nrange in size from 1000s up to 1,000,000 km2. Detailed studies of today's oceanic\nmicroplates show that they behave as independent plates but are continuously\ndeforming structures (MacLeod et al., 2017; Baxter et al., 2020).”\nYou say “the abstract is re-written” but the track-changes show only four\nchanges that are extremely cosmetic. The two statements you make in the\nabstract are not factually correct (lines 9-10 and lines 20-21) and these need to be\nrevised or toned down. It will not affect your ms as far as I can see but be more\naccurate to current Archean geoscience. The reality is, we cannot be sure of\nmany things in Archean geology and geodynamics, and hence we need to be\nopen about that as we try to push things forward.\nI think I made this point in the first review – the first line of the abstract is not\nappropriate (line 9-10). It makes a statement as fact, that we do not know. We do\nnot know that plate tectonics started at 2.7 Ga – this is not a fact and needs to be\nremoved or edited. I’m sorry, but I insist on this. This is because there are a whole\nrange of opinions on when PT started, from the Hadean, Mesoarchean,\nNeoarchean, to Neoproterozoic. Even if you mean “non-subduction, mobile-lid\nplate tectonics”, this statement will not necessarily be read this way – if you do\nmean this, you should at least clarify it. Furthermore, this would also be debated\n– some think a stagnant lid is appropriate for the Neoarchean. I do not see why\nmicroplates and rifts could not exist in a stagnant lid, with localised extension\ntaken up by ‘crumpling’ at the margins, similar to Phanerozoic ‘intraplate\norogens’ such as the Alice Springs\nOrogen. Again, this is what is theorised for Venus. A connection of rifts does not\nnecessarily mean moving plates – if you want to infer this, you probably need\nfurther evidence. Referencing some paleomagnetics from Archean rocks would\nbe a start, such as these papers:\nhttps://pmc.ncbi.nlm.nih.gov/articles/PMC7176424/\nhttps://www.sciencedirect.com/science/article/pii/S0012821X0700516X?casa_toke\nn=DtodWmKuBKkAAAAA:m3VPwWcIdcMmDoZTCtttjlHUPMWgYhWKBOT2fOHDY\nOjj_2PxuoNDRNMycU4YqaBYfDIXx8lyDA\nThey show evidence of plate movement in the archean, although as you can\nimagine, this is contested. I personally like the idea of episodic subduction, and\nthis matches the data in Mole et al 2021 for the SE Superior. However, these rifts\nappear to be ‘pre-subduction’, if subduction started at 2.695 Ga in the Abitibi, so\nin that context they would potentially need another driver.\nOn top of this stuff, some of the hottest magmas on Earth are known from the\nNeoarchean, which suggests this may have been a thermal peak, rather than a\ncooler inflexion (see Herzberg et al 2000). You can say this, but not as a\nstatement. You need to add ‘may’, or ‘possibly’ etc. If you feel this is a result of\nyour study, that’s fine, but don’t open with it. Right now, as I said in the previous\nreview, this is not correct, and a poor way to start the paper.\nYou might rather say that “The nature of Archean global tectonics, and the\nassociated geodynamic regime, is much debated in modern geoscience, despite\ndecades of research. In this study, we use a new geophysical dataset to show\nthat, by the Neoarchean, convective forces etc…”\nThat way you can state lines 11-14 more as your result and findings, which they\nare.\nWe have modified the first sentence of the abstract with something similar to what the\nreviewer has suggested, avoiding a categorical statement about the start of subduction\nat 2.7 Ga.\n“The nature of Archean tectonics and the associated geodynamic regime is much\ndebated in modern geoscience, despite decades of research. In this study, we present a\nnew geophysical model to show that, by the Neoarchean, convective forces Towards\nthe end of the Archean, approximately 2.7 billion years ago, the Earth’s crust had\nsufficiently cooled to begin a transition to plate tectonics. Convective forces from rising\nmantle plumes or early forms of plate subduction caused widespread extension,\ncreating linear zones of crustal growth.”\nWhere appropriate, we have replaced \"horizontal tectonics\" with \"mobile-lid tectonics\" to\navoid suggesting any inferences about the drivers.\n“The connectivity of the interpreted relict rifts is possible evidence for the existence of\nNeoarchean plate boundaries and triple junctions and supports a model of at least local\nhorizontal mobile-lid tectonics during this stage of Earth’s history.”\nApart from these corrections, we do not believe the full discussion of the topic\nsuggested here by the reviewer can be reasonably covered in the abstract.\nLine 18: timing is important for the Abitibi – is this model strictly for the syn-\nvolcanic period i.e., 2750-2695 Ma? If so, a few lines on how the compressive\nphase 2695-2670 Ma influenced it might be useful to complete the circle. You say\nin the abstract your microplates are evidence for horizontal tectonics (i.e.,\ncompression) but there is no evidence for this structurally until ~<2690 Ma – how\ndoes this work?\nWe have addressed this further into the manuscript, in the last paragraph of the\nIntroduction. There we discuss the compressive phase the Abitibi experienced, how that\naffected the upper crust and how, with the imaging we have at our disposal (seismic),\nthe lower crust/Moho was not similarly affected. As noted above, we have replaced\n“horizontal tectonics” with “mobile-lid tectonics” to avoid any inferences about a\nparticular geodynamic regime apart from synvolcanic rifting.\nLines19-20: You have found rifts – or plate boundaries in a mobile-lid tectonic\nsetting – this not direct support for horizontal tectonics. Archean tectonics is\nlittered with these kinds of statements. Report on what you actually have – rifts\nare evidence for extension. Horizontal tectonics will be interpreted as subduction\n– if you don’t mean this, then state ‘mobile-lid’ or whatever you do mean.\nDone. As noted above.\nLines 25-26: you have not referenced this. Please be more exact when describing\nwhat you mean by plate tectonics – just mobile lid or also subduction? – if the\nlatter, say ‘modern-style plate tectonics’. Rifting does not = modern-style plate\ntectonics, so don’t infer that. If you mean mobile-lid and horizontal movement –\nthen be specific. Don’t leave ambiguity. This a massively important point – be\nvery clear on what you mean.\nDone. As noted above.\n“A proposed lack of mobile-lidplate tectonics was suggested, in part, from an apparent\nlack of Archean rifting (Hamilton, 1998).”\nLine 35-36: I do not understand why you keep making statements. There are very\nfew things in Archean geoscience that can be said with certainty, as a statement.\nThis ‘may’ have been a pivotal time – we don’t know for certain – new data\nsuggests the Neoarchean was pivotal. Don’t state facts that we don’t have for the\nArchean. Half of these statements would be corrected if you just inserted ‘may’ or\n‘likely’. It’s a problem in Archean geoscience that two papers next to each other\ncan make two contrasting absolutist statements that contradict each other. It’s\nmade the science messy, don’t be one of them.\nThe wording has been changed to avoid this problem.\n“The Meso- to Neoarchean is seen by some as a pivotal time in Earth’s history, that was\nsomehow transitional between stagnant-lid and mobile-lid tectonics (Brown et al., 2020;\nPalin et al., 2020).”\nAh ha! Line 43 we get to the point! So, when you say mobile-lid, you DO mean\nmobile-lid +/- subduction. You should remove references to ‘horizontal tectonics’,\nand just say mobile-lid. Horizontal tectonics is disliked as a term by many (JF\nMoyen for one) and it is mostly associated with modern-style plate tectonics. You\ncan remove all ambiguity by simply stating ‘mobile-lid’ tectonics. Many see\nhorizontal vs vertical as problematic language, when it is likely to be a dynamic\ncontinuum.\nDone. We have removed “horizontal tectonics” and replaced it with “mobile-lid tectonics”\nthroughout the manuscript.\nOf course, relatively small-scale rifts of this nature do not necessarily mean there\nwas a global mobile-lid either… but at least local mobility. On Venus rifts are\ntaken up by localised folded mountains, a system known as “tesserae” – these\nare regional dynamic forces in the mantle, and not associated with a true mobile-\nlid. In fact, you should probably mention such features as part of your\ninterpretation – they are a valid interpretation for what we see in the Archean. I\nthink it would be really valuable to clearly present all these options, even as a\nlist if it’s easier, and then choose your preferred option (with reasoning). You have\ndone this in the ‘origin of rifting” section, but you could do more. I suspect you\nhave more words available – don’t be afraid to add more detail.\nA full discussion of this topic is far beyond the scope of this paper. We offer one piece\nof evidence for Archean rifting. We also emphasize that our conclusions do not\nnecessarily support a global tectonic style, but a more local phenomenon for the Abitibi.\nWe have emphasized this the end of the introduction:\n“Although rifting by itself does not prove a large-scale horizontal mobile-lid tectonic\nregime, the existence of crustal fragments that are encircled by rifts are a strong\nindication of at least a local microplate regimemosaic in the Abitibi.”\nLine 50: parts of the Superior are younger than 2690 Ma.\nWe are referring only to the syn-volcanic stage of the Abitibi was between 2.75 and 2.69\nGa:\n“The Abitibi greenstone belt, situated in the southern Superior Province of Canada, is\nthe largest and one of the best preserved Neoarchean greenstone belts in the world\n(Monecke et al., 2017) (Fig. 1). It formed between 2.75 and 2.69 Ga in a series of\nsubmarine volcanic events thought to have been centered in Archean rift basins.”\nLine 58 – spelling error ‘ruft’\nApologies. Corrections were made in the final version of the manuscript but not in the\n\"track-change\" version.\nLine 64 – change to ‘mobile-lid’\nDone.\nLine 85 – Mole et al 2022 should be here, Mole et al 2021 should be at the end of\nthis line. Mole et al 2021is the original source of the isotopic and WR dataset. Use\nMole et al 2022 for any mineral systems inferences. Some mistakes across the ms\nwith these references.\nMole et al. (2022) was the reference used here. This has been corrected throughout the\nrest of the manuscript.\nLine 119-124: It is good you have made this point, but it feels like a ‘we hope this\nisn’t the case” – can you be more specific? It seems a key point – can you prove\nthat we aren’t just seeing the surface effects? Apologies if this is already in the\nMethods. Maybe a reference here would help.\nWe have added the reference to the new Hayward et al. (2024) study that produced a\ngravity inversion model of the Pilbara. There they show, using the same type of\nmodelling, that near-surface mafic volcanic belts, felsic plutons, then density features in\nthe mantle can all be modelled independently of each other.\nLine 131: if the resolution of your model is 10 km, and the range in moho depth is\n32-45 km (line 149) – this range is very close to the uncertainty… does this affect\nhow resolvable the features are?\nThank you. I should have specified 10 km “horizontal” resolution, not vertical, which was\nthe largest spacing between the gravity stations that I modelled. The vertical resolution\nis controlled by the cell size of the model as shown in Figure S1 and described in the\nsupplementary data. They were generally 3 km wide (the tetrahedra aren’t regular, so\nthere’s minor variation in their dimensions) and 3km tall. Unlike cubic cells that are\nstacked on top of each other, tetrahedra can vertically overlap, so the vertical resolution\nof the model is much less than 3 km and closer to 1 or 2 km.\nLine 153: partial melting of the crust\nThis is corrected.\nLine 161: good point\nThank you for making that suggestion in the last round of reviews.\nLine 175: Matheson\nThank you. Another error not changed in the tracked document.\nLine 180-181: length is used twice here, re-write\nOne of those should have been a width:\n“(between 30 and 50 km and in length width between 60 and 130 km in length)”\nLine 183: time? (spelling)\nThank you. Another error not changed in the tracked document.\nLine 183: some actual examples here would be useful, rather than just citations.\nWe have added some:\n”Although regional north-south shortening has likely affected the overall geometry, the\nlengths and widths of the inferred rift zones (between 30 and 50 km and in length width\nbetween 60 and 130 km in length) are comparable to modern continental (e.g., East\nAfrican Rift: Rosendahl, 1987) and oceanic rifts (e.g., in the Lau Basin: Galley et al.,\n2024) (Galley et al., 2024; Rosendahl, 1987).”\nLine 185: hmm is this true? A stronger argument might be that the Archean crust\nwas hotter, and therefore behaved differently during compression – see the paper\nbelow:\nhttps://www.nature.com/articles/s41561-018-0138-0\nI suspect that the rift zones were more likely to survive in a crust that collapsed\nlaterally, rather than thickened vertically. This might provide some better evidence\nfor your point.\nLine 230: apologies, you have already seen the Calvert paper. I still think you\nmight look to use it in the point I mention above. Apologies if I already said this\nstuff last time!\nWe have clarified this statement, adding that lower crustal flow does not change the\ntopography of the Moho.\n“At the same time, we expect the relative topography of the Moho to be largely\npreserved through the deformation owing to the ductile behavior of the lower crust. The\nlower Archean crust has been shown to flow under stress (Calvert and Doublier, 2018),\na condition that leads to the thickening of the lower crust when undergoing post-rifting\ncompression. However, the shallowing of the Moho is still preserved despite the relative\npost-rifting thickening of the lower crust with respect to the upper crust (Bertotti et al.,\n2000).”\nI’m not sure if you mention it, but it seems intuitive that the width of your mapped\nrifts are minimums, and that they have been closed at least partially by the later\ncompression.\nYes, we state “Although regional north-south shortening has likely affected the overall\ngeometry, …”\nFigure 5: As I said – this is a really nice figure. Combining the separate systems\nendorses your model and validates those existing models into a holistic system.\nHowever, for some reason you have cited Mole et al’s Yilgarn paper for these\ndata. The correct reference should be the Mole et al 2021 paper (Precam Res).\nMole et al references seem to be mixed up a bit in their usage: The\nisotopic/geochemical dataset itself is from Mole et al 2021. When using it to refer\nto mineral systems, use Mole et al 2022. If you want to compare your dataset to\nsimilar systems in other cratons, or cite an example of other Archean rifts, use\nMole et al 2019 (i.e., Kalgoorlie-Kurnalpi Rift; KKR).\nThank you for that correction.