Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin

Domain: earth_science
Published: 2026-01-13
Editorial outcome: minor_revision

Abstract
The Continent-Ocean Transition (COT) in the young Tyrrhenian basin documents mantle exhumation punctuated with multiple episodes of discrete oceanic crust formation. This observation challenges prevailing models of magma-poor COTs, which typically describe mantle exhumation preceding the emplacement of oceanic crust. Notably, this COT developed without the conventional conditions associated with magma-poor rifted margins, such as slow rifting velocities and chemically depleted mantle sources. A key observation is the low shear-wave velocity observed in the uppermost mantle of the Tyrrhenian basin and its adjacent onshore regions, which correlates with subduction-related volcanism, suggesting the presence of a hydrated mantle wedge with low rheological strength. Here, we show that, based on 3D magmatic-thermomechanical numerical modeling, the episodic formation of oceanic crust within the Tyrrhenian basin’s COT results from the mechanical weakness of the mantle. The lithospheric mantle is exhumed to the surface through exhumation channels initiated within the weak mantle zone. The subsequent flow of partially molten mantle toward these channels leads to the development of multiple short-lived spreading centers. Our findings shed light on characteristics and mechanisms shaping the COT of marginal basins, where their opening is influenced by subduction processes.

Introduction
The discovery of exposed mantle by drilling in the COT west of Iberia prompted the development of conceptual models for magma-poor rifted margins. These margins are characterized by minimal syn-rift magmatism and, in some cases, by a broad zone of exhumed mantle that shows little to no magmatism prior to the formation of mature oceanic crust1–5. However, limited by the scarce evidence obtained in ultra-deep water drilling and low-resolution geophysics, the nature and genesis of the magma-poor COT is still controversial6–9. Two primary conditions have been proposed for the formation of magma-poor rifted margins: (1) Slow extension results in low upwelling rates, which in turn enhance asthenospheric cooling and reduce melt production10,11; (2) Upwelling mantle with chemically depleted composition, too refractory to melt10,12.

The Tyrrhenian basin in the Mediterranean Sea was formed by extension of continent lithosphere above the retreating Ionian slab during the Neogene13,14 (Fig. 1a). Facilitated by the shallow water and thin sediment cover, high-quality geological and geophysical data have been obtained and analyzed13–22. The Tyrrhenian basin therefore, constitutes an ideal natural laboratory for investigating the formation of the COT in magma-poor settings9. It has been proposed that the basement of the Tyrrhenian basin is composed of extensive exhumed mantle14–21 (Fig. 1c). This observation conflicts with current wisdom on magma-poor rifted margins, as mantle exhumation in the Tyrrhenian basin is punctuated with multiple discrete events of magmatic emplacement (~5–2 Ma)19,20 (Fig. 1c).

The conventional two conditions to explain the formation of magma-poor margins (slow rifting or depleted mantle) can hardly be reconciled with the COT in the Tyrrhenian basin for the following reasons: (1) The full extension rate is about 4-6 cm/yr during the opening of the Vavilov and Marsili sub-basins of the Tyrrhenian basin23–25 (Fig. 1b), which is about 3 to 5 times faster than the half extension velocities of <0.6 cm/yr required for magma-poor mantle exhumation at rifted margins10; (2) The peridotites sampled in the Tyrrhenian COT are heterogeneous and on average mineralogically fertile26. Magni et al.27 suggested that episodic extension, driven by intermittent trench retreat, could explain the ridge jump from Vavilov Basin to Marsili Basin. However, within the Vavilov Basin, continuous opening is observed20,25. Why this continuous extensional regime produced mantle exhumation punctuated with multiple short-lived spreading centers (magmatic emplacement) remains unresolved.

