Northern Scandinavian mountains supported by a low-grade eclogitic crustal keel

Domain: earth_science
Published: 2025-01-11
Editorial outcome: minor_revision

Abstract
Plate tectonics predicts that mountain ranges form by tectono-magmatic processes at plate boundaries, but high topography is often observed along passive margins far from any plate boundary. The high topography of the Scandes range at the Atlantic coast of Fennoscandia is traditionally assumed isostatically supported by variation in crustal density and thickness. Here we demonstrate, by our Silver road seismic profile, that the constantly ~44 km thick crust instead is homogenous above the Moho, and Pn-velocity abruptly change from 7.6 kms−1 below the Scandes to >8.2 kms−1 below the Proterozoic shield. By modelling gravity anomalies and topography, based on the seismic model, we demonstrate that this change corresponds to an increase in metamorphic eclogitic grade from 35% below the high-topography Scandes to 70% below the low-topography shield. The sharp contrast between the low-grade, reduced-density and the high-grade, high-density eclogitic bodies below the uniform seismological Moho explain the enigmatic topography of the mountain range without a crustal root.

Introduction
High topography is observed at a number of passive margins such as in North and South America, eastern and southern Africa, eastern Australia, southern India, and around the Atlantic Ocean1. These regions are all located far from plate boundaries, which makes the observed high-topography enigmatic, and dynamic topography is a possible cause2,3. The Scandinavian mountain range (the Scandes) along the Atlantic coast of Fennoscandia is a classic example. It is a NNE-SSW striking mountain range in the Caledonian deformed part of western Fennoscandia (Baltic Shield), consisting of two domes with elevation up to 2500m separated by a lower relief section with topography less than 1000m (Fig. 1a). The Caledonian orogeny at ca. 490–390 Ma was the last collisional event, and the area has been tectonically stable since the North Atlantic break-up at ~55 Ma4.

The origin and timing of the Scandes is debated5. Most models suggest that their high topography formed during the Cenozoic1 but a competing model suggests that the topography has sustained since the Caledonian orogeny6. Analysis of apatite fission track, geomorphological, and sedimentation data indicates that the high topography is young and that major uplift of the Scandes compared to the shield and shelf occurred during the Meso-Cenozoic in a series of uplift, relaxation, subsidence and final uplift events which exhumed deep parts of the Caledonian orogen5,7,8. However, climatic change also influences the observed uplift considerably, and isostatic rebound related to increased erosion may lead to peak uplift while the average topography decreases9.

Given the uncertainty of the timing of uplift, it is a key question if the change in topography across the Scandes may be explained by isostasy, as indicated by the near-zero free air gravity anomalies throughout Fennoscandia. In this case, the origin of the present mountain range should be related to the latest major tectono-magmatic event that modified the thickness and density of the crust and lithospheric mantle. Although non-unique, previous gravity analysis has suggested Airy-type crustal isostasy for the northern Scandes and isostatic compensation by low-density mantle in the southern Scandes, possibly enhanced by flexure of the lithosphere10. However, seismic identification of a 10–12km thick crustal root under the southern Scandes highlands suggests dominant Airy-type isostasy with compensation at the Moho11. The extensional collapse of the Caledonian orogen and later Permian extension may have had major effect on delamination or convective removal of the lower crust below part of the Scandes12. Seismic Receiver Function studies in both the northern and southern Scandes indicate that the low elevation of the shield is controlled by the presence of a thick, high-density, mafic lower crust13,14. The presence of a pronounced low-velocity (δVp ~−5% with respect to regional average) layer at depths of 60 to ~120 km below the whole Scandes range may indicate partial deep isostatic support of the high topography from this level15.

Postglacial uplift of Fennoscandia is well documented16,17, but it cannot explain the existence of the Scandes since it affects the low-topography shield more than the margin of Fennoscandia. Proposed models for recent (Mesozoic or Cenozoic) uplift include tectonic uplift due to far-field compressional stresses or shoulder uplift due to oceanic break-up8, isostatic response to accelerated glacial erosion following climate change18, dynamic uplift due to asthenospheric diapirism19, or progression of ‘fingers’ of the Icelandic hotspot20. In any case, the cause of uplift must be found above the mantle transition zone that has constant thickness throughout the region which excludes that a deeper thermal anomaly may affect the lithosphere21.