\nLine 214: be careful when using ‘most reasonable’ – I know what you mean, but\nunfortunately what is reasonable is subjective… Better to say, something like, ‘in\nconjunction, the geophysical and geochemical evidence best supports a rift\nmodel….”\nWe have changed this first sentence of the Discussion to:\n“The presented geophysical, geologic, and geochemical evidence best supports an\nextensional model of the Abitibi whereThe most reasonable interpretation of the linear\nzones of shallow Moho in the Abitibi is that they are the root zones of relict Archean\nrifts.”\nLine 279: you need a reference here\nWe have added the citation:\nJackson, S. L., & Cruden, A. R. (1995). Formation of the Abitibi greenstone belt by arc-\ntrench migration. Geology, 23(5), 471–474. https://doi.org/10.1130/0091-\n7613(1995)023<0471:FOTAGB>2.3.CO;2\nLine 295: “Ey” should be Eu\nCorrected, thanks.\nLines 292 to 295: you have these Eu/Eu*/Y relationships backwards. Values >1\nindicate a deeper source/thicker crust (possibly – some contention on this) and\n<1 shallower crust. This is because, for deep crust we expect high Eu values (no\nplagioclase), and low Y values (garnet present). The high Eu = higher Eu/Eu*,\nand lower Y values result in a higher final number. Hence deep crust = >1. Vice\nversa\nfor shallow crust with plagioclase present, which takes out the Eu, no garnet =\nhigher Y. See below figure from Mole et al 2022. You seem to have this correct in\nFig 5D, so just an error in the text I think. Note the Ui/Yb map in Mole et al 2022\ncorrelates quite nicely with your N and S rifts.\nCorrected, thanks. That did appear to be just a typo in the text, the figure and its\ncaption/legend was correct.\nLine 291-298: This section is good but feels rushed. You talk about La/Sm, but in\nthe Figure 5D, you have Sr/Y labelled (which is generally a depth indicator).\nPlease make the figure and text consistent.\nThe low La/Sm could also indicate less crustal contamination i.e., from passing\nthrough thinner or no silicic crust. Contamination indicator (Th/Yb) correlates\nwith decreasing eNd in Mole et al 2021, possibly suggesting the trend is due to\ncontamination with an older, existing crust.\nWe have added the reference to Sr/Y ratios. The reference La/Sm ratios was\nintentional, but was not accompanied by a figure in this manuscript. We still wanted to\nmention the spatial relationship it has with our modelled rifts.\n“In contrast, the interpreted rift zones lie generally below crust with a more depleted\nmantle source indicated by εHf values (Fig. 5C), thinner crust (i.e., (Eu/Euy*)/Y*10,000\nratios <> 1; Fig. 5D) (Mole et al., 2021), more shallow melts (low Sr/Y ratios; Fig. 5D),\nand less fractionated melts (low La/Sm ratios (Mole et al., 2021)), all consistent with\nrifting of the Archean crust (Mole et al., 2021, 2022).”\nLine 307 – again clarify what you mean when referencing horizontal tectonics.\nThis last section is where you need to be very clear what you are inferring; is it\nmobile-lid tectonics? Papers by Gerya et al have shown how plumes can form\nmobile lids, which then induce short subduction events – this might fit the\ntimeline of events in the Abitibi:\nAs suggested in previous comments we have changed “horizontal tectonics” to “mobile-\nlid tectonics” to avoid this confusion.\nOrigin of rifting section: I know what you are trying to say here but I think it could\nbe more clearly written. I think removing vertical vs horizontal tectonics will help\nit be more specific – talk about the processes you mean directly i.e.,\nsagduction/drip tectonics and the other options discussed in this response. If\nyou are talking around mobile microplates, then say that. You have done a good\njob and trying to extricate the data from going too far, which is great, but I think it\nneeds a bit more polish. For Nat comms, you need to provide more, just saying it\nmeans horizontal tectonics is too vague – show us how well you understand the\ncomplexities of Archean geoscience and go further with your model/process.\nRemember, just because you have evidence of regional mobile microplates does\nnot mean it was a completely mobile-lid – as I said before, Venus has evidence for\nthis and is generally seen as a stagnant lid. I think ‘locally mobile-lid’ covers\npretty much all bases. In the plume model above (Gerya et al), the rifting is\nactually driven by a vertical process, with lateral spread of the ‘plates’ – so I don’t\nthink committing to horizontal tectonics is necessary or even correct at this\npoint. I think you could include this, or at least present this further alternative.\nIf you must use “horizontal tectonics” at least use it after the explanation you\nprovide. That would mean removing it from the abstract to avoid confusion.\nDone. As suggested in previous comments we have changed “horizontal tectonics” to\n“mobile-lid tectonics” to avoid any assumptions about the driver of extension. In the\nfollowing statement, we clarify our understanding of the significance of the geophysical\nmodelling relative to different notions of horizontal or vertical tectonics already\ndiscussed extensively in the literature.\n“Rifting in a modern plate tectonic context is often linked to extensional forces driven by\nplate subduction, whereas in the Meso- to early Neoarchean other processes may have\nbeen more important, ranging from sagduction/drip tectonics (e.g., Mareschal and West,\n1980; Collins et al., 1998) to plume-driven processes (e.g., it is believed mantle plumes\nand mantle convection drove extension (Gerya et al., 2015; Harris and Bedard, 2014;\nPiccolo et al., 2020). Rifting could also have occurred in regimes exhibiting only\nsagduction/drip tectonics driven by density contrasts in the crust (Mareschal and West,\n1980). The emplacement of dense mafic material over less dense crust and partially\nmelted lower crust leads to the sinking, or sagduction, of the mafic material (Collins et\nal., 1998) (so-called dome-and-keel structures). Rigid crust, typical of horizontal The\nresults presented here provide evidence of at least local mobile-lid tectonics Mobile-lid\ntectonic regimes allows for where rifting was to be focused along discrete corridors,\nwhereas more ductile crust in vertical diapirism and sagduction driven other tectonic\nregimes leads to disseminated extension across large areas (Choukroune et al., 1995).\nThe Moho topography of the Abitibi best fits a horizontal mobile-lid tectonic regime with\ndistinct corridors of shallow Moho encircling regions of thicker crust (Fig. 5). This\ncontrasts with the coupled shortening and extension of the crust associated with\ndiapirism and sagduction in a vertical tectonic regime (Johnson et al., 2014).”\nLine 320-321 – you use ‘although’ and then ‘however’ in next sentence – these are\nsynonyms, reword this.\nThis sentence was replaced in the conclusion.\nLine 321: why? What about the model suggests this?\nThis question is addressed in the concluding remarks.\nLine 323: don’t need ‘widespread’ here, it’s ambiguous and I’m not sure you have\nevidence for it.\nIt has been removed.\nLine 323: This is not the best way to end the paper. If you have words left, a\nparagraph bringing the reader back to the main point, and its implications, would\nleave a better impression. Perhaps use the last sentence to summarise the\ndiscussion of different models/ideas from the section above? Or leave the reader\nwith what further implications may be? i.e., “ our results show a strong empirical\nrelationship between mantle plume activity, microplate formation, and major\nmineral systems for the Abitibi, which imply at least a locally mobile-lid tectonic\nenvironment existed in the Neoarchean. These results have implications for the\nformation and evolution of all cratons, as well as their host mineral systems.”\nSomething like that?\nWe have revised the last paragraph to highlight the possible implications of our findings\nfor the larger audience.\n\"This study presents a new gravity-derived model of the Moho surface beneath the\nAbitibi greenstone belt that reveals a possible Neoarchean rift network. The geometry of\nthe proposed microplate mosaic is supported by trace element and isotopic data of\nassociated volcanic rocks and the occurrence of rift-related mineral deposits. The\nresults show a strong empirical relationship between mantle plume activity, microplate\nformation, and major mineral systems in a local mobile-lid tectonic environment.”\nI think it would also be worth making the point that deep sea hydrothermal\nsystems have been associated with these rifts, as exemplified by the VMS\nsystems. The origin of early life may be connected to these type of settings, and\nhence these new maps of Archean rift systems map out potential locations for\nearly life i.e.,\nhttps://www.sciencedirect.com/science/article/pii/S0301926814001454\nThis might be a nice way to end the paper, to show it has profound implications\nfor the wider Earth system and environment.\nThere is also a new paper in Geology (screen shot of a figure below):\nhttps://pubs.geoscienceworld.org/gsa/geology/article/53/3/269/650953/Inception-\nof-ridge-ridge-ridge-triple-junctions\nIt might be good to reference and/or include this evidence for rifts in an arc\nenvironment, in the interest of balance. It doesn’t mean you need to commit either\nway, but it would mean you present possibilities for multiple settings. It also\nsupports your ‘mobile-lid’ / ’laterally-dynamic’ tectonic process. Perhaps with the\nkomatiites occurring at the same timing of rifting, and komatiites connected to\nplumes (ambient mantle was too cool; Nisbet et al.), the plume model is more\nlikely; but you should decide what you prefer.\nThank you for these suggestions. We added the particular references and suggest\npossible implications of a mobile-lid regime within the Abitibi in the previous revised\nparagraph. However, we prefer to focus on the merits of our modelling for others to\nconsider rather than a review of the literature on Archean geodynamics. We trust that\nknowledgeable readers will grasp the significance for early Earth tectonics, mineral\nsystems, and \"origins of life\" without a full discussion of all the models here.\nMajor changes required:\nRemove all occurrences of ‘horizontal tectonics’ – replace with what you mean,\nwhich is ‘local mobile-lid tectonics’. I know what you mean, but horizontal\ntectonics as a term is closely associated with subduction and modern-style plate\ntectonics, so better to use something else.\nDone.\nThere are other possibilities you need to explain and explore, to make the origin\nof rifting section more thorough and less vague.\nWe have expanded on the first paragraph in this section with reference to rifting in a\nmodern context and (in the revised paragraph) with reference to other processes\nranging from sagduction/drip tectonics (e.g., Mareschal and West, 1980; Collins et al.,\n1998) to plume-driven processes (e.g., Gerya et al., 2015; Harris & and Bedard, 2014;\nPiccolo et al., 2020).\nAs stated in the first review, do not make statements that are not currently valid in\nArchean geoscience. The reality is we don’t know enough on many aspects to\nmake statements – for most points there is a counterpoint. This is easily fixed by\nqualifying statements by adding ‘may’ or ‘likely’ to take the absolutism out of the\nsentence.\nThank you for helping with this and in refining the manuscript and its message.\nTidy up the quality of the writing – it feels a bit rushed. There are numerous\nspelling mistakes, and some sentences could be clearer. Be very careful when\npresenting geochemical data – there are some major blunders in there.\nWe have made an effort to polish the manuscript. While there regrettably were some\nspelling mistakes in the “Tracked” version of the document, they were all caught in the\nfinal un-marked version.\nReviewer 2 comments\nThe authors made appropriate revisions to the paper, which is now suitable for\npublication.\nTaras Gerya, Zurich, 17.02.2025\nThank you.\nReviewer 3 comments\nI have looked over the revised paper of Galley et al. Unfortunately only a few of\nmy comments have been addressed.\nThe assumption of 3070 kg/m3 for a mantle density has still not been justified. The\ndensity, which is more typical of lower crustal is too low for the mantle – it is even\nlower than the density (3180 kg/m3) normally assumed for the mantle at an\nactively spreading mid-ocean ridge!\nThe 3070 kg/m3 density contour is not assumed to be the density of the mantle, but\nrather the density at the interface between the lower crust and the mantle. You are\ncorrect, typically when the upper mantle is assumed to have a homogeneous density it\nis closer to ~3300 kg/m3, and a homogeneous crust can be typically ~2670 kg/m3 for the\nupper crust (continental) and ~2900 kg/m3 for the lower crust. The density at the\ninterface between the lower crust and the mantle would be represented by a value\nsomewhere between that of the upper mantle and the lower crust; i.e., between the\n2900 kg/m3 lower crust and 3300 kg/m3 upper mantle, or 3100 kg/m3. Model testing\ndone by myself and others (such as Dr. Kim Welford who advanced the gravity\nmodelling method for a heterogeneous density model of the crust/upper mantle to\nresolve the Moho) have found that the 3070 kg/m3 contour most accurately fits the\nMoho.\n“The Moho interface was then extracted from the 3D density inversion by isolating the\n3.07 g/cm3 iso-density surface. This iso-density layer has been shown to be an\naccurate proxy for the Moho when performing minimum-structure inversion modeling\n(Galley et al., 2024; Welford, 2024; Welford & Hall, 2007), in contrast to the commonly\nused surface-based Parker-Oldenburg modeling method (Oldenburg, 1974; Parker,\n1973). The latter solves for the depth of an interface between an assumed\nhomogeneous-density crust (e.g., 2.85 g/cm3 (Chappell & Kusznir, 2008)) and the\nmantle (e.g., 3.33 g/cm3 (Chappell & Kusznir, 2008)). We opted not to use the surface-\nbased modeling method, instead using the more sophisticated, albeit more\ncomputationally demanding, minimum-structure inversion method (see Methods for\ndetails), to account for the known heterogeneity in the density of the Abitibi crust\n(Esmaeil Eshaghi et al., 2023). Our more heterogeneous density model would then\nresolve a Moho interface at a density value between that of the crust and mantle.”\nThe inverted Moho is still being compared to travel-time seismic reflection data.\nThere should be an explanation of how these data have been converted to depth\nand what the uncertainties on these depths might be. Readers should not have to\nsearch through the literature for this information.\nWe have added more information on the processing of the seismic reflection data to the\nsection “Comparison with Seismic Profiles”, as well as more details for the other two\nmodels we used to compare with our gravity model. We also added a comparison to a\nseismic refraction model of the Moho to the same section and Figure 3.