The pronounced low shear-wave velocity anomaly in the uppermost mantle (~40–80 km) beneath the Tyrrhenian basin and onshore regions is well documented18,28,29 (Fig. 1b). This anomaly spatially correlates with subduction-related volcanism (14-0 Ma) suggests the presence of a hydrated mantle wedge formed by present or past slab dehydration18 (Fig. 1b). Also, the magmatic products (Mid Ocean Ridge type basalts and alkaline lavas) observed in the Tyrrhenian basin suggest the presence of fluids released from the dehydration of the subducted plate20. Hydrated mantle is likely characterized by low rheological strength29,30. A weak crust has been recognized for promoting depth-dependent extension and strain migration during continental rifting31–37. In line with this concept, we hypothesize that a weak mantle wedge may exert a similar influence, potentially facilitating mantle exhumation and the migration of spreading centers in the Tyrrhenian Basin.

In this work, we conduct 3-D magmatic-thermomechanical numerical models tailored to the Tyrrhenian Basin’s geology. Our simulations demonstrate how a weakened mantle wedge can trigger mantle exhumation punctuated with multiple discrete oceanic crust. Systematic parameter sensitivity studies further identify key controls governing these geodynamic processes.

Results
We conducted numerical models to explore the effects of a weak mantle zone on mantle exhumation and magmatism during continental rifting. The initial model configuration (Supplementary Fig. 1), rock rheologies, and material properties are described in the Methods and the Supplementary Table 1. In our models, the weak mantle (water-saturated) zone with a low friction coefficient is positioned in the northeastern part (x=292–584 km). In contrast, the southwestern model domain (x=0-292 km) retains intact mantle rheology (Supplementary Fig. 1). This geological contrast enables us to examine how lateral variations in rheological properties affect the style of rifting and continental break-up. To simulate the trench retreat process, a high extension rate of 4 cm/yr is imposed on the southeastern boundary (y=0), consistent with the southeastward trench retreat velocity during the opening of the Vavilov and Marsili sub-basins23,25 (Fig. 1b).

Figures 2 and 3 illustrate the process of how a weak mantle zone induces lithospheric mantle exhumation punctuated with multiple discrete oceanic crust, in the reference model (mf3). This weak mantle zone is defined by a low friction coefficient (μ=0.001), spanning 40 km laterally (y=560–600 km) and vertically (z=60–100 km). In this model, crustal break-up occurs at 2.09 million years (Myr), whereby lithosphere thinning and asthenosphere upwelling are focused above the weak mantle zone (Figs. 2a, 3a). During this process, two exhumation channels with highest extensional strain rate emerge within the weak mantle zone beneath the southeast and northwest continental margins (Fig. 3b). The lithospheric mantle is exhumed to the surface along the exhumation channels (Figs. 2a, 3a). As the extension continues, a spreading center is firstly formed with oceanic crust emplacement atop the partially molten mantle (Figs. 2b; 3c). This marks the formation of mantle exhumation punctured with magmatic emplacement. The exhumation channel beneath the southeastern margin continues to be active, exhibiting peak extensional strain rates in this region. This rapid extension facilitates the inflow of low-density, low-viscosity, partially molten mantle material toward the channel, where it fills the newly formed space (Fig. 3d). By 3.40 Myr, a larger volume of the lithospheric mantle is exposed (Figs. 2c, 3e). Concurrently, as the partially molten mantle consistently flows toward the exhumation channel, the model successively forms new spreading centers (atop of the molten mantle) toward the southeast, with oceanic crust being emplaced independently above each spreading center (Fig. 3e, f). Eventually, at 4.57 Myr, as the weak mantle beneath the southern margin diminishes, the exhumation channel ceases activity. The molten mantle gathers below the new spreading center and the system transitions to a steady-state configuration (Figs. 2d, 3g, h). The youngest spreading center remains active, while the old spreading centers become abandoned as the partially molten mantle flow to the new established spreading center (Figs. 2d, 3g, h). Our models therefore suggest that mantle exhumation punctuated with multiple discrete oceanic crust emplacements is triggered by the weak mantle zone.