The objective of this paper is to assess the relative importance of the mechanisms that cause the high topography of the Scandes by interpretation of a seismic profile, integrated with gravity and topographic data, across the northern dome. This assessment is complicated by the complex geology of Fennoscandia, which formed by amalgamation of a series of terranes and micro-continents during the Archean to the Palaeoproterozoic, followed by significant modification in the Neoproterozoic and Palaeozoic by the Sveconorwegian (Grenvillian) and Caledonian orogenies22,23. The Trans-Scandinavian Igneous Belt (TIB, Fig. 1b) is marked by granite-porphyry intrusions that were emplaced during the Gothian Orogeny (1.75–1.5 Ga) with a general NS trend at the transition between the Palaeoproterozoic Svecofennian (~1.9–1.8 Ga) and the Meso- to Neoproterozoic Sveconorwegian (~1.0 Ga) domains of southeastern Fennoscandia and also traced as sporadic outcrops northwards within the presently thin sheets of Caledonian orogenic structures, with one outcrop observed in the Silver-Road profile (Fig. 1b). The surface geology across Fennoscandia is exposed in the Precambrian part of the Baltic Shield, which generally lacks a sedimentary cover. The Caledonian Deformation Front (CDF) is observed as a major transition between the Caledonian deformed area (today exposed as a middle crustal part of the orogen) and the Svecofennian shield.

Here we present the Silverroad seismic velocity profile of the crustal and uppermost mantle structure across the northern Scandes topographic dome (ND in Fig. 1a) which shows that the entire onshore crust is largely homogeneous across the northern Scandes (coinciding with the Caledonides) and into the Svecofennian Baltic Shield, such that crustal isostasy cannot explain the topography as traditionally proposed10. Instead, the model suggests that the high topography of the Scandes is isostatically supported by a layer with relatively low density due to low proportion of metamorphic eclogite below the Moho in sharp contrast to high-density, high-grade eclogite below the cratonic Svecofennian domain with low elevation.

Definition of some geological terms has been included at the beginning of “Methods” section.

Results
Seismic data and interpretation
Our 380km long, WNW to ESE striking, controlled source, seismic refraction/wide-angle reflection, Silver-Road profile is located in northwestern Fennoscandia perpendicular to the coast (Fig. 1a). It extends across a narrow coastal, offshore sedimentary basin and the onshore Caledonian and Palaeoproterozoic Svecofennian domains. The model is interpreted based on data from 272 onshore stations at nominally 1.1 km distance for 5 onshore explosive sources detonated in ~50m deep boreholes with 400kg explosives at the profile ends and 200kg explosives in-between, as well as 12 stacks of multiple airgun shots from the ~70km offshore part of the profile (Fig. 1a, “Methods” section). The data are interpreted by travel time picking of the main seismic phases and subsequent ray tracing travel time modelling of the crustal and sub-Moho P-wave velocity structure (Fig. 2) as well as extensive resolution tests (“Methods” section, Supplementary Figs. 1–12).

Seismic velocity model
Theseismic velocity model (Fig. 2) constrains the velocity structure of the crust and upper mantle to ~50 km depth with a depth and velocity resolution better than ±2km and ±0.2 kms−1, respectively (Supplementary Fig. 1, “Methods” section). The offshore part includes an up-to-10 km thick sedimentary cover with variable P-wave velocity between 2.1 kms−1 (top) and 5.8 kms−1 (bottom) constrained by an earlier seismic profile24. There is no evidence for sedimentary layers in the Caledonian and Svecofennian domains.

Our model (Fig. 2b) challenges earlier models in the region (Fig. 3) in two aspects:
1. Crustal thickness (~44 km) and velocity distribution show almost no lateral difference between the Svecofennian shield and the high-topography Scandes.
2. The upper mantle Pn velocity changes at ~125 km horizontal distance between 7.6 kms−1 below the high-topography area and 8.2 kms−1 below the Svecofennian shield.