\n“The results of the gravity inversion are compared to Moho depths from the Lithoprobe\nreflection seismic experiments (John Ludden & Hynes, 2000), a Lithoprobe seismic\nrefraction model (Grandjean et al., 1995), those from the CRUST 1.0 model (Laske et\nal., 2013), and from teleseismic data (Rondenay et al., 2000) (Fig. 3B). The reflection\nseismic sections were depth-migrated using a constant velocity model of 6 km/s,\ncreating an approximate depth model of the Moho and overlying crustal features\n(Ludden & Hynes, 2000). The seismic refraction line modeled by Grandjean et al.\n(1995) resolved a Moho line from a 2D velocity model of the crust and upper mantle,\nproviding more accurate depth information on the Moho. The teleseismic measurements\nled to a series of velocity models of the Abitibi crust and upper mantle, (Rondenay et al.,\n2000). The CRUST 1.0 model is the coarsest of the Moho models with a spatial\nresolution of 1 arc-degree (~111 km) and Moho depths derived from a velocity model of\ngeneralized Archean crust (upper crust: 6.2 km/s; middle crust: 6.4 km/s; and lower\ncrust: 6.8 km/s) (Laske et al., 2013).”\nThe authors have now provided a Bouguer anomaly profile, but the vertical scale\nrange of 1000-1060 mGal in Figure 4D makes no sense. I could not look up what\nthe Bouguer anomalies are in the study area because none of the maps in the\npaper have latitude and longitude (they should do!) but Government maps show\nBouguer anomalies in the range -20 to -100 mGal southeast of Hudson Bay.\nWe used “processed Bouguer anomaly data” and have noted this in the figures and text.\nIn the section “Further Bouguer Data Processing” in the Methods we have explained\nwhat steps were taken to process the Bouguer data.\n“To model the Moho topography in the Abitibi, we inverted processed Bouguer gravity\ndata compiled by Natural Resources Canada (Canadian Geodetic Survey, 2022) (Fig. 2;\nsee Methods for details on data processing) to produce a three-dimensional (3D)\ndensity model of the Earth’s crust (Fig. 3).”\nThe output of the inversion modeling is a density structure that has still not been\nverified by forward modeling. It should be a relatively straight-forward process to\ntake the density contours such as those shown in Figures 3B and 4D and\ncalculate the gravity effect of each contour using a line-integral method and the\ndensity contrast between layer above the contour and the layer below.\nIn summary, I am not convinced that the undulations in the Moho proposed in the\npaper are required to fit the observed gravity data. The authors need to\ndemonstrate more clearly that the sources of the Bouguer gravity anomalies are\nnot within the crust and that Moho is not flat.\nIn response to the previous review we did verify the inversion model with forward\nmodelling, see Figure S4, and explained that the gravity anomalies resolved are much\nlarger that the model residual, and are therefore significant and not just noise (the\namplitudes of the forward signal of the mantle component range from ~20 to 50 mGal).\nWe did not produce a forward model of contours because that would be an\napproximation of our model. Our model is not composed of homogeneous layers\nseparated by contours, but rather tetrahedral cells. So, it was more accurate to calculate\nthe forward model of the cells that lie above the Moho (Fig. S4C) that represents the\ngravity anomaly produced from the crust, and compare that to the forward signal from\nthe cells below the Moho (Fig. S4D). As suggested, most of the signal of the modelled\ndata is fitted by features in the crust, but there is still a large, regional component that\nneeded to be fit by deep features, which we interpret as the Moho/upper mantle.\nWith further explanation of the forward modelling (see below) we believe we have\nshown that the undulations in the Moho are required to fit the observed gravity data,\nthat not all of the sources of the Bouguer gravity anomalies are within the crust, and that\nMoho is not flat. Density features in the crust were not enough to fit the modelled data,\nand that the interpreted undulations in the Moho are supported by the teleseismic,\nseismic refraction, and seismic reflection profiles.\nWe added a new section to the Methods section to explain this further:\n“Forward Model Verification\nTo verify that the density anomalies in the upper mantle"}, "author_response": "Reviewer # 1’s comments\nReviewer 1 found the manuscript to provide important insight into Archean crustal and tectonic activity, with a dataset and imagery that were convincing and match other datasets related to the Abitibi crust. The reviewer was not convinced that this paper needed to get into \"the Archean geodynamics controversy\", and we agree. Rather, the impact of the paper is in imaging relict Archean rifts alone, and seemed less concerned about the plate tectonic debate. Nevertheless, we have added a fuller discussion of how alternative tectonic scenarios would influence the interpretation of our results. The abstract is re-written, and the paper has been edited to be less ambiguous about the general findings.\n\nMajor points\n1. More figures to backup the arguments and models – at least one schematic figure clearly showing what they consider ‘microplates’\nA new illustration has been included as Figure 5.\n2. A figure showing how they see the temporal evolution of the Abitibi, in regard to their model/data, including the late multi-phase compression.\nWe have further discussed the impact of the late compression event and show the age progression of crustal growth in our new synoptic Figure 5B.\n3. More links to the isotopic maps of Mole et al (2021) to reinforce the arguments.\nWe agree and have discussed the similarities to Mole et al. more in the revised text and in Figure 5.\n4. Are microplates required to explain your data? The Reviewer called for more justification of a model whereby the proposed complex network of plates and triple junctions is really a microplate mosaic, citing examples from Iceland, Lau Basin, Japan.\nDifferent views have been expressed about what constitutes a \"microplate mosaic\", ranging from subduction driven fragmentation to plume induced fragmentation. To clarify, we have included a straightforward definition (i.e., crustal fragments that are encircled by rifts) that do not imply a particular tectonic regime. In particular, proximity to subduction is not necessary for microplate formation. We cite the key literature on this topic (Mammerickx & Klitgord, 1982; Mueller et al., 2017) that describes the typical sizes of microplates and their kinematics that closely resemble the structures inferred from our model.\n5. You have 41 refs, but Nat comms allows up to 70, hence I have added a number of suggestions.\nThe suggested references have been added, and the text and figures have been revised accordingly.\n6. Include more on the geochemical and isotopic mapping that has been done, which supports your arguments.\nWe added more references to the extensive work of Mole et al. to strengthen the interpretations of the geophysical model results.\n7. The reviewer did not see why rift networks and triple junctions could not occur in a stagnant lid or \"non-plate\" scenario, citing additional details in the comments below.\nThis is addressed in comment #17 below and in the revised manuscript.\n8. The reviewer was not sure this paper needs to get into the Archean geodynamics controversy – the data on rifts is of high impact enough. From what I can see, it could be sued to make arguments for multiple settings, and does not support the existence of plate tectonics, perse.\nOur model focuses on physical evidence in the deep crust and mantle for the existence of Archean rifts in the Abitibi, and how they could have been connected. The geometry and potential connectivity of the rifts point to a microplate model, and we describe how this could apply to a variety of tectonic processes from plate-dominated to non-plate dominated, including a plume setting.\n9. In the end, the reviewer thought it would be good to discuss various settings, AFTER presenting the main finding of the rift architecture, including what might be the best modern analogy.\nWe added a new section specifically addressing tectonic scenarios for the modelled rift zones, including specific comparisons to the microplate architecture of present-day Iceland as an example. We argue the Moho topography of the Abitibi best fits a horizontal tectonic regime, which contrasts with the coupled shortening and extension of the crust that would be associated with diapirism and a vertical tectonic regime.\n10. Use of geology to support the model (e.g., the occurrence of komatiite).\nMany known occurrences of komatiite align with the rifts, but some don’t. This is partly because the modelled rifts are deep features, and komatiites, like the major faults, are near-surface features that may be displaced relative to the Moho topography due to lateral crustal flow (as see in in the Yilgarn). This is discussed in the revised text, but a more detailed description of the surface geology, vis-à-vis the rift model, is being prepared for another manuscript.\n11. There is good evidence that these rifts were continental in nature (syn-volcanic zircon from these rifts do not suggest an oceanic origin, as well as rare, but constrained Mesoarchean zircon inheritance). How does that affect the model?\nSee response to comment #4.\n12. Can you infer plate tectonic scale features based on the scale of features you have mapped – i.e., in a diagram that summarises your findings and clarifies the scaling.\nThe new Figure 5 clarifies these relationships and the scales. We describe in the revised text (and in the response to comment #17) our interpretation of the rift geometry with comparisons to different tectonic settings.\n13. Line 10: Whether plate tectonics was indeed occurring in the late Archean is still debated – it should not be presented as fact.\nWe have adjusted the language accordingly.\n14. Line 27: This model has been published by others for the Superior and other cratons and should be cited.\nWe have added the references.\n15. Line 28: Weak hot crust would be more likely to flow laterally, which could possibly aid rifting.\nWe have clarified this statement, emphasizing the comparison of hot Archean crust to modern seafloor spreading centers and subduction-driven rifting.\n16. Line 32: Again, be careful stating debated points as facts.\nWe have changed the sentence to emphasize the general consensus, rather than a statement of fact.\n17. Line 36: Rifts do not necessarily = mobile lid.\nSee response to comment #4. We have clarified the distinction between horizontal (or mobile-lid) and vertical tectonics, as objectively as possible. In horizontal tectonics the crust comprises many rigid or semi-rigid lithospheric plates bounded by divergent, convergent, and/or transform margins. We believe this is what our model shows and emphasize that this is the extent of our results, without reference to the motion of the plates (only the extension and fragmentation). We think most readers already recognize that the broader aspects of subduction are still a matter of debate, and we have tried to characterize them as such.\n18. Line 37: Previous work on Abitibi rifting.\nWe have revised the text to more fully describe the previous work on Abitibi rifting, based on isotopic mapping, and agree it is great that these two completely independent datasets are showing such similar features.\n19. Line 50: What is the evidence that the Moho topography have survived 2.6 Ga of modification?\nWe have added a comparison of our model to the time-resolved isotope data from Mole et al. (2022) and also to the distribution of VMS, as two independent pieces of evidence for survival of the Archean Moho topography and the corresponding rifts, as suggested by the reviewer. The time-resolved εHf map, in particular, shows the existence of a rift-like feature at >2695 Ma that corresponds closely to the Moho topography shown in Figure 5.\n20. Line 62: Why is the mid-lower crust not affected by the deformation?\nWe explain in the revised manuscript that the N-S shortening thickened the crust by upper crustal imbrication above the brittle-ductile transition zone. Seismic profiles have shown that this imbrication does not extend to the Moho, thus the Archean Moho topography is preserved. The mid-lower crust is also not affected because is it largely below the BDZ.\n21. Line 83. Is it possible the intrusive (less dense) and volcanic areas (more dense) are creating the appearance of belts of deep/shallow Moho? Dense volcanic rocks are understandably more abundant in rifts, but why would intrusives dominate the flanks?\nDeeper features, such as Moho topography, which we interpret as rifts, produce long wavelength, low amplitude anomalies in the gravity data. Near-surface features produce shorter wavelength, higher amplitude anomalies. The iterative process of the inversion modelling is able to resolve these in 3D. We explain in the revised text (see also comment #24) that the abundance of felsic plutons at the flanks of the rift zones may reflect shallower levels of erosion at the rift flanks compared to the rift valleys, the latter subsiding and being filled with more dense mafic volcanics. Subsequent erosion would also have led to a greater exposure of the pluton infused crust between the rifts resulting in the pattern we observe today, acknowledging that some intrusions may be younger than the rift system.\n22. Line 93: Why do you not show any 3D images?\nThe model is 3D, but we find it is more informative to show aspects of the model in plan view or cross sections (Fig. 3B and 4D) rather than in a 3D perspective. We could extract a 3D surface from the model to represent the Moho, but studying it is best done in section and elevation/depth maps.\n23. Line 94: I would argue here that you have added further resolution, and confirmed the architecture, but it is not the first rift architecture map of the Abitibi.\nWe have revised to text throughout the manuscript to more accurately describe the contribution made here relative to the sparse Moho information available from seismic studies and isotope mapping.\n24. Line 116: If the rifts are dominated by volcanics, perhaps this is because they were localised into grabens, which filled with volcanic material and form a lower crustal level. The plutons are represented by the felsic intrusion dominated flanks, which are horsts. There are likely felsic intrusions under the grabens, they just aren’t exposed yet. This is important because many people think we cannot image the rifts as has been done here and isotopically.\nWe have added this discussion to the text and in response to comment #21.\n25. Line 119: What about the thickness of volcanic rocks?\nWe have clarified this in the text and in response to comments #21 and 24.\n26. Line 122: Can we presume these rifts would have been much wider, and what we are seeing is the inverted, closed versions? Also the ‘depth’ of the moho is presumably a minimum?