As a comparison, in the southwestern part of the model (x=0–292 km) where there is no weak lithospheric mantle, the extensional deformation is rather symmetric. As the lithosphere thins, the continental crust is coupled with the lithospheric mantle and breaks apart simultaneously (Fig. 2a, b; Supplementary Figs. 2a–f), leading to symmetric rifted margins and negligible exposure of the continental lithospheric mantle. During the spreading phase, the strain is always localized at the mid-ocean ridge (Fig. 2c, d; Supplementary Figs. 2g–l). The oceanic crust on both sides of the ridge is symmetrically accreted, and only one spreading center developed during the simulation.

To investigate the conditions governing mantle exhumation punctuated by discrete oceanic crust emplacement, we systematically vary key parameters of the weak mantle zone across numerical models. These parameters include: friction coefficient (mf series: mf1–mf4), lateral width (mw series: 20–50 km), vertical thickness (mt series: 20–50 km), and extension velocity (mv3), with detailed specifications provided in Supplementary Table 2. In model mf1 (weak mantle friction coefficient = 0.1), short-lived exhumation channels develop within the weak mantle zone. This configuration produces symmetric rifted margins with minimal lithospheric mantle exposure (Fig. 4a; Supplementary Fig. 3a). Model mf2 (weak mantle friction coefficient = 0.01) develops exhumation channels within the weak mantle zone, and lithospheric mantle is exposed along the exhumation channel beneath the southeastern margin, accompanied by southeastward migration of the spreading center (Fig. 4a; Supplementary Fig. 3b). The end-member model mf4 (weak mantle friction coefficient = 0), exhibits a long-lived exhumation channel activity. This prolonged tectonic regime results in extensive lithospheric mantle exposure at the surface and facilitates multiple discrete oceanic crust emplacement events (Fig. 4a; Supplementary Fig. 3c).

Model mw20 (weak mantle width = 20 km) fails to develop exhumation channels, resulting in minimal lithospheric mantle exposure and a single spreading center (Fig. 4b; Supplementary Fig. 4a). Model mw30 (weak mantle width = 30 km) exhibits transient exhumation channel activity, moderate mantle exposure, and dual spreading centers (Fig. 4b; Supplementary Fig. 4b). Model mw50 (weak mantle width = 50 km) develops long-lived exhumation channels, promoting extensive mantle exposure and multiple spreading centers (Fig. 4b; Supplementary Fig. 4c).

Model mt20 (weak mantle thickness = 20 km) shows coupled crust-mantle rupture, negligible mantle exhumation, and a single stable spreading center (Fig. 4c; Supplementary Fig. 5a). Model mt30 (weak mantle thickness = 30 km) forms short-lived exhumation channels, moderate mantle exposure, and dual spreading centers with gradual abandonment of the older center (Fig. 4c; Supplementary Fig. 5b). Model mt50 (weak mantle thickness = 50 km, no gap between the weak mantle and the crust) preserves a large volume of weak mantle beneath continental margin, facilitating long-lived exhumation channels, extensive mantle exposure, and multiple spreading centers (Fig. 4c; Supplementary Fig. 5c).

In summary, our models show that a weak mantle zone with a smaller friction coefficient, larger width or thickness favors the formation of long-lived exhumation channels, thereby increasing the area of exhumed mantle lithosphere and the events of oceanic crust emplacement (Fig. 4).

Given that the rotational pole is located northeast of the Vavilov basin, the rifting rate in the northeast part of the basin is lower than the 4 cm/yr imposed in our reference model38. To investigate the effects of a slower extension velocity, we implemented a low-velocity model (mv3) with a rate of 3 cm/yr. In this model, the slower extension suppresses decompression melting, leading to decreased magmatic productivity. As a result, the COT developed only a single spreading center (Supplementary Fig. 6).