The Moho depth and Pn-velocities are well-constrained by reversed Pn coverage and strong PmP reflections in the distance interval km 50–250 along the profile. This coverage documents that the Moho is flat and that the Pn velocity is fully constrained in this distance interval. More details are shown in Supplementary Figs. 2–12.

While the results of early profiles across the northern Scandes also suggest little lateral variation in Moho depth, the resolution in these studies is low and does not allow determination of the Pn velocity, which was assumed to be ~8.2 kms−1 (see refs. 25,26). In contrast, profiles in the high-topography, southern Caledonides section (Fig. 3) show the presence of a felsic to intermediate, 38–42 km thick crust11 with abrupt thinning towards the ~19 km thick crust below the shelf12 and thickening to ~50 km in the Svecofennian shield13, where it includes an up-to 24 km thick lower crust with velocity >6.9 kms−1 (see refs. 11,27). The southern high-resolution profiles show Pn velocities of ca. 8.1 kms−1 across the southern Scandes and into the Svecofennian shield11. These results on the crustal and upper mantle structure of southern Fennoscandia are in sharp contrast with our seismic model for the northern Scandes (Fig. 2).

It is unusual to observe a mountain range without a crustal root, as in the northern Scandes, and to observe constant thickness and structure of the crust across two very different crustal domains of Caledonian and Proterozoic origin. The observed crustal velocities (Fig. 2) are consistent with a Precambrian cratonic crustal type with a felsic to intermediate granulite facies lower crust along the whole seismic profile, including the Caledonian orogen, which therefore must be thin-skinned.

The low Pn velocity of 7.6 kms−1 below the Caledonian domain has not been observed in previous studies along the Norwegian coast28 (Fig. 3). We suggest that this low Pn velocity extends to a depth of 60 km, which is the depth to a strong positive velocity discontinuity below the seismic Moho in a recent S-wave receiver function (S-RF) profile29, and we test this model by gravity modelling. This S-RF profile coincides with our profile and is calculated from the ScanArray broad-band seismic data30, covering the whole of Fennoscandia with a nominal station spacing of 50 km, together with additional broad-band seismic stations at higher density in the region around the Silver-Road profile14. The data shows clear P-RF and S-RF conversions from the Moho at the same depth as in our profile, and the S-RF include a clear converter at 60 ± 4 km depth29.

Density model
A distinct difference in the relation between topography and Moho depth, in the northern, central and southern Scandes as well as the Svecofennian shield (Fig. 3i), indicates differences in isostatic compensation. Low Pn velocity below the northern Scandes indicates that the high topography may be isostatically supported by low-density material below the Moho. To test this hypothesis, we calculate a density model down to 60-km depth (Fig. 4) from the seismic model by 2-D density modelling (“Methods” section) constrained by a standard velocity to density relation31,32. The model includes a distinct change in sub-Moho density from 3.22 g cm−3 below the Scandes to 3.47 g cm−3 below the high-velocity Svecofennian shield, and it explains the observed Bouguer gravity anomaly along the profile (Fig. 4).

Assuming isostasy with a compensation level at the base of the density model, we calculate the topography variation predicted by the model. The difference between predicted and observed topography is up to 2.9 km in the less-constrained offshore part, up to 2.5 km in the high-topography area of the Scandes, and <1 km in the Svecofennian shield (Fig. 4a). Therefore, isostatic equilibrium can only be achieved at a deeper level which we assume to be at the base of the teleseismic low-velocity layer (δVp ~−5% compared to regional average) at ~120 km depth below the Scandes15.