\nYes, this is correct, and we have emphasized that the modelled features are post-tectonic. We have tried to restore the N-S compression and E-W shearing but chose to leave that for another paper, given that it is not directly related to the gravity modelling. We also avoid the term “crustal thickness”, as opposed to \"Moho depth\" to avoid any assumptions about the depth of erosion and thickness of the crust at ~2.7Ga. Instead, we focus only on the topography of the Moho.\n27. Line 139: Please clarify what crust you think is being rifted.\nWe note that the rifts are similar in their architecture to continental rifts and may have developed in crust with a thickness similar to that of modern continental crust. However, we do not discuss compositional of petrological implications that would be far beyond the scope of this paper.\n28. Line 152: Lateral ‘flow’ of hot mafic crust could explain how the lower and mid-upper crust can become dissociated.\nWe have added this point to the discussion.\n29. Line 158: Rifting versus discrete plates and triple junctions.\nSee response to comment #4 and #17. We have provided the accepted definition of microplates in the paper and discuss them several times in the context of crustal fragments that are encircled by rifts. We do not imply a particular tectonic regime and consider that they can form in either crustal type (oceanic or continental). We acknowledge that rifting can occur without discrete plates or triple junctions, but we show here that triple junctions likely existed, requiring a microplate architecture.\n30. Line 160: Why are we only hearing about this oblique zone now?\nWe have moved the description of the SW-NE trending corridor of shallow Moho to the results to better introduce this feature in the context of the modelling.\n31. Line 166: How continuous are these rifts?\nWe have described the segmentation of the rifts (and the scale of segmentation) which is similar to modern continental and ocean rifts, including the boundaries of microplates.\n32. Line 168: It would be good to see the rifts presented as one continuous system.\nWe have illustrated the potential continuity of the rifts in the new synoptic Figure 5.\n33. Line 171: Please reference points that are not original.\nReferences have been provided where indicated.\n34. Line 175: This is the interpretation of Mole et al 2021, 2022.\nReferences have been provided where indicated.\n35. Line 175: Note the occurrence of these kind of transform systems in Iceland.\nWe have added the comparison.\n36. Line 180: Reference the isotopic mapping which imaged this feature and supports your identification of it.\nWe have added the comparison.\n37. Line 180: You have excellent evidence for a complex rift system, but can you demonstrate the rifts separate ‘plates’? Isotopic and geochronologic evidence suggests pre-existing crust, so how does this factor into your model? Triple junctions are usually between oceanic or mafic plates.\nWe have addressed this question in our definition of microplates, which are closed polygons encircled by rifts. Triple junctions are a requirement of microplate formation, regardless of the makeup (oceanic or continental) or age of the crust. The oblique shallow Moho anomaly in the east creates the possibility of triple junctions in our model. We have tried to re-emphasize these points in the revised text.\n38. Line 180: If you are going to make the link to horizontal tectonics, here would be best.\nThis discussion has been moved to another location.\n39. Line 183: The link to ore deposits requires more references, including in the introduction.\nDone.\n40. Line 195: Be more clear about what crust you think is being rifted (oceanic versus continental).\nSee responses to comments #4 and #29. We chose not to include a discussion of the makeup of the crust, but focus instead on the Moho topography as the main control on rifting.\n41. Line 196: You have not discussed to any extend the nature of the crust.\nSee response to comment #40.\n42. Line 204: Wording\nAdjusted.\n43. Line198-204: You do not need to get into the Archean geodynamics controversy with this paper. The identification of the rift architecture is big enough news.\nWe have clarified that the rift model does not imply a particular geodynamic, including subduction. For detailed comments regarding the different scenarios see responses to #4 and 17.\n44. Line 207: The end of the paper is not decisive about linking the data to plate tectonics and ‘plate boundaries’.\nWe have modified the ending to better match the tone of the paper throughout.\n45. Figure 2: Improvements\nWe have added the outline to the data figure, and the corresponding geographical details are provided in Fig. 1. We have added landmarks and labels to the other figures, including the new Fig. 5.\n46. Figure 3: The red-blue colour profiles used are not generally good for colour-blind people and may not present data clearly. Also be consistent across all figures.\nWe are familiar with Crameri at al. 2020 and use the CET colour ramps the paper discusses. The red-white-blue colour ramp in Fig. 3 is standard for density figures, but the same bounds are not used in Fig. 4. Crust is mainly blue and the mantle red. This is explained in the caption. Variations in density are the main focus of Fig. 4, and particularly Fig. 4D, where we removed the mantle component to highlight just the variations in the crust.\n47. We encourage the authors to ensure all raw data are available for others to repeat and test their model.\nWe have added the data and instructions on how to access the modelling program.\n\nReviewer # 2’s comments\nRevier #2 found the manuscript to contain important new data and hypotheses that potentially offer better understanding of Archean geodynamics. However, improvements are suggested for better presentation of the materials in support of model, in particular several details of the figures and a conceptual sketch representing the proposed geometry of microplates, rift centers and triple junction(s).\n\nSpecific comments\n1. Fig. 4. The districts are not indicated on the figure.\nWe have added the labels to the figure.\n2. Line 139. The linear zones of shallow Moho need to be clearly indicated in Fig.4A, which is otherwise difficult to interpret. Geometry of the proposed triple rift junction needs to be outlined.\nWe have created a new Figure 5 to better outline the rifts, and compare them to the Moho depth model. We also introduce the role of oblique extension earlier in the paper that highlights the connectivity of the rifts and the locations of possible triple junctions that support the microplate model.\n3. Line 160. All these names are not in Fig.4A and it is difficult to guess which features are being referred to.\nWe have added all this information in the new Figure 5.\n4. Line 207. This discussion is not supported by an adequate conceptual figure containing the interpreted sketch based of Fig. 4A, the indicated microplates, rift centers and triple junction(s).\nThese features are all indicated in the new Figure 5.\n\nReviewer # 3’s comments\n1. Line 49. There is an assumption in this paper that studies of today’s gravity field can be used to infer crustal structure in billion-year-old cratons. This assumption should be stated and justified.\nWe have revised the text to better explain how the Archean Moho topography is preserved through subsequent deformation (see response to comment #20 of Reviewer #1). We outline in the introduction how gravity inversion modelling can reveal the original Archean Moho and that any subsequent deformation has not erased the original topography. We have also discussed how studies have shown the lower crust/ Moho is largely unobscured and can indeed support large stresses on long geological timescales (Benn, 2006).\n2. Lines 68/69. The statement that “Inversion modelling creates a petrophysical model” is misleading. Inversion in gravity interpretation is a procedure that creates a body with a certain specified density contrast with its surroundings directly from an observed gravity anomaly.\nWe have clarified this statement in the Methods section. There are two main types of inversion modelling parametrizations: geometric and voxel-based. In geometric inversion modelling, the physical properties in the model are fixed (assigned) and the geometry of the body is solved for. We perform minimum-structure inversion modelling, which uses a voxel-based parameterization. This type of inversion modelling produces a pixelized petrophysical model of the sub-surface.\n3. Line 71/72. Why is it relevant that the vidi inversion code was run on Canada’s narval cluster? Is the code limited to certain platforms? Is it publicly available?\nWe mention the Narval cluster to document the infrastructure and resources used in the modelling. The code is not limited to certain platforms. We have added information on how the program can be accessed in the Code Availability section. It was developed at Memorial University of Newfoundland, and is now the main tool employed by Geotexera Inc., a commercial company. The executables can be shared by contacting the senior author.\n4. Line 73. Give the reasons why the “3.07 g/cm3 iso-density surface” was selected for the Moho.\nWe elaborate on our choice in the revised manuscript. We used 3.07 g/cm3 as the iso-density surface, which has been shown to accurately fit the seismic Moho when performing minimum-structure inversion modeling 19,21,26. This contrasts with the commonly used surface-based Parker-Oldenburg modeling method 27,28, that solves for the depth of an interface between an assumed homogeneous-density crust and mantle. Due to the known heterogeneity in density in the Abitibi crust33 we opted not to use the surface-based modeling method, instead using the more sophisticated, albeit more computationally demanding, minimum-structure inversion method.\n5. Line 78. Explain what the “signal” is that was output from the inversion code. The measured Bouguer anomaly is referred to the geoid and assumes in its reduction a certain density for the rock between the physical surface and mean sea level. Was the “signal” similarly referred to sea level?\nWe have modified the figure caption to explain the nature of the signal. The inversion code outputs the forward signal of the inversion model. The difference between the inversion model's forward signal and the measured Bouguer anomaly is the model residual, which is plotted in this figure. The forward signal of the inversion model would be similarly referred to sea-level.\n6. Line 100/101. The Moho derived from the inversion model is compared to CRUST1.0, a teleseismic model and five Lithoprobe seismic lines. However, there are issues associated with each of these sources of seismic data. There is [1] no evidence provided in the paper that there is any active source data available within the region of the study area. The telesesismic data is limited to Receiver Function data which is based on an assumption of P wave crustal velocity between the physical surface and the Moho. There is [2] no evidence provided in the paper of what velocity was used or discussion on whether or not this velocity is an appropriate one to use in the study area. Finally, the Lithoprobe data are seismic reflection profiles and so are in two-way travel time, not depth. There is [3] no evidence provided in the paper of what velocities were used to convert the reflection data to depth.\n1) We provide evidence in Fig. 3 showing the locations of each of the seismic surveys (which were located within the Abitibi), and the citation to each of the respective studies.\n2) Each of the published surveys used in the paper provides details on the processing and modelling that were done to resolve a Moho interface. The seismic studies all focused on the Abitibi and used velocity models best tailored to this region’s crust.\n3) All the velocity information is provided in the original studies. The reflection profiles are originally in two-way travel time, but they were converted to depth as in other Lithoprobe studies.\n7. Figures 3 and 4. The relationship between the observed Bouguer anomalies and the inferred Moho undulations is very difficult to see in the paper. At the very least profiles of the observed Bouguer anomaly should be shown in both Figures 3 and 4 so that the reader can see the relationship.\nWe have added the Bouguer data near the Figure 4 profile to better show this relationship. Since this is not a geometric inversion there won’t be a 1:1 relationship between the Moho topography and the Bouguer anomaly. We show that many density variations in the crust contribute to the overall Bouguer anomaly signal (see the new Fig. S4). We hope the change to Fig. 4 and the new Fig. S4 clarify the relationship.\n8. Line 105. Was it not assumed in the model that the sub-surface density structure was layered?\nWe did not assume that the sub-surface density distribution in the Abibiti was layered. As explained in the “Gravity Modelling of the Moho” section, our modelling produces a 3D density distribution in the sub-surface, from which a contour level can be isolated to represent the Moho. But the model is not itself a collection of layers. To help clarify how we parameterized our model we added a figure of the model mesh to the Methods section, showing the distribution of “pixels” that made up the model. We also added further description on the parameterization used to produce the model.\n9. Acknowledgments. Makes no mention of data availability or access to the inversion software used in the paper. Web links should be provided that link to the Bouguer anomaly and topography data and to the vidi inversion code.\nWe have added these components to the Data Availability and Code Availability sections.\n10. In summary, the reviewer had many concerns about the validity of our results because of the lack of forward modelling.\nTo address the reviewer's concerns, we performed forward modelling to show that the undulations in the Moho produce a signal much greater than the model residual, and therefore is significant. However, we believe these concerns are related to a misunderstanding of the parameterization used in our modelling (see response to comment #2). Because we did not perform a geometric inversion, we were not solving for a (or a collection of) iso-density layer(s). If we had, indeed forward modelling the signal from each layer would inform how each layer contributes to the total signal. Instead, we solved for a 3D distribution of density throughout a model of tetrahedral cells. Isolating the component of the model that lies below the resolved Moho, and forward modelling its signal shows the undulations in the Moho (and density anomalies in the upper mantle). The anomalies have amplitudes of 20-30 mGal, well above the model residual, which has a mean of 0 mGal and standard deviation of 2 mGal (see the new Fig. S4).