Methods
Governing equations
We use the geodynamic code I3ELVIS, which employs finite difference and marker-in-cell methods with irregular staggered Eulerian grids44–46 to solve the continuity and momentum equations for incompressible flow and the energy equation. The governing equations of conservation of mass, momentum and energy are:

∂vi / ∂xi = 0 (1)

∂σij / ∂xj - ∂P / ∂xi + ρg = 0 (2)

ρcp (∂T / ∂t + vi ∂T / ∂xi) = ∂ / ∂xi (k ∂T / ∂xi) + Hr + Ha + Hs + HL (3)

where vi is the velocity, σij is the deviatoric stress tensor, P is the pressure, ρ is the density, g is the gravity acceleration, cp is the heat capacity, T is the temperature, k is the thermal conductivity, Hr, Ha, Hs and HL are radiogenic, adiabatic, frictional, and latent heating source, respectively.

Method setup
The scale of the computational domain is 584 km (x-dimension) × 808 km (y-dimension) × 270 km (z-dimension) (Fig. 2a). The whole domain is resolved by 293 × 405 × 117 Eulerian nodes with a resolution of 2 × 2 × 2.5 km3. More than 240 million Lagrangian markers are randomly distributed in the model domain to track the physical properties, such as viscosity, plastic strain, and temperature. Free-slip boundary conditions are applied at the back wall (y=808 km), top wall (z=0 km), and at both the left and right walls (x=0 and 584 km). The volume of material flowing out of the front boundary (y=0) is compensated by the volume of material influx through the lower boundary (z=290 km).

The compositional fields are shown with colors in Fig. 2a. The initial material field consists of a weak 20-km-thick sticky air layer, which allows the crustal surface to approximate a free surface47. The lithosphere is divided into a 15-km-thick upper crust (wet quartzite rheology), a 15-km lower crust (Felsic granulite rheology), a 70-km-thick lithospheric mantle (dry olivine rheology), and an underlying 170-km-thick asthenospheric mantle (dry olivine rheology) (Fig. 2). The initial geotherm in the crust increases linearly from 0 °C at the surface to 500 °C at the Moho. The initial geotherm in the lithospheric mantle increases linearly from 500 °C at the Moho to 1320 °C at the bottom of the lithosphere (Fig. 2). We assume an adiabatic gradient of 0.5 °C/km for the subcontinental asthenospheric mantle. The visco-plastic thermo-mechanical parameters of each lithology are based on a list of laboratory experiments.

Rheology
A visco-plastic rheology is applied in the entire model domain with different rheological parameters for compositionally distinct layers. The viscous deformation in our models mainly follows the flow laws of the dislocation creep.

ηdisl = 1/2 (ε_II)^((1-n)/n) exp(Ea / nRT) A^(-1/n) exp(PVa / nRT) (4)

where A is the pre-exponential factor, Ea is the activation energy, Va is the activation volume, and n creep exponent. These values are listed in Supplementary Table 1.

Brittle/plastic rheology is controlled by fracture-related strain weakening46. It is implemented by using a Drucker-Prager criterion:

ηplas = σyield / 2 ε_II (5)

σyield = Cγ + φ(P - Pf) (6)

Where ηplas is the effective viscosity of plastic rheology. σyield is the plastic yield stress (Pa). P is the dynamic pressure on solids (Pa), Pf is the hydrostatic fluid pressure, Cγ is the plate strength at P - Pf = 0 (for both tensile and confined fracture) that depends on the plastic strain γ, defined by:

Cγ = C0, when γ ≤ γ0 (7)

Cγ = C0 + (C1 - C0) (γ - γ0) / (γ1 - γ0), when γ0 < γ ≤ γ1 (8)

Cγ = C1, when γ > γ1 (9)

C0 and C1 are the initial and final strength values for the fracture-related weakening, respectively. γ0 and γ1 are the upper and lower strain limits for the fracture-related weakening, respectively. φ is the internal friction coefficient, defined by:

φ = φ0, for γ ≤ γ0

φ = φ0 + (φ1 - φ0) (γ - γ0) / (γ1 - γ0), for γ0 < γ ≤ γ1

φ = φ1, when γ > γ1 (10)

φ0 and φ1 are the upper and lower limits, respectively. Strain weakening is obtained by strain-dependent linear interpolation between parameterized minimum and maximum values of φ and Cγ.