We determine densities in the interval between 60 and 120 km depth by fitting both Bouguer anomaly and topography with only minor change to the previously determined crustal and sub-Moho densities (Fig. 4). The free air gravity (FA) is close to zero along most of the profile, including the highest topography (Fig. 5b) with some variation near the coast, where the section is not fully isostatically balanced. We, therefore, include the FA variation in the calculation of the isostatic topography (“Methods” section). The resulting model (Fig. 5) is derived by initially assuming a peridotitic mantle below 60-km depth in the Svecofennian shield with an in-situ density of 3.36 g cm−3 (S2) below the sub-Moho, high-velocity (Vp ~8.2 kms−1), high-density (3.47 g cm−3) body (S1). By this choice, the model requires a high density of 3.47 g cm−3 below the high Scandes between 60-km and 120 km depth (C2) in order to explain the observed topography (see “Gravity test” in “Methods”), despite the seismic velocity in this low-velocity zone is 5% lower than the average in the same depth interval throughout Fennoscandia15. With these choices and a slight decrease of densities to 2.87 g cm−3 and 3.00 g cm−3 for the lower crustal part, and to 3.20 g cm−3 for the shallow sub-Moho layer below the Scandes (C1), the model predicts the topography within ±60 m elevation and the Bouguer anomaly within an RMS-average of 23 mGal along the profile (Fig. 5). While a direct assessment of the uncertainty in gravity inversion is not possible, our sensitivity tests for various combinations of parameters demonstrate that the uncertainty of the model densities is of the order of 0.04 g cm−3 for the model values above 60 km depth and less than 0.02 g cm−3 between 60 and 120 km depth (Supplementary Figs. 13–16, “Methods” section), and tests show that the chosen exact compensation depth cannot influence the determined densities beyond the estimated uncertainties (Supplementary Figs. 17 and 18).

Methods
Definition of some important geological terms
Moho – the seismic interface where the P-wave velocity increases to above ~7.6 kms−1; it is mostly a strong seismic reflector. Reflections from the Moho are termed PmP and the velocity directly below the Moho is termed Pn velocity. Crust-mantle interface – the petrological transition from mafic to ultra-mafic rocks, which not always coincides with the Moho. Eclogites – here used to refer to eclogitic rocks formed from lower crustal, mafic rocks by high-pressure metamorphism. Fennoscandia – the geographic area consisting of Scandinavia and Finland; the Baltic Shield – the major, cratonic part of Fennoscandia which lacks a sedimentary cover and includes an Archaean nucleus in the north, to which a series of terranes and micro-continents amalgamated during the Svecofennian, Sveconorwegian and Caledonian orogenies; Svecofennian shield – the Svecofennian orogenic part of the Baltic Shield; Caledonides – the Caledonian deformed western part of Fennoscandia, possibly representing a series of nappes over original shield type crust; Scandes – the mountain range in western Fennoscandia along the North Atlantic coast which largely coincides with the Caledonides.

Seismic data
Our 380km long refraction/wide-angle reflection profile, sub-perpendicular to the coastline, was acquired in July 2016 in northern Scandinavia between 67.6°N/13°E and 65.6°N/19.5°E. It extends from ca. 80 km offshore, across the Caledonides in the region of the high topography of the northern Scandes, and into the Svecofennian unit of Fennoscandia with moderate topography (Fig. 1a, b). Our seismic profile is based on data from 272 onshore stations, which recorded the seismic signal from five onshore explosive sources and twelve stacks of airgun shots (AGS) in the offshore part of the profile, sourced from vessel Hakon Mosby (Fig. 1a). The nominal station spacing is 1.1 km, the onshore source spacing is ~75 km, and airgun stacks are calculated at 3 km intervals. The 170 Texan and 102 Sercel instruments were equipped with 4.5 Hz vertical component geophones (Supplementary Table 1). Data were resampled to 6 ms before processing and modelling. The onshore seismic sources consist of two end-shots each charged with 400 kg chemical explosives (shots SP1 and SP5) and 200 kg charges at shot points SP2, SP3, and SP4 (Fig. 1a and Supplementary Table 2). All shots included multiple boreholes, each charged with ~50 kg of explosives above the base of the boreholes at a depth of ~50m. Each airgun stack is calculated by stacking of 18 individual recordings of signals from a 22 liter large airgun array (Supplementary Table 2).