\nRegarding the undulations observed in the Moho, the many seismic studies performed in the Abitibi have all shown changes in Moho depth, as we showed in Fig. 3B, which we verified our model against. Our model would be wrong if it presented a flat Moho. If forced to have a flat Moho, the model would still produce a density distribution in the crust to fit the observed Bouguer anomaly data. However, we verified our model depths to those previously published (and derived from methods independent of gravity modelling)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Introduction\nThe revision of this paper by Galley et al represents an improvement on the previous version. The majority of the revisions are excellent – however a few further comments are provided below. This paper deserves to be published in Nat Comms, but it needs to be more exact in its points in key places. Parts of the paper and its major points on tectonics are still far too vague.\n\nThis is a good paper, but it could be a great paper with a bit more work and care and attention. The are numerous typos and errors, and the prose itself could be clearer in places. However, the science is sound and I think this will be a really nice contribution after these revisions. I envisage that after this relatively straight-forward revisions the paper will be suitable for acceptance.\n\nResponse to previous comments\nI do not particularly like the way the authors have summarised the reviewer’s (my) points and then made their comment or rebuttal. Please paste the actual comment, verbatim, into your report, and then address it. This avoids any ambiguity; the editor and I can see the original comment next to the response.\nApologies. This has been changed.\n\nIn general, the revisions were thoughtfully performed and comments addressed or rebutted well. However, I have further questions regarding the responses below:\nThank you for adding figure 5. This is excellent and I think is a very powerful link between the geology, geochemistry, and geophysics. This is something that is, unfortunately, quite rare. I suspect this figure will be used a lot.\nThank you.\nI still find the microplate argument ambiguous. Were these pieces of crust actually totally separate plates? Or different terranes? I am not sure… but I think calling them microplates will at least generate some interesting discussion. Your definition is progress, but the rifts surely have to be ‘mantle taping’? i.e., like at plate margins? I wonder if the situation in East Africa is a good analogy. Some discussion on the size and geometry of microplates needs to be in the main text. Apologies if I missed it.\nWe have expanded the description of microplates in the introduction\n“Bird (2003) defined modern microplates as small, mostly rigid areas of lithosphere that range in size from 1000s up to 1,000,000 km2. Detailed studies of today's oceanic microplates show that they behave as independent plates but are continuously deforming structures (MacLeod et al., 2017; Baxter et al., 2020).”\n\nYou say “the abstract is re-written” but the track-changes show only four changes that are extremely cosmetic. The two statements you make in the abstract are not factually correct (lines 9-10 and lines 20-21) and these need to be revised or toned down. It will not affect your ms as far as I can see but be more accurate to current Archean geoscience. The reality is, we cannot be sure of many things in Archean geology and geodynamics, and hence we need to be open about that as we try to push things forward.\nI think I made this point in the first review – the first line of the abstract is not appropriate (line 9-10). It makes a statement as fact, that we do not know. We do not know that plate tectonics started at 2.7 Ga – this is not a fact and needs to be removed or edited. I’m sorry, but I insist on this. This is because there are a whole range of opinions on when PT started, from the Hadean, Mesoarchean, Neoarchean, to Neoproterozoic. Even if you mean “non-subduction, mobile-lid plate tectonics”, this statement will not necessarily be read this way – if you do mean this, you should at least clarify it. Furthermore, this would also be debated – some think a stagnant lid is appropriate for the Neoarchean. I do not see why microplates and rifts could not exist in a stagnant lid, with localised extension taken up by ‘crumpling’ at the margins, similar to Phanerozoic ‘intraplate orogens’ such as the Alice Springs\nOrogen. Again, this is what is theorised for Venus. A connection of rifts does not necessarily mean moving plates – if you want to infer this, you probably need further evidence. Referencing some paleomagnetics from Archean rocks would be a start, such as these papers:\nhttps://pmc.ncbi.nlm.nih.gov/articles/PMC7176424/\nhttps://www.sciencedirect.com/science/article/pii/S0012821X0700516X?casa_token=DtodWmKuBKkAAAAA:m3VPwWcIdcMmDoZTCtttjlHUPMWgYhWKBOT2fOHDYOjj_2PxuoNDRNMycU4YqaBYfDIXx8lyDA\nThey show evidence of plate movement in the archean, although as you can imagine, this is contested. I personally like the idea of episodic subduction, and this matches the data in Mole et al 2021 for the SE Superior. However, these rifts appear to be ‘pre-subduction’, if subduction started at 2.695 Ga in the Abitibi, so in that context they would potentially need another driver.\nOn top of this stuff, some of the hottest magmas on Earth are known from the Neoarchean, which suggests this may have been a thermal peak, rather than a cooler inflexion (see Herzberg et al 2000). You can say this, but not as a statement. You need to add ‘may’, or ‘possibly’ etc. If you feel this is a result of your study, that’s fine, but don’t open with it. Right now, as I said in the previous review, this is not correct, and a poor way to start the paper.\nYou might rather say that “The nature of Archean global tectonics, and the associated geodynamic regime, is much debated in modern geoscience, despite decades of research. In this study, we use a new geophysical dataset to show that, by the Neoarchean, convective forces etc…”\nThat way you can state lines 11-14 more as your result and findings, which they are.\nWe have modified the first sentence of the abstract with something similar to what the reviewer has suggested, avoiding a categorical statement about the start of subduction at 2.7 Ga.\n“The nature of Archean tectonics and the associated geodynamic regime is much debated in modern geoscience, despite decades of research. In this study, we present a new geophysical model to show that, by the Neoarchean, convective forces Towards the end of the Archean, approximately 2.7 billion years ago, the Earth’s crust had sufficiently cooled to begin a transition to plate tectonics. Convective forces from rising mantle plumes or early forms of plate subduction caused widespread extension, creating linear zones of crustal growth.”\nWhere appropriate, we have replaced \"horizontal tectonics\" with \"mobile-lid tectonics\" to avoid suggesting any inferences about the drivers.\n“The connectivity of the interpreted relict rifts is possible evidence for the existence of Neoarchean plate boundaries and triple junctions and supports a model of at least local horizontal mobile-lid tectonics during this stage of Earth’s history.”\nApart from these corrections, we do not believe the full discussion of the topic suggested here by the reviewer can be reasonably covered in the abstract.\n\nLine 18: timing is important for the Abitibi – is this model strictly for the syn-volcanic period i.e., 2750-2695 Ma? If so, a few lines on how the compressive phase 2695-2670 Ma influenced it might be useful to complete the circle. You say in the abstract your microplates are evidence for horizontal tectonics (i.e., compression) but there is no evidence for this structurally until ~<2690 Ma – how does this work?\nWe have addressed this further into the manuscript, in the last paragraph of the Introduction. There we discuss the compressive phase the Abitibi experienced, how that affected the upper crust and how, with the imaging we have at our disposal (seismic), the lower crust/Moho was not similarly affected. As noted above, we have replaced “horizontal tectonics” with “mobile-lid tectonics” to avoid any inferences about a particular geodynamic regime apart from synvolcanic rifting.\n\nLines19-20: You have found rifts – or plate boundaries in a mobile-lid tectonic setting – this not direct support for horizontal tectonics. Archean tectonics is littered with these kinds of statements. Report on what you actually have – rifts are evidence for extension. Horizontal tectonics will be interpreted as subduction – if you don’t mean this, then state ‘mobile-lid’ or whatever you do mean.\nDone. As noted above.\n\nLines 25-26: you have not referenced this. Please be more exact when describing what you mean by plate tectonics – just mobile lid or also subduction? – if the latter, say ‘modern-style plate tectonics’. Rifting does not = modern-style plate tectonics, so don’t infer that. If you mean mobile-lid and horizontal movement – then be specific. Don’t leave ambiguity. This a massively important point – be very clear on what you mean.\nDone. As noted above.\n“A proposed lack of mobile-lidplate tectonics was suggested, in part, from an apparent lack of Archean rifting (Hamilton, 1998).”\n\nLine 35-36: I do not understand why you keep making statements. There are very few things in Archean geoscience that can be said with certainty, as a statement. This ‘may’ have been a pivotal time – we don’t know for certain – new data suggests the Neoarchean was pivotal. Don’t state facts that we don’t have for the Archean. Half of these statements would be corrected if you just inserted ‘may’ or ‘likely’. It’s a problem in Archean geoscience that two papers next to each other can make two contrasting absolutist statements that contradict each other. It’s made the science messy, don’t be one of them.\nThe wording has been changed to avoid this problem.\n“The Meso- to Neoarchean is seen by some as a pivotal time in Earth’s history, that was somehow transitional between stagnant-lid and mobile-lid tectonics (Brown et al., 2020; Palin et al., 2020).”\n\nAh ha! Line 43 we get to the point! So, when you say mobile-lid, you DO mean mobile-lid +/- subduction. You should remove references to ‘horizontal tectonics’, and just say mobile-lid. Horizontal tectonics is disliked as a term by many (JF Moyen for one) and it is mostly associated with modern-style plate tectonics. You can remove all ambiguity by simply stating ‘mobile-lid’ tectonics. Many see horizontal vs vertical as problematic language, when it is likely to be a dynamic continuum.\nDone. We have removed “horizontal tectonics” and replaced it with “mobile-lid tectonics” throughout the manuscript.\n\nOf course, relatively small-scale rifts of this nature do not necessarily mean there was a global mobile-lid either… but at least local mobility. On Venus rifts are taken up by localised folded mountains, a system known as “tesserae” – these are regional dynamic forces in the mantle, and not associated with a true mobile-lid. In fact, you should probably mention such features as part of your interpretation – they are a valid interpretation for what we see in the Archean. I think it would be really valuable to clearly present all these options, even as a list if it’s easier, and then choose your preferred option (with reasoning). You have done this in the ‘origin of rifting” section, but you could do more. I suspect you have more words available – don’t be afraid to add more detail.\nA full discussion of this topic is far beyond the scope of this paper. We offer one piece of evidence for Archean rifting. We also emphasize that our conclusions do not necessarily support a global tectonic style, but a more local phenomenon for the Abitibi.\nWe have emphasized this the end of the introduction:\n“Although rifting by itself does not prove a large-scale horizontal mobile-lid tectonic regime, the existence of crustal fragments that are encircled by rifts are a strong indication of at least a local microplate regimemosaic in the Abitibi.”\n\nLine 50: parts of the Superior are younger than 2690 Ma.\nWe are referring only to the syn-volcanic stage of the Abitibi was between 2.75 and 2.69 Ga:\n“The Abitibi greenstone belt, situated in the southern Superior Province of Canada, is the largest and one of the best preserved Neoarchean greenstone belts in the world (Monecke et al., 2017) (Fig. 1). It formed between 2.75 and 2.69 Ga in a series of submarine volcanic events thought to have been centered in Archean rift basins.”\n\nLine 58 – spelling error ‘ruft’\nApologies. Corrections were made in the final version of the manuscript but not in the \"track-change\" version.\n\nLine 64 – change to ‘mobile-lid’\nDone.\n\nLine 85 – Mole et al 2022 should be here, Mole et al 2021 should be at the end of this line. Mole et al 2021is the original source of the isotopic and WR dataset. Use Mole et al 2022 for any mineral systems inferences. Some mistakes across the ms with these references.\nMole et al. (2022) was the reference used here. This has been corrected throughout the rest of the manuscript.\n\nLine 119-124: It is good you have made this point, but it feels like a ‘we hope this isn’t the case” – can you be more specific? It seems a key point – can you prove that we aren’t just seeing the surface effects? Apologies if this is already in the Methods. Maybe a reference here would help.\nWe have added the reference to the new Hayward et al. (2024) study that produced a gravity inversion model of the Pilbara. There they show, using the same type of modelling, that near-surface mafic volcanic belts, felsic plutons, then density features in the mantle can all be modelled independently of each other.\n\nLine 131: if the resolution of your model is 10 km, and the range in moho depth is 32-45 km (line 149) – this range is very close to the uncertainty… does this affect how resolvable the features are?\nThank you. I should have specified 10 km “horizontal” resolution, not vertical, which was the largest spacing between the gravity stations that I modelled. The vertical resolution is controlled by the cell size of the model as shown in Figure S1 and described in the supplementary data. They were generally 3 km wide (the tetrahedra aren’t regular, so there’s minor variation in their dimensions) and 3km tall. Unlike cubic cells that are stacked on top of each other, tetrahedra can vertically overlap, so the vertical resolution of the model is much less than 3 km and closer to 1 or 2 km.\n\nLine 153: partial melting of the crust\nThis is corrected.\n\nLine 161: good point\nThank you for making that suggestion in the last round of reviews.\n\nLine 175: Matheson\nThank you. Another error not changed in the tracked document.\n\nLine 180-181: length is used twice here, re-write\nOne of those should have been a width:\n“(between 30 and 50 km and in length width between 60 and 130 km in length)”\n\nLine 183: time? (spelling)\nThank you. Another error not changed in the tracked document.\n\nLine 183: some actual examples here would be useful, rather than just citations.\nWe have added some:\n”Although regional north-south shortening has likely affected the overall geometry, the lengths and widths of the inferred rift zones (between 30 and 50 km and in length width between 60 and 130 km in length) are comparable to modern continental (e.g., East African Rift: Rosendahl, 1987) and oceanic rifts (e.g., in the Lau Basin: Galley et al., 2024) (Galley et al., 2024; Rosendahl, 1987).”