Integrated plastic strain γ (t is time and ε_ij(plastic) is the plastic strain rate tensor):

γ = ∫ (2 (ε_ij(plastic))2)1/2 dt (11)

The effective viscosity is defined as the minimum value between the brittle/plastic and ductile viscosities, with further limitations imposed by the cut-off values. In the case of partially molten materials (such as partially molten mantle and crustal rocks), Gerya (2013)46 investigated the effects of constant viscosities and the dry olivine flow law on the resulting spreading patterns. In light of the aforementioned results, a constant viscosity of 1018 Pa·s (i.e., the lower cut-off viscosity) of the partially molten materials has been incorporated into the model.

Magmatism
In order to model plate break-up and oceanic spreading, magma-related processes (melt generation, extraction, percolation and accretion) are implemented in our models in a simplified instantaneous process as described by Gerya (2013)46. The melt differentiation and interaction of the melts and the mantle are not included. The melt fraction (M) of the mantle is based on the parameterized melting model from Katz et al.48:

M = 0, for T < Ts

M = (T - Ts) / (Tl - Ts), for Ts ≤ T < Tl

M = 1, for T ≥ Tl (12)

Where Ts, Tl are the mantle solidus, mantle liquidus, respectively. Partial melting occurs in a wide area similar to the melt pooling region49, and melt is extracted and stored in the shallowest part of the partial melting area. Lagrangian markers track the amount of melt extracted during the model evolution. A total amount of extracted melt is calculated at each time step:

δM = M - ∑i=1n Mext (13)

where n is the previous extraction episodes, and Mext is the extracted melt at each time step. The rock is considered non-molten when the extracted melt fraction is larger than the standard melt fraction (δM < 0). Extracted mantle melts first travel vertically until they reach the mantle solidus surface under the lithosphere and then migrate along this surface toward regions with the highest local surface topography, where they form a crustal magma chamber. To ensure melt volume conservation and account for mantle compaction and subsidence in response to the melt extraction, melt addition to the bottom of the magma region is performed at every time step by converting shallowest markers of hot partially molten mantle into magma markers. At each time step, the volume of converted markers matches that of the extracted assembled melts computed for the time step. As a result of this treatment, the partially molten mantle always contains a small amount (<1 vol.%) of melt as this melt is continuously extracted and added to crustal magma chambers. The extracted melt at the next time step will be added to the bottom of the magma chamber.