Seismic data analysis
The interpretation of the seismic data includes phase correlation and travel time picking of the refraction and reflection seismic phases (Pg, P1, P2, Pc1, Pc2, PmP, Pn) from the crust and uppermost mantle (Fig. 2 and Supplementary Figs. 2–10), and subsequent forward modelling of the crustal velocity structure by raytracing travel time fitting. The Szplot software package67, modified by P. S ́ roda, was used for phase correlation and travel time picking, taking reciprocity into account. The seismic model was modelled with the rayinvr software67 through the graphical user interface Pray68, which allows interactive editing of the velocity model. The software uses a ray tracing method for forward calculation of travel times and synthetic seismograms and for iterative inversion for optimum determination of selected model parameters. The 2-D velocity model is derived by applying a top-down scheme where the shallowest velocity layers are first determined, followed by subsequently deeper layers for the P-wave travel times.

Resolution of the seismic model
Overall, the model resolves the upper 50 km vertically. Vp velocities are constrained down to this depth at profile distance 0 to 250 km horizontally. The diagonal values of the resolution matrix are used to determine the depth and velocity resolution of the model69. Parameters in the velocity model are considered well-constrained if the diagonal value of the resolution matrix exceeds 0.5 within a depth range of Δd = ±2 km or a velocity range of ΔV = ±0.2 kms−1. The area covered by seismic rays encompasses almost all parameter nodes, which all are constrained within the resolution criteria, i.e. they are resolved within ±2 km and ±0.2 kms−1 (Supplementary Fig. 1a). However, the top of the lower crust is slightly less well resolved due to a limited number of travel time observations for P2 and Pc2 (e.g. Supplementary Fig. 1, 5, 6, and 9). The offshore part of the model is constrained by the results of an earlier OBS profile24.

Travel time fit of seismic phases
The seismic sections from onshore explosions (SP) and stacks of airgun shots (AGS) are of high quality. The correlated P-wave phases have clear onsets, which allow precise determination of their arrival times. The first arrival, refracted phases are observed across the whole onshore profile. The refracted phases from the crust (Pg, P1, P2) are correlated as first arrivals to around 200 km offset, where the sub-Moho Pn becomes first arrival out to 300 km offset. The refracted phase P2 from the lower crust is also identified as a secondary arrival behind the P1 where it merges with the PmP reflection. We identify two intra-crustal reflection phases Pc1, Pc2, and the PmP reflection from the Moho. Reciprocity is checked for all interpreted phases. All phases can be reliably picked and the the refracted, first-arrival phases have high signal-to-noise ratio.

The Pg phase from the upper crust is observed out to 100–120 km offset in all sections. P1 is observed on all sections to around 210 km offset with a higher apparent velocity than Pg, ranging between 6.3 and 6.5 kms−1. The corresponding reflection phase Pc1 is observed at 80–120 km offset in all onshore shot sections. The refraction P2 from the lower crust is observed in the offset interval 200–300 km in the long-range shot point sections (Sp1, Sp2 and SP5) with an apparent velocity of 6.7–6.8 kms−1 with the matching Pc2 reflection only observed in the SP1 section in the western part of the model.

The PmP phase is observed along the whole profile (Supplementary Figs. 10–12) with variable waveform and amplitude providing reflection coverage of the Moho from km 0 to km 250. The amplitude is weaker below the Scandes than in the Svecofennian part of the profile, where the model includes a strong velocity contrast across the Moho.

The Pn phase is observed as a clear first-arrival phase at offsets between 200 km and the end of each section in all sections with reversed coverage of the profile from 60 to 250 km (Supplementary Figs. 2–12). It provides good ray coverage of the uppermost mantle along the whole profile. The observed Pn phases all show direct connection to the critical PmP reflection demonstrating that the picked PmP phase are reflections from the Moho.