\n\nLine 185: hmm is this true? A stronger argument might be that the Archean crust was hotter, and therefore behaved differently during compression – see the paper below:\nhttps://www.nature.com/articles/s41561-018-0138-0\nI suspect that the rift zones were more likely to survive in a crust that collapsed laterally, rather than thickened vertically. This might provide some better evidence for your point.\nLine 230: apologies, you have already seen the Calvert paper. I still think you might look to use it in the point I mention above. Apologies if I already said this stuff last time!\nWe have clarified this statement, adding that lower crustal flow does not change the topography of the Moho.\n“At the same time, we expect the relative topography of the Moho to be largely preserved through the deformation owing to the ductile behavior of the lower crust. The lower Archean crust has been shown to flow under stress (Calvert and Doublier, 2018), a condition that leads to the thickening of the lower crust when undergoing post-rifting compression. However, the shallowing of the Moho is still preserved despite the relative post-rifting thickening of the lower crust with respect to the upper crust (Bertotti et al., 2000).”\n\nI’m not sure if you mention it, but it seems intuitive that the width of your mapped rifts are minimums, and that they have been closed at least partially by the later compression.\nYes, we state “Although regional north-south shortening has likely affected the overall geometry, …”\n\nFigure 5: As I said – this is a really nice figure. Combining the separate systems endorses your model and validates those existing models into a holistic system. However, for some reason you have cited Mole et al’s Yilgarn paper for these data. The correct reference should be the Mole et al 2021 paper (Precam Res). Mole et al references seem to be mixed up a bit in their usage: The isotopic/geochemical dataset itself is from Mole et al 2021. When using it to refer to mineral systems, use Mole et al 2022. If you want to compare your dataset to similar systems in other cratons, or cite an example of other Archean rifts, use Mole et al 2019 (i.e., Kalgoorlie-Kurnalpi Rift; KKR).\nThank you for that correction.\n\nLine 214: be careful when using ‘most reasonable’ – I know what you mean, but unfortunately what is reasonable is subjective… Better to say, something like, ‘in conjunction, the geophysical and geochemical evidence best supports a rift model….”\nWe have changed this first sentence of the Discussion to:\n“The presented geophysical, geologic, and geochemical evidence best supports an extensional model of the Abitibi whereThe most reasonable interpretation of the linear zones of shallow Moho in the Abitibi is that they are the root zones of relict Archean rifts.”\n\nLine 279: you need a reference here\nWe have added the citation:\nJackson, S. L., & Cruden, A. R. (1995). Formation of the Abitibi greenstone belt by arc-trench migration. Geology, 23(5), 471–474. https://doi.org/10.1130/0091-7613(1995)023<0471:FOTAGB>2.3.CO;2\n\nLine 295: “Ey” should be Eu\nCorrected, thanks.\n\nLines 292 to 295: you have these Eu/Eu*/Y relationships backwards. Values >1 indicate a deeper source/thicker crust (possibly – some contention on this) and <1 shallower crust. This is because, for deep crust we expect high Eu values (no plagioclase), and low Y values (garnet present). The high Eu = higher Eu/Eu*, and lower Y values result in a higher final number. Hence deep crust = >1. Vice versa for shallow crust with plagioclase present, which takes out the Eu, no garnet = higher Y. See below figure from Mole et al 2022. You seem to have this correct in Fig 5D, so just an error in the text I think. Note the Ui/Yb map in Mole et al 2022 correlates quite nicely with your N and S rifts.\nCorrected, thanks. That did appear to be just a typo in the text, the figure and its caption/legend was correct.\n\nLine 291-298: This section is good but feels rushed. You talk about La/Sm, but in the Figure 5D, you have Sr/Y labelled (which is generally a depth indicator). Please make the figure and text consistent.\nThe low La/Sm could also indicate less crustal contamination i.e., from passing through thinner or no silicic crust. Contamination indicator (Th/Yb) correlates with decreasing eNd in Mole et al 2021, possibly suggesting the trend is due to contamination with an older, existing crust.\nWe have added the reference to Sr/Y ratios. The reference La/Sm ratios was intentional, but was not accompanied by a figure in this manuscript. We still wanted to mention the spatial relationship it has with our modelled rifts.\n“In contrast, the interpreted rift zones lie generally below crust with a more depleted mantle source indicated by εHf values (Fig. 5C), thinner crust (i.e., (Eu/Euy*)/Y*10,000 ratios <> 1; Fig. 5D) (Mole et al., 2021), more shallow melts (low Sr/Y ratios; Fig. 5D), and less fractionated melts (low La/Sm ratios (Mole et al., 2021)), all consistent with rifting of the Archean crust (Mole et al., 2021, 2022).”\n\nLine 307 – again clarify what you mean when referencing horizontal tectonics.\nThis last section is where you need to be very clear what you are inferring; is it mobile-lid tectonics? Papers by Gerya et al have shown how plumes can form mobile lids, which then induce short subduction events – this might fit the timeline of events in the Abitibi:\nAs suggested in previous comments we have changed “horizontal tectonics” to “mobile-lid tectonics” to avoid this confusion.\n\nOrigin of rifting section: I know what you are trying to say here but I think it could be more clearly written. I think removing vertical vs horizontal tectonics will help it be more specific – talk about the processes you mean directly i.e., sagduction/drip tectonics and the other options discussed in this response. If you are talking around mobile microplates, then say that. You have done a good job and trying to extricate the data from going too far, which is great, but I think it needs a bit more polish. For Nat comms, you need to provide more, just saying it means horizontal tectonics is too vague – show us how well you understand the complexities of Archean geoscience and go further with your model/process. Remember, just because you have evidence of regional mobile microplates does not mean it was a completely mobile-lid – as I said before, Venus has evidence for this and is generally seen as a stagnant lid. I think ‘locally mobile-lid’ covers pretty much all bases. In the plume model above (Gerya et al), the rifting is actually driven by a vertical process, with lateral spread of the ‘plates’ – so I don’t think committing to horizontal tectonics is necessary or even correct at this point. I think you could include this, or at least present this further alternative.\nIf you must use “horizontal tectonics” at least use it after the explanation you provide. That would mean removing it from the abstract to avoid confusion.\nDone. As suggested in previous comments we have changed “horizontal tectonics” to “mobile-lid tectonics” to avoid any assumptions about the driver of extension. In the following statement, we clarify our understanding of the significance of the geophysical modelling relative to different notions of horizontal or vertical tectonics already discussed extensively in the literature.\n“Rifting in a modern plate tectonic context is often linked to extensional forces driven by plate subduction, whereas in the Meso- to early Neoarchean other processes may have been more important, ranging from sagduction/drip tectonics (e.g., Mareschal and West, 1980; Collins et al., 1998) to plume-driven processes (e.g., it is believed mantle plumes and mantle convection drove extension (Gerya et al., 2015; Harris and Bedard, 2014; Piccolo et al., 2020). Rifting could also have occurred in regimes exhibiting only sagduction/drip tectonics driven by density contrasts in the crust (Mareschal and West, 1980). The emplacement of dense mafic material over less dense crust and partially melted lower crust leads to the sinking, or sagduction, of the mafic material (Collins et al., 1998) (so-called dome-and-keel structures). Rigid crust, typical of horizontal The results presented here provide evidence of at least local mobile-lid tectonics Mobile-lid tectonic regimes allows for where rifting was to be focused along discrete corridors, whereas more ductile crust in vertical diapirism and sagduction driven other tectonic regimes leads to disseminated extension across large areas (Choukroune et al., 1995). The Moho topography of the Abitibi best fits a horizontal mobile-lid tectonic regime with distinct corridors of shallow Moho encircling regions of thicker crust (Fig. 5). This contrasts with the coupled shortening and extension of the crust associated with diapirism and sagduction in a vertical tectonic regime (Johnson et al., 2014).”\n\nLine 320-321 – you use ‘although’ and then ‘however’ in next sentence – these are synonyms, reword this.\nThis sentence was replaced in the conclusion.\n\nLine 321: why? What about the model suggests this?\nThis question is addressed in the concluding remarks.\n\nLine 323: don’t need ‘widespread’ here, it’s ambiguous and I’m not sure you have evidence for it.\nIt has been removed.\n\nLine 323: This is not the best way to end the paper. If you have words left, a paragraph bringing the reader back to the main point, and its implications, would leave a better impression. Perhaps use the last sentence to summarise the discussion of different models/ideas from the section above? Or leave the reader with what further implications may be? i.e., “ our results show a strong empirical relationship between mantle plume activity, microplate formation, and major mineral systems for the Abitibi, which imply at least a locally mobile-lid tectonic environment existed in the Neoarchean. These results have implications for the formation and evolution of all cratons, as well as their host mineral systems.” Something like that?\nWe have revised the last paragraph to highlight the possible implications of our findings for the larger audience.\n\"This study presents a new gravity-derived model of the Moho surface beneath the Abitibi greenstone belt that reveals a possible Neoarchean rift network. The geometry of the proposed microplate mosaic is supported by trace element and isotopic data of associated volcanic rocks and the occurrence of rift-related mineral deposits. The results show a strong empirical relationship between mantle plume activity, microplate formation, and major mineral systems in a local mobile-lid tectonic environment.”\n\nI think it would also be worth making the point that deep sea hydrothermal systems have been associated with these rifts, as exemplified by the VMS systems. The origin of early life may be connected to these type of settings, and hence these new maps of Archean rift systems map out potential locations for early life i.e.,\nhttps://www.sciencedirect.com/science/article/pii/S0301926814001454\nThis might be a nice way to end the paper, to show it has profound implications for the wider Earth system and environment.\nThere is also a new paper in Geology (screen shot of a figure below):\nhttps://pubs.geoscienceworld.org/gsa/geology/article/53/3/269/650953/Inception-of-ridge-ridge-ridge-triple-junctions\nIt might be good to reference and/or include this evidence for rifts in an arc environment, in the interest of balance. It doesn’t mean you need to commit either way, but it would mean you present possibilities for multiple settings. It also supports your ‘mobile-lid’ / ’laterally-dynamic’ tectonic process. Perhaps with the komatiites occurring at the same timing of rifting, and komatiites connected to plumes (ambient mantle was too cool; Nisbet et al.), the plume model is more likely; but you should decide what you prefer.\nThank you for these suggestions. We added the particular references and suggest possible implications of a mobile-lid regime within the Abitibi in the previous revised paragraph. However, we prefer to focus on the merits of our modelling for others to consider rather than a review of the literature on Archean geodynamics. We trust that knowledgeable readers will grasp the significance for early Earth tectonics, mineral systems, and \"origins of life\" without a full discussion of all the models here.\n\nMajor changes required:\nRemove all occurrences of ‘horizontal tectonics’ – replace with what you mean, which is ‘local mobile-lid tectonics’. I know what you mean, but horizontal tectonics as a term is closely associated with subduction and modern-style plate tectonics, so better to use something else.\nDone.\nThere are other possibilities you need to explain and explore, to make the origin of rifting section more thorough and less vague.\nWe have expanded on the first paragraph in this section with reference to rifting in a modern context and (in the revised paragraph) with reference to other processes ranging from sagduction/drip tectonics (e.g., Mareschal and West, 1980; Collins et al., 1998) to plume-driven processes (e.g., Gerya et al., 2015; Harris & and Bedard, 2014; Piccolo et al., 2020).\nAs stated in the first review, do not make statements that are not currently valid in Archean geoscience. The reality is we don’t know enough on many aspects to make statements – for most points there is a counterpoint. This is easily fixed by qualifying statements by adding ‘may’ or ‘likely’ to take the absolutism out of the sentence.\nThank you for helping with this and in refining the manuscript and its message.\nTidy up the quality of the writing – it feels a bit rushed. There are numerous spelling mistakes, and some sentences could be clearer. Be very careful when presenting geochemical data – there are some major blunders in there.\nWe have made an effort to polish the manuscript. While there regrettably were some spelling mistakes in the “Tracked” version of the document, they were all caught in the final un-marked version.", "reviewer_2": "The authors made appropriate revisions to the paper, which is now suitable for publication.\n\nTaras Gerya, Zurich, 17.02.2025\nThank you.", "reviewer_3": "I have looked over the revised paper of Galley et al. Unfortunately only a few of my comments have been addressed.\n\nThe assumption of 3070 kg/m3 for a mantle density has still not been justified. The density, which is more typical of lower crustal is too low for the mantle – it is even lower than the density (3180 kg/m3) normally assumed for the mantle at an actively spreading mid-ocean ridge!\nThe 3070 kg/m3 density contour is not assumed to be the density of the mantle, but rather the density at the interface between the lower crust and the mantle. You are correct, typically when the upper mantle is assumed to have a homogeneous density it is closer to ~3300 kg/m3, and a homogeneous crust can be typically ~2670 kg/m3 for the upper crust (continental) and ~2900 kg/m3 for the lower crust. The density at the interface between the lower crust and the mantle would be represented by a value somewhere between that of the upper mantle and the lower crust; i.e., between the 2900 kg/m3 lower crust and 3300 kg/m3 upper mantle, or 3100 kg/m3. Model testing done by myself and others (such as Dr. Kim Welford who advanced the gravity modelling method for a heterogeneous density model of the crust/upper mantle to resolve the Moho) have found that the 3070 kg/m3 contour most accurately fits the Moho.