Spontaneous cooling and crystallization of melts are implemented at the walls of the lower-crustal magma chambers by simply converting the rock back to the model lithology. This simple approach accounts for cooling and crystallization of melts. The magmatic products intruded along the shear zones are not included in our model.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors use numerical modelling to investigate how weak mantle rheology influences mantle exhumation and oceanic spreading during back-arc extension and continental breakup, aiming to explain the basement formation of the Central Tyrrhenian basin. The authors test different parameters namely, width and thickness of weak mantle areas, as well as several values for friction coefficient to conclude that weak regions in the mantle exerts first-order controls on the coexistence of oceanic spreading and mantle exhumation processes. While the concept of mantle inheritance influencing continental breakup is not new, few numerical studies to my knowledge have explicitly addressed its impact on the intermittent nature of these processes during continental margin evolution.\nOverall, the paper is well written and logically structured. However, I believe two major issues must be addressed—either through additional modelling or more extensive discussion—for the work to be suitable for publication in Nature Communications:\n• The authors convincingly argue that the weak mantle nature results from interaction with slab-derived by-products. However, such interactions are not a systematic feature in the evolution of magma-poor margins globally. This raises an important question: Can the findings of this study be generalized to other magma-poor margins (e.g., the Iberian margin), or are they specific to marginal basins? Clarification on the global applicability of these results is needed.\n• The Tyrrhenian basin is well known for its complex 3D geodynamic evolution (Faccenna et al., 2001). Variations in trench retreat between the Adriatic and Ionian sectors led to the formation of a triangular-shaped back-arc system, especially during the early Pliocene when the Magnaghi and Vavilov Basins developed (Faccenna et al., 2001). These basins opened with a rotational pole located near the Campanian margin (Milia et al., 2017; https://doi.org/10.1016/j.tecto.2016.10.017). Yet, in the current study, the authors assume constant kinematic parameters (4 cm/yr) along the basin axis. While I understand this choice likely aims to isolate the role of mantle rheology, a more detailed discussion is needed to address how along-axis variations in extension rates—reflecting the real 3D dynamics of the Tyrrhenian basin—could influence the modelled processes\nMinor comments:\n• Lines 37-38: The evolution of the Tyrrhenian basin is mostly constrained from previous DSDP and ODP samples in addition to extensive seismic reflection data. Rather than Faccenna et al. 2001, I would refer to original ODP and DSDP papers which tackled this aspect before. For instance:\no Kastens, K. A., & Mascle, J. 44. THE MAIN RESULTS OF ODP LEG 107 IN THE FRAME OF NEOGENE TO RECENT GEOLOGY OF PERITYRRHENIAN AREAS1.\n• Lines 41-42: Previous papers already speculated on the presence of exhumed mantle, such as Sartori et al. (2004; https://doi.org/10.1016/j.tecto.2004.02.008). To be fair, I would refer to some of them as well.\n• Fig.1 (Line 76): Insert -> Inset.\nI don’t wish to remain anonymous,\nManel Prada", "reviewer_3": "Review on “Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin” by Su et al.\nDear authors, dear Editor,\nThank you for inviting me to review this interesting manuscript. The paper focuses on the effect of a weak mantle wedge on magma-poor COT development based on the Tyrrhenian basin example. The key finding is that the weakened mantle wedge during previous subduction fosters the development of magma-poor Continental-Ocean Transition (COT) in the rapidly extending back-arc basins such as the Tyrrhenian basin. This finding provides a new perspective on the mechanisms controlling the magma-poor COT and is certainly of interest to the broad audience of Nature Communications.\nHowever, several critical aspects (described below) require clarification before the manuscript is considered for publication.\nMinor comments and suggestions can be found in the attached pdf.\nThe manuscript would generally benefit from a more focused connection between geological observation and model results. The result section requires improvements and verifications of the presence of the observed inconsistencies. The literature review seems to spotlight the discussion part, which lacks a discussion of results and their critical evaluation.\nIntroduction. The introduction section will benefit from a brief definition of magma-poor COT.