The resulting 2-D ray tracing velocity model (Fig. 2b) explains 99% of the picked travel times (Supplementary Table 3). The RMS travel time residual (tRMS) is 76 ms, corresponding to 88 ms for onshore sources and 53 ms for the airgun shots (Supplementary Tables 2 and 3). We estimate the uncertainty of the picked travel times to be 75 ms for the refracted crustal phases (Pg, P1 and P2) and 100 ms for all other phases, which is a conservative estimate. The calculated χ2 of the travel time residual is 0.9, corresponding to 1.3 for the onshore sources and 0.4 for the airgun stacks (Supplementary Tables 3–5).

Gravity test
We convert initially the P-wave velocity profile to a two-dimensional density section by using a standard velocity-density relation31,32. This section is then used for 2-D modelling the density distribution along the seismic profile by iteratively fitting calculated gravity for the model with the observed Bouguer gravity anomalies from the EGM2008 gravity model70 by using the GM-SYS (Geosoft Oasis Montaj) software. The initial density model basically fits all gravity data points. By slight modification of the initial density model (by less than 0.10 g cm−3 and generally less than 0.05 g cm−3), we obtain a fit to the gravity data better than 20 mGal.

Our first model extends to 60-km depth, where a recent regional S-wave receiver function study shows a strong positive converter in the area around our profile29. The density model consists of four main layers including almost homogeneous top, middle, and lower crust with densities of 2.72, 2.84, and ~3.00 g cm−3. Similar to the velocity model, the main density variation is found in the upper mantle with densities of 3.24 g cm−3 below the thinned offshore crust, a similar low-density body (3.22 g cm−3) in the sub-Moho low-velocity region below the high topography of the Scandes, and an exceptionally high density of 3.47 g cm−3 in the Svecofennian unit (Fig. 4c). This model fits the observed Bouguer gravity anomaly within ±11 mGal (Fig. 4b). However, a calculated hypsometry profile, assuming vertical isostasy, shows very large elevation differences to the observed topography, i.e. the model predicts a ~2.9 km high mountain range, where the observed height is <1 km (Fig. 4a).

To fit the observed topography, we extend the model to 120 km depth, keeping its upper part similar to the first model. The 120 km compensation depth is chosen to incorporate the results of a recent teleseismic tomography study15 which includes a very low P-wave velocity anomaly (−5% with respect to the regional average) in the upper mantle down to a depth of ~120 km.

The free air gravity anomaly (FA) is close to zero along most of the profile and below the highest topography such that we may assume isostatic equilibrium. The FA variation between +10 and -25 mGal in the coastal zone (Fig. 5b) may be caused by lithosphere flexure. To account for possible deviation from isostatic equilibrium, we include the FA values into our calculation of predicted topography. Similar to isostatic calculations, where FA is assumed zero, the topography (t) above sea level is calculated from the equation

FA = 2πG(tρ1 + Σhi ρi) (1)

where G is the gravity constant, ρ1 is the density of the upper layer in the model, and the second term is the sum of the products of density ρi and thickness hi of the layers between sea level and the compensation depth.

To fit both the observed Bouguer gravity anomaly and topography, the mantle interval between 60 and 120 km depth requires high densities of 3.47 g cm−3 in unit C2 below the high-topography Scandes, located between zones with “normal” peridotitic upper mantle densities (~3.36 g cm−3) both towards the ocean and in the Svecofennian shield (Fig. 5c). In comparison to our first gravity model, only minor changes are required for the crustal and mantle densities in the upper 60 km. The adjustment of densities includes changes from 2.94 to 2.87 g cm−3 and from 3.03 to 3.00 g cm−3 for the lower crustal part in the Caledonian and Svecofennian parts, respectively, as well as from 3.22 to 3.20 g cm−3 for the shallow sub-Moho layer below the Scandes (C1), while keeping unchanged the sub-Moho density below the Svecofennian unit (S1). With these slight changes, the model explains both the observed Bouguer anomaly and the hypsometry (Fig. 5).