\n“The Moho interface was then extracted from the 3D density inversion by isolating the 3.07 g/cm3 iso-density surface. This iso-density layer has been shown to be an accurate proxy for the Moho when performing minimum-structure inversion modeling (Galley et al., 2024; Welford, 2024; Welford & Hall, 2007), in contrast to the commonly used surface-based Parker-Oldenburg modeling method (Oldenburg, 1974; Parker, 1973). The latter solves for the depth of an interface between an assumed homogeneous-density crust (e.g., 2.85 g/cm3 (Chappell & Kusznir, 2008)) and the mantle (e.g., 3.33 g/cm3 (Chappell & Kusznir, 2008)). We opted not to use the surface-based modeling method, instead using the more sophisticated, albeit more computationally demanding, minimum-structure inversion method (see Methods for details), to account for the known heterogeneity in the density of the Abitibi crust (Esmaeil Eshaghi et al., 2023). Our more heterogeneous density model would then resolve a Moho interface at a density value between that of the crust and mantle.”\n\nThe inverted Moho is still being compared to travel-time seismic reflection data. There should be an explanation of how these data have been converted to depth and what the uncertainties on these depths might be. Readers should not have to search through the literature for this information.\nWe have added more information on the processing of the seismic reflection data to the section “Comparison with Seismic Profiles”, as well as more details for the other two models we used to compare with our gravity model. We also added a comparison to a seismic refraction model of the Moho to the same section and Figure 3.\n“The results of the gravity inversion are compared to Moho depths from the Lithoprobe reflection seismic experiments (John Ludden & Hynes, 2000), a Lithoprobe seismic refraction model (Grandjean et al., 1995), those from the CRUST 1.0 model (Laske et al., 2013), and from teleseismic data (Rondenay et al., 2000) (Fig. 3B). The reflection seismic sections were depth-migrated using a constant velocity model of 6 km/s, creating an approximate depth model of the Moho and overlying crustal features (Ludden & Hynes, 2000). The seismic refraction line modeled by Grandjean et al. (1995) resolved a Moho line from a 2D velocity model of the crust and upper mantle, providing more accurate depth information on the Moho. The teleseismic measurements led to a series of velocity models of the Abitibi crust and upper mantle, (Rondenay et al., 2000). The CRUST 1.0 model is the coarsest of the Moho models with a spatial resolution of 1 arc-degree (~111 km) and Moho depths derived from a velocity model of generalized Archean crust (upper crust: 6.2 km/s; middle crust: 6.4 km/s; and lower crust: 6.8 km/s) (Laske et al., 2013).”\n\nThe authors have now provided a Bouguer anomaly profile, but the vertical scale range of 1000-1060 mGal in Figure 4D makes no sense. I could not look up what the Bouguer anomalies are in the study area because none of the maps in the paper have latitude and longitude (they should do!) but Government maps show Bouguer anomalies in the range -20 to -100 mGal southeast of Hudson Bay.\nWe used “processed Bouguer anomaly data” and have noted this in the figures and text. In the section “Further Bouguer Data Processing” in the Methods we have explained what steps were taken to process the Bouguer data.\n“To model the Moho topography in the Abitibi, we inverted processed Bouguer gravity data compiled by Natural Resources Canada (Canadian Geodetic Survey, 2022) (Fig. 2; see Methods for details on data processing) to produce a three-dimensional (3D) density model of the Earth’s crust (Fig. 3).”\n\nThe output of the inversion modeling is a density structure that has still not been verified by forward modeling. It should be a relatively straight-forward process to take the density contours such as those shown in Figures 3B and 4D and calculate the gravity effect of each contour using a line-integral method and the density contrast between layer above the contour and the layer below.\nIn summary, I am not convinced that the undulations in the Moho proposed in the paper are required to fit the observed gravity data. The authors need to demonstrate more clearly that the sources of the Bouguer gravity anomalies are not within the crust and that Moho is not flat.\nIn response to the previous review we did verify the inversion model with forward modelling, see Figure S4, and explained that the gravity anomalies resolved are much larger that the model residual, and are therefore significant and not just noise (the amplitudes of the forward signal of the mantle component range from ~20 to 50 mGal). We did not produce a forward model of contours because that would be an approximation of our model. Our model is not composed of homogeneous layers separated by contours, but rather tetrahedral cells. So, it was more accurate to calculate the forward model of the cells that lie above the Moho (Fig. S4C) that represents the gravity anomaly produced from the crust, and compare that to the forward signal from the cells below the Moho (Fig. S4D). As suggested, most of the signal of the modelled data is fitted by features in the crust, but there is still a large, regional component that needed to be fit by deep features, which we interpret as the Moho/upper mantle.\nWith further explanation of the forward modelling (see below) we believe we have shown that the undulations in the Moho are required to fit the observed gravity data, that not all of the sources of the Bouguer gravity anomalies are within the crust, and that Moho is not flat. Density features in the crust were not enough to fit the modelled data, and that the interpreted undulations in the Moho are supported by the teleseismic, seismic refraction, and seismic reflection profiles.\nWe added a new section to the Methods section to explain this further:\n“Forward Model Verification\nTo verify that the density anomalies in the upper mantle shaped the topography of the modeled Moho, we created a series of forward models of components of our inversion model (Fig. S4). The first forward model calculated the gravity signal due to anomalies above the Moho, i.e., crustal anomalies (Fig S4C). Next, a forward model was calculated from the component of the inversion model below the Moho, i.e., the upper mantle/Moho anomalies (Fig. S4D). Comparing the forward signals of the crustal anomalies and Moho anomalies to the modeled data (Fig. S4A) it is possible to quantify how much of the modeled data was fit due to density features in the crust versus features in the upper mantle. As expected, most of the data was fit by crustal density anomalies, but a significant, more regional component of the data had to be fit by deep, upper mantle density features. The amplitude of the forward signal of the mantle component was ~20 to 50 mGal (Fig. S4D), much greater than either the Model Data Residual (Fig. S4B) or the uncertainty that averaged 0.3 mGal (Canadian Geodetic Survey, 2022).”\n\nThe source code of the computer program used in the inversion is still not being made publicly available. Readers will therefore be unable to understand how the code works or, more significantly, be able to reproduce the results in the paper.\nThe computer program is publicly available, but with the legal condition that it must be used only for academic purposes. As such, we cannot keep it on a repository for all to access at any time, but are more than happy to make it available to researchers by contacting Dr. Lelièvre, to verify this work or conduct different research. The code is currently in use at Memorial University of Newfoundland, University of Ottawa, Mount Allison University, and Woods Hole Oceanographic Institute.\nFrom Nature’s reporting standards and availability of data, materials, code and protocols:\n“An inherent principle of publication is that others should be able to replicate and build upon the authors' published claims. A condition of publication in a Nature Portfolio journal is that authors are required to make materials, data, code, and associated protocols promptly available to readers without undue qualifications.”\nWe feel like we have met these requirements."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_1": "This is the third time I have reviewed this paper, and I am now happy for the paper to be accepted. Thank you to the authors for making the corrections in good faith. I think this will be an interesting contribution.\n\nHowever, as I said in my initial review, this is based on the assumption that a geophysicist has reviewed and cleared this manuscript, as I do not have the expertise to check that element of the paper.\n\nA few final comments:\nLine 30-31: remove somehow, add 'potentially'\nLine 162: This should be 'deeper' level of erosion - if these areas represent the 'horsts' then they eroded more deeply to reveal the intrusive rocks (as they where at a higher level), whereas the grabens or basins still contain the volcanic sequences. Line 163 can still be true in this scenario. However, what you say in line 164 suggests you already know this, so perhaps just changing the 'levels of erosion' to 'levels of subsidence' is more accurate.\nLine 297-299: Nice way to finish!", "reviewer_4": "Please note, that is not a complete review of the manuscript, but some thoughts on the gravity modelling performed as part of the study and the concerns by one reviewer.\nAs such, the modelling seems to be correct, but in my view is explained in unnecessary complicated matter.\n1) Processed Bouguer anomaly\nThe processed Bouguer anomaly is simply a shifted Bouguer anomaly by adding a constant. Hence, I would recommend to show the Bouguer anomaly instead of the processed one.\n2) The shift has implications for modelling. Figure S3 illustrates that by showing that both models have the same gravity anomaly disregarding the shift (and I would omit this figure). Such a shift is often treated manually, by an average value or a reference model depending on the application. Here, a reference model would be most appropriate and this is sort of expressed by a 65 km thick slab of 400 kg/m**3 (and I suggest that authors use SI units for density). This shift in the inversion is somewhat similar to setting a reference depth in methods like Parker-Oldenburg.\n3) This is also seen in the resulting density maps in Figure S2 (in which depth I wonder), which for result A shows that simple depth weighting is not sufficient. Only when the anomalies Are shifted to positive values the inversion provides increasing densities with depth.\n4) Coming to densities. A contrast of 400 kg/m**3 does not indicate a mantle density of 3070 kg/m**3 as its well known that 2670 kg/m’**3 is an okay value for near-surface density, but that density and velocity increase with depth (e.g. Zoback & Mooney 2002) and that there is a jump at the Moho. I have the feeling and this can’t be seen from your colour scale, that you smear out the contrast (as densities in the mantle continue to increase). That would be fine, if you demonstrate that the model response is the same, when all densities >3.07 are put to a constant value of 0.2-0.4 larger than the lower crust.\n5) “We opted not to use the surface-based modelling method, instead using the more sophisticated, albeit more computationally demanding, minimum-structure inversion method (see Methods for details), to account for the known heterogeneity in the density of the Abitibi crust”\nThis sentence is a bit a summary of the authors opinion, but I would argue that the word sophisticated is stretching it a bit. Yes, inverting for the density change with depth is okay and its advantage is that it provides crustal densities which appear reasonable. However, I would argue that you get the same result, when you A) filter the data, B) invert the long-wavelength part for the Moho (with reference depth of 35 km and density contrast of 400 kg/m**3), and C) invert the short-wavelength residual for crustal internal variations.\n6) Furthermore, I disagree with Lines 381-388 and that you corrected for something done wrong in a normal correction. I applaud your honesty, that you state this correction as most modellers simply apply a shift, but this is also not a meaningless quantity, and we are simply speaking about a reference model, which also has isostatic implications.\n7) The inversion itself is well described, but the setting of the slab is actually the main factor for your results. I had to careful think about what actually has been done and can completely understand the concerns and confusion of one of the reviewers.\nIn summary, I would omit Figure S3, put some of the technical descriptions on the inversion in the supplement, but discuss the role of the slab or reference model in the main text as this is the most crucial factor for the resulting models and refer to a normal use of anomalies. The processed Bouguer anomaly creates an unnecessary level of confusion and does not contain any new information. For gravity experts, shifts and reference models are nothing new. For non-experts that simply mystifies the gravity modelling.\nJörg Ebbing"}, "author_response": "Reviewer # 1 (Remarks to the Author):\nThis is the third time I have reviewed this paper, and I am now happy for the paper to be accepted. Thank you to the authors for making the corrections in good faith. I think this will be an interesting contribution.\nHowever, as I said in my initial review, this is based on the assumption that a geophysicist has reviewed and cleared this manuscript, as I do not have the expertise to check that element of the paper.\nA few final comments:\nLine 30-31: remove somehow, add 'potentially'\nDone.\nLine 162: This should be 'deeper' level of erosion - if these areas represent the 'horsts' then they eroded more deeply to reveal the intrusive rocks (as they where at a higher level), whereas the grabens or basins still contain the volcanic sequences. Line 163 can still be true in this scenario. However, what you say in line 164 suggests you already know this, so perhaps just changing the 'levels of erosion' to 'levels of subsidence' is more accurate.\nThis should have been “deeper”, thank you.\nLine 297-299: Nice way to finish!\nThank you!\n\nReviewer # 3 (Remarks to the Author):\nI have looked over the revised paper of Galley et al. Unfortunately only a few of my earlier comments have been addressed.\n1. The 3070 kg/m3 density contour. I checked the Galley et al. (2024), Welford (2024 and Welford & Hall (2007) papers but could not find any reference in them to the 3070 kg/m3 contour as Moho. The Parker methodology is the most commonly used way to compute the effect of Moho because seismic refraction data indicate that the transition from crust and mantle is generally associated with abrupt change in density, not a gradual one. At the very least, a comparison should be made between the model predictions and one assuming the crust and mantle had homogeneous densities of, say 2850 and 3300 kg/m3 respectively.