\nFurthermore, a brief and more focused elaboration on magmatic products could improve the paragraph (lines 51-67) on the Tyrrhenian basin. Namely, the motivation to include weak mantle could be justified additionally by saying that magmatic products (Mid Ocean Ridge type basalts and alkaline lavas, e.g., Prada et al., 2016) observed in the study area suggest the presence of fluids released from the dehydration of the subducted plate. From the current text, the temporal relationship between subduction-related magmatism and the exhumation of mantle and oceanic crust is unclear. Adding, maybe in the brackets, age constraints for the described events. Are the magmatic products intruded along the shear zones in the mantle? What is the contact between the exhumed mantle and magmatic products? Is there an explanation why the oceanic crust bodies are “tilted to the east” (Fig. 1c, this study)?\nResults. The model result section does not always explain the motivation for the parameter choice. For example, it is not clear why the weak mantle is placed in the NE. Why is it not continuous along the strike? If a retreating Ionian slab affects the entire region, i.e., leads to a constant extension rate along the strike (following your model set up), I would expect that slab dehydration will weaken the mantle above along its strike. Geological records indeed show along-strike heterogeneities (perhaps extension rate varies) in the studied system, which is best observed in the triangular form of the Vavilov Basin (see Milia et al., 2017). The rift basin in your model indeed has a triangular form, but rotated by 180°.\nAnother critical question is why there is a break between the weak mantle and the crust. It looks like the weak mantle is engulfed within the lithospheric mantle. I would not expect gaps in the metasomatised mantle above the slab “right after” arc magmatism (e.g., Dorendorf et al., 2000). This seems needed to decouple crust and mantle, which eventually leads to an asymmetric rift and “punctuated” magmatic activity crosscutting the exhumed mantle. Would the result be the same if the weak mantle started at the Moho?\nFollowing the colour code, the exhumed mantle seems not lithospheric (dark green in Fig. 2) but weak (blue), which is inconsistent with the result description.\nRegarding the partially molten mantle, it is unclear whether the model differentiates between lithospheric (I guess not affected by slab dehydration) or weak (affected by slab dehydration). Fig. 2 seems to show partial melting of lithospheric (dark green) and (afterwards) asthenosphere mantle (dark blue), which is regarded in the model as depleted at least in the SW part of the model domain. This is important because, based on the literature, punctuated magmatic products, i.e. basalts developed by partial melting of fertile–subduction–like mantle (blue in the model domain). This points out the inconsistency between geological observation and model results, as in your model, basalts are formed by the partial melting of the depleted mantle. Furthermore, in I3Elvis, melts are instantaneous, meaning that differentiation and interaction of the melts and potentially weak mantle are not considered. This has to be discussed.\nCan the term “exhumation channels” be explained? Can these be comparable with the low-angle detachments seen in many COTs? If not, what is the difference?\nDiscussion. The subchapters in the discussion part do not reveal clear messages, making it difficult to distinguish their main messages. Fewer subchapters with clearer and stronger messages are needed. It seems like a long revision of previous work, with some information on the current results appearing only at the end.\nThe comparison with available geological observations could be better exploited in the discussion, and the processes could be better explained. Are there similarities in the magnitude of the spreading ridge migration between models and observations? The Oceanic Crust looks like ridges in Fig. 1. Do the models resemble their amplitude (basin bathymetry) and frequency?\nExplanation of the jumping spreading centres is unclear and difficult to grasp as something novel. The partially molten mantle moves following the “exhumation channel”. In fact, the exhumation channel “jumps” following the velocity boundary. So once the shear zone is established in the high-strain area, it follows the velocity boundary until the weak mantle completely disappears (pinches out). It is visible by having a weak mantle between spreading ridges, getting thinner and thinner, and the velocity boundary moving.\nSome questions to be explained in the discussion are 1) why these high-strain areas jump even though the velocity boundary is constant, and 2) why it terminates.\nFigures – font is very small and figure parts are densely packed. It is difficult to read.\nBest regards,\nNevena Andrić-Tomašević\nReferences:\nDorendorf, F., Wiechert, U., Wörner, G. 2000. Hydrated sub-arc mantle: a source for the Kluchevskoy volcano, Kamchatka/Russia, Earth and Planetary Science Letters, 175, Issues 1–2, 69-86, https://doi.org/10.1016/S0012-821X(99)00288-5.