Extensive sensitivity tests of the key parameters for units C1, C2, S1, and S2 (Supplementary Figs. 13–16) show that the final density model shows a perfect match of topography within 130 m (Fig. 5a) and the Bouguer anomaly within 24 mGal (Fig. 5b). The densities of units C1 and S1 between ~45 and 60 km depth are well-constrained within 0.04 g cm−3, and the densities of the large units C2 and S2 between 60 and 120 km depth are constrained within 0.02 g cm−3. Teleseismic tomography has a relatively low vertical resolution, so we test the importance of the chosen exact depth to the base of the density model by sensitivity analysis, by which we find that the exact choice is insignificant for the determined densities (Supplementary Figs. 17 and 18).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper presents seismic and gravity modelling evidence for the nature of the upper lithosphere beneath the Scandes\nmountain range and argues that the topography is controlled by variations in density associated with variable amounts of\neclogite at particular levels. The role of metamorphism in the support of topography is an interesting active area of research,\nand in my view this is an valuable contribution to this field. I enjoyed reading it and I am confident it will be publishable. I\nhave two concerns in its present form. The first is presentational but requires a careful review of the way critical concepts are\nintroduced, and the second relates to the justification of assumptions of the gravity modelling.\nThe subject inevitably blurs some boundaries in terminology which are used casually in places. For example, the eclogite\ncontaining bodies are interpreted to be of lower crustal origin but are below the Moho discontinuity interpreted in the seismic\ndata. This is difficult to explain, and the manuscript only really addresses this in the discussion. As a result, for much of the\npaper it is unclear whether the authors are discussing mantle eclogite, lower-crustal eclogite mixed with mantle or a lower\ncrustal root rich in eclogite that is below the ‘seismological’ Moho. Clarifying this is not that easy, and it is clear the authors\nare aware of the problem and have tried to address it (e.g. lines 341-343), but I do not think they have been entirely\nsuccessful in this version (or at least it could still be made easier for the reader). I think more precise definitions of ‘Moho’,\n‘mantle’, the nature of the eclogite-rich body etc. in the introduction and throughout are needed. These issues are covered in\nthe discussion but need to be addressed earlier. I understand they may feel that this preempts the discussion too much, but I\nthink this will greatly help fresh readers. Some other terms (e.g. ‘uplift’) are also used loosely and could be clarified. These\nreferences occur throughout the manuscript and I think reconsidering how these terms are used and how to lead the reader\nthrough the concepts would be beneficial.\nThe gravity modelling is non-unique and so the choices of depth ranges where variation is allowed will make major\ndifferences to the results. The seismic data presented here allows them to constrain the upper 40 km as well as the layer\nimmediately below this. This layer is assumed to be 20 km thick beneath both the Scandes and the shield on the basis that\n‘a recent S-wave receiver function profile includes a strong positive velocity anomaly below the seismic Moho’ at 60 km. It is\nnot clear where this profile is from, the reference is to a conference abstract so it is not possible to assess it, and it is not\nclear to me why this layer should be the same thickness in both locations (the shield and the Scandes). Would it be possible\nto show the receiver function and plot its location? Is it reasonable to assume a layer of constant thickness in both regions?\nEqually another layer is placed below this to explain the rest of the variation in the gravity data in the interval 60-120 km.\nThis is assumed because this is the base of a ‘teleseismic low velocity layer’ below the Scandes (incidentally it is the\nmethodology rather than then layer which is tele seismic). It is assumed that there is no variation below this. These decisions\nseem somewhat arbitrary (or at least require better documentation). The association between low velocities and high\ndensities beneath the Scandes in the 60-120 km depth range is very surprising, but must also be somewhat dependent on\nthe assumptions made. It seems possible that some small component of dynamic uplift could also produce the gravity signal\nwithout requiring the high densities at this level?\nMinor comments\nLine 29: Seismological Moho?\nLines 47-51: When referring to uplift it Is important to specify what is uplifted relative to what baseline: See England, P. and\nMolnar, P., 1990. Surface uplift, uplift of rocks, and exhumation of rocks. Geology, 18(12), pp.1173-1177.\nLine 53-54: The lack of a significant FA gravity anomaly is important and so this should include a reference to a figure and\nbe quantified.\nLine 68: ‘Deep’ is very vague: ‘at these depths'?.\