\nTo address the reviewer's concerns about the 3070 kg/m3 density contour, we have added to the manuscript: “Previous studies similarly modeling the Moho have used the 3020 kg/m3 iso-density contour25,26,32, however through further testing we found that the 3070 kg/m3 contour better fit known Moho depth in the Abitibi.” Lines 90-92.\nWe agree that the Moho in Archean terranes (as observed at present) is more abrupt than the Moho in the modern oceanic crust, for example. To address this, we previously added a second modelling step outlined in the Method’s section “A Priori Moho Structure Constraints”. Our final density model does, in fact, represent the Moho as a discrete change in density. This was not clear in earlier drafts, in part due to a lack of explanation in the main text (it was only in the Methods section) but also because of the choice of colour bars emphasizing density variation in the crust but washing out the Moho and mantle. Figures 3 and S2 have been adjusted to better explain the sharp density boundary across the Moho. We have also provided the additional explanation in the main text, under the “Gravity modelling of the Moho” section (see lines 92-115).\nWe chose not to construct another density model composed of a discrete Moho interface separating a homogeneous crust and mantle (with respective densities 2850 and 3300 kg/m3). It is known that there are large variations in density in the crust (reflecting the greenstone belts, felsic plutons, and ultramafic intrusives), and so it would be incorrect to assume a homogeneous density. Such a model would incorrectly require density variations in the mantle topography that likely don't exist.\n2. Comparison with seismic data. The absence of controlled source seismic refraction data remains an important missing constraint on the gravity modeling. I am not convinced that seismic reflection profile data can provide any useful constraints on Moho depth in the absence of detailed velocity information. Teleseismic data can provide estimates of Moho depth, albeit for an assumed average velocity of the crust, but requires closely spaced seismic recording stations and techniques such as H-k stacking, which I do not know were employed in this case. Finally, CRUST 1.0 is a 111 × 111 km grid of existing, mainly seismic refraction, data which be of limited use in resolving the subtle, short wavelength undulating variations in Moho depth inferred in this paper.\nWe still do not understand the reviewer's comment that seismic data are lacking. The refraction profile was included in Figure 3 with the edits from the last review, and discussed in the “Comparison with seismic profiles” section. We also further emphasized that the CRUST1.0 model is regional in resolution, and discussed how the assumption of a homogeneous crust with respect to velocity can lead to misleading Moho features in the seismic profile. To emphasize this, Figure 3 was updated to include a second profile better aligned with the refraction and teleseismic line. The colour bar was also fixed as to not wash out the density in the mantle, but rather show variations in density across the whole model. Previous versions of the colour bar obscured the sharp transition in density across the Moho, which was causing confusion.\nFor added text see section “Comparing with Seismic Profiles”, lines 125-153.\n3. Observed and ‘processed’ Bouguer anomalies. I am afraid that simply adding the word ‘processed’ to ‘Bouguer anomalies’ that are of the order of 1000 mGal makes no sense to this reviewer. The observed Bouguer gravity anomalies in the study region are approximately -20 to -100 mGal (according to Canadian government maps) and all figures that purport to show the observed Bouguer anomaly should reflect this.\nWe have revised all the data figures to show the original Bouguer anomaly. Although this was the data that was modelled, showing it the way we did in the manuscript has created too much confusion. Reviewer #4 made a similar comment. The use of the shift in gravity is further explained in our response to point 6 of reviewer #4.\n4. Forward modeling. The modeling in Figure S4 is interesting, but it is not a verification of the density structure deduced from inversion by forward modeling. Rather, it shows the modeled contributions to the Bouguer anomaly of sources in the crust compared that of the mantle. Interestingly the figure, together with Figure 4D, shows how the inversion places so much of the source of the short wavelength gravity anomalies in the upper crust. The lower crust appears to make little contribution. Such a density structure for the crust in a cratonic setting is surprising and should be verified, for example, by spectral modeling. I was more confused, however, about what Figure S4 reveals about the long wavelength contribution. Is it mainly from the Moho (ie the crust/mantle boundary), the upper mantle or both? If the latter, then would we not expect a ‘trade-off’ between the two sources such that the Moho could be flat and provide no contribution while sources in the mantle provide the main contribution?\nWe stress that the modelled contributions of the crust and mantle to the Bouguer anomaly are forward models. To emphasize this, we expand on the forward modelling of the mantle component by separating the signal due to the Moho from the signal below the Moho (see lines 449-470 and Fig. S3). In particular, we show that the forward model of the Moho produces significant gravity anomalies that were needed to fit the Bouguer anomaly data, and that density variations in the upper mantle below the Moho had little contribution. The additional forward modelling of the Moho surface shows that the density variations in the upper mantle are much lower in amplitude than the gravity anomalies produced from the modelled Moho, thus the \"trade-off\" mentioned by the referee does not appear to be the case with our model. See the Method’s section “Forward Model Verification” for changes to the text and Figure S3 (also included in previous versions of the manuscript as Fig. S4).\nThe observation that the lower crust shows little contribution to the Bouguer anomaly (i.e., appears strongly layered with little lateral variation in density), has been addressed in the “Crustal Density Structure” section of the manuscript, lines 174-177, in particular with reference to Ludden & Hynes (2000), who also identified how the lower crust in the Abitibi appeared to be much more strongly layered than the upper crust.\n5. Software availability. I repeat my earlier statement that the source code for the inversion method should be made freely available to all researchers so that the community has the opportunity to verify the methodology and repeat the calculations made in this paper.\nIt is freely available, so long as the researchers agree to only use it only for academic purposes. Details for making a request are provided.\nIn summary, my concern about the gravity modeling remains. Therefore, I question the main result of the paper: namely that the regions of shallow Moho reflect narrow linear belts of thin rifted crust that are bounded by the broader regions of thicker crust of one or more Archean ‘microplates’.\nWe hope these latest revisions have adequately addressed the remaining questions of this reviewer. The main concerns about the gravity modelling were:\n- the sharpness of the density change at the crust-mantle boundary\n- the lack of controlled-source seismic refraction data\n- the processing of the Bouguer anomalies\n- the lack of forward modelling\nWe have addressed each of these concerns by:\n- further clarifying that the Moho in our model is represented by a discrete change in density, through changes to the text and figures\n- updated Figure 3 to better show that we had already provided controlled-source seismic refraction data\n- revised all the data figures to show the original Bouguer anomaly and to avoid any confusion about the processing\n- performed additional forward modelling to verify the gravity inversions\n\nReviewer #4 (Remarks to the Author):\nPlease note, that is not a complete review of the manuscript, but some thoughts on the gravity modelling performed as part of the study and the concerns by one reviewer.\nAs such, the modelling seems to be correct, but in my view is explained in unnecessary complicated matter.\n1) Processed Bouguer anomaly\nThe processed Bouguer anomaly is simply a shifted Bouguer anomaly by adding a constant. Hence, I would recommend to show the Bouguer anomaly instead of the processed one.\nDone. We originally thought showing the data we modelled was the more honest approach, but we recognize how that created confusion.\n2) The shift has implications for modelling. Figure S3 illustrates that by showing that both models have the same gravity anomaly disregarding the shift (and I would omit this figure). Such a shift is often treated manually, by an average value or a reference model depending on the application. Here, a reference model would be most appropriate and this is sort of expressed by a 65 km thick slab of 400 kg/m**3 (and I suggest that authors use SI units for density). This shift in the inversion is somewhat similar to setting a reference depth in methods like Parker-Oldenburg.\nWe have removed Figure S3. The reviewer's description of the use of our “slab forward signal” is accurate. All densities in the manuscript have been reformatted to SI units.\n3) This is also seen in the resulting density maps in Figure S2 (in which depth I wonder), which for result A shows that simple depth weighting is not sufficient. Only when the anomalies Are shifted to positive values the inversion provides increasing densities with depth.\nThis is also correct. We have made changes to the text to better explain this shift as the addition of a reference model, rather than simply the signal of an infinite slab (see Method’s section “Reference Model” and lines 113-115).\n4) Coming to densities. A contrast of 400 kg/m**3 does not indicate a mantle density of 3070 kg/m**3 as its well known that 2670 kg/m’**3 is an okay value for near-surface density, but that density and velocity increase with depth (e.g. Zoback & Mooney 2002) and that there is a jump at the Moho. I have the feeling and this can’t be seen from your colour scale, that you smear out the contrast (as densities in the mantle continue to increase). That would be fine, if you demonstrate that the model response is the same, when all densities >3.07 are put to a constant value of 0.2-0.4 larger than the lower crust.\nWe have adjusted Figure 3 as to better show the Moho interface. We respond below to the concern about the use of the “infinite slab”, in how it can be better represented as a reference model (after comment #6).\n5) “We opted not to use the surface-based modelling method, instead using the more sophisticated, albeit more computationally demanding, minimum-structure inversion method (see Methods for details), to account for the known heterogeneity in the density of the Abitibi crust”\nThis sentence is a bit a summary of the authors opinion, but I would argue that the word sophisticated is stretching it a bit. Yes, inverting for the density change with depth is okay and its advantage is that it provides crustal densities which appear reasonable. However, I would argue that you get the same result, when you A) filter the data, B) invert the long-wavelength part for the Moho (with reference depth of 35 km and density contrast of 400 kg/m**3), and C) invert the short-wavelength residual for crustal internal variations.\nWe acknowledge that other equally sophisticated approaches could be used and have modified the text to reflect this (see lines 92-99). We chose the methodology described in this paper to avoid any filtering, which requires assumptions about what components of the data correspond to which features in the crust/upper mantle. We believe it is a strength of our 3D density model to be able to model the geometry of the Moho while simultaneously considering density variations in both the crust above it and upper mantle below it.\n6) Furthermore, I disagree with Lines 381-388 and that you corrected for something done wrong in a normal correction. I applaud your honesty, that you state this correction as most modellers simply apply a shift, but this is also not a meaningless quantity, and we are simply speaking about a reference model, which also has isostatic implications.\nWe have revised our use of the word “correction” to avoid any confusion that we believe the classic gravity data corrections are wrong. Simply, we applied a \"shift\", exactly as the reviewer describes it, in order to resolve a realistic density model of the subsurface.\nWe have changed the previous “Further Bouguer Data Processing” to “Reference Model” in the Method section to better explain the implications of adding the 1092 mGal signal to the Bouguer anomaly data in order to resolve a Moho interface that closely matches previously published seismic sections.\n7) The inversion itself is well described, but the setting of the slab is actually the main factor for your results. I had to careful think about what actually has been done and can completely understand the concerns and confusion of one of the reviewers.\nWe hope that we have addressed the concerns and confusion of reviewer #3, as described above and in the summary below.\nIn summary, I would omit Figure S3, put some of the technical descriptions on the inversion in the supplement, but discuss the role of the slab or reference model in the main text as this is the most crucial factor for the resulting models and refer to a normal use of anomalies. The processed Bouguer anomaly creates an unnecessary level of confusion and does not contain any new information. For gravity experts, shifts and reference models are nothing new. For non-experts that simply mystifies the gravity modelling.\nWe have removed Figure S3, and included more information on the role of adding in the reference model (previously described as a slab, see “Reference Model” section in the Methods). Adding the forward signal from the “65 km thick 400 kg/m3 slab” is now described in terms of a linear reference model. We have also omitted any mention of the processing of the “Bouguer anomaly” in the figures, as we now see how this could create confusion, as explained by the reviewer."}, {"round": 4, "reviewer_comments": {"reviewer_4": "The revised version has improved a lot as now the gravit yinversion is exlained in a clear and transparent way. One could still argue about the dis- and advantages of the inversion procdure compared to other approaches., As such the approach is sound and as with all potential field inversions, not one recipe is applicable to all areas. The authors admit this and reflect on this in the discussion adequately. The methods part describes all details for the expert reader, so that I recommend publication."}, "author_response": ""}]

Decision letter