\nMilia, A., Torrente, M.M., Tesauro,M. (2017) From stretching to mantle exhumation in a triangular backarc basin (Vavilov basin, Tyrrhenian Sea, Western Mediterranean), Tectonophysics, 710–711, 108-126, https://doi.org/10.1016/j.tecto.2016.10.017.\nPrada, M., Ranero, C. R., Sallarès, V., Zitellini, N. & Grevemeyer, I. (2016) Mantle exhumation and sequence of magmatic events in the Magnaghi–Vavilov Basin (Central Tyrrhenian, Italy): New constraints from geological and geophysical observations. Tectonophysics, 133-142.\nTheunissen, T., Huismans, R.S. Mantle exhumation at magma-poor rifted margins controlled by frictional shear zones. Nat Commun 13, 1634 (2022). https://doi.org/10.1038/s41467-022-29058-1", "reviewer_4": "In this paper \" Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin\", Su et al. present 3D magmatic-thermomechanical numerical models of the fast back-arc extension occurring in the Tyrrhenian basin. They aim to model the effect of a large and very weak mantle in the initial lithospheric mantle. The model approach is based on previous observations of present-day slow shear waves at 80 km depth below the Tyrrhenian Sea, that are attributed to hydrated mantle during the subduction process and the retreating slab. The results are interesting and pertinent. Further discussion is needed to assess the role of melting in the weak mantle material. Indeed, mantle hydration favorizes melting and it is unclear if the models consider this behavior. As a whole, the significance of the results is very important for the field (if the point about weak mantle melting is discussed) as it provides a new pattern of mantle exhumation processes.\nI think the manuscript needs a more detailed discussion about the limits of the modelling. The manuscript reads well. The figures are clear and complete the text adequately.\nGeneral :\n- Line 13 and 208-210: The model does not describe only a COT but also spreading as soon as the initial lithosphere is broken at 2.72 My. So, I am not sure that it is necessary to say it challenges prevailing models of magma-poor COT.\n- Line 27: The model rather gives information about continental mantle exhumation during spreading, which is new too.\n- Line 52 to 58: It is not clear to me, how the results of Magni et al. (2021) are in contradiction with the record of the Vavilov sub-basin?\n- Line 98: The reference 29 seems not relevant here.\n- Line 178: I would rather say “magma-poor rifting and spreading can develop under fast-extension”\n- Line 232 to 237: Here, the list of possible causes is unclear. It would be more interesting to group them by common processes. Indeed, it would highlight the novelty of your theory.\nModelling approach:\n- It occurred to me that hydrated mantle is not implemented in the model. You only define a weak mantle. Did you assess the consequences of mantle hydration on its melting capacities?\n- Does the weak mantle (and the lithospheric mantle) melt at all in the model? If not, why?\n- To me the main question is why the exhumation of mantle (allowed by the weak mantle) does not generate more melt? It should appear in the discussion.\nFigures:\n- Fig. 2: The colour code for oceanic crust and lithospheric mantle makes unclear if lithospheric mantle is exhumed to the surface with weak mantle.\n- Fig. 3 (a, d, g, j): make it clear that the colours are the same as in fig. 2\n- Fig. 4: make it clear that the squares refer to the number of spreading centres and that the circles refer to the area of exposed lithospheric mantle\n- Fig. 3 (suppl. mat.): the text is « b model mf3 (u = 0.01)» instead of « b model mf2 (u = 0.01)»"}, "author_response": "Response to Reviewers\n\n[... Authors' point-by-point response to Round 1 reviewers ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The paper now tackles most of my previous comments, but caution is needed when comparing modelling results with specific volcanic features, as their nature is complex and not all volcanoes where formed during spreading center phases.\nAlso I think that the text would benefit from some minor modifications when referring to the Iberian margin.\nI attach here the rebuttal letter and manuscript with some comments.", "reviewer_3": "Dear authors, dear Editor,\nThe manuscript has been significantly improved, well done. The authors adequately addressed my points. I have no further comments that could hinder publication of this work.\nBest regards,\nNevena Andrić-Tomašević", "reviewer_4": "The replies to the comments on the paper \" Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin\", by Su et al., answered all my concerns. The authors provided clear explanations about the terms/phrasing used in the paper and clarified their text accordingly. The processes at play were better discussed thanks to new numerical models including melting during exhumation.\nI have no further comments, and I congratulate the authors for their nice work\n."}, "author_response": "Response to Reviewers\n\n[... Authors' point-by-point response to Round 2 reviewers ...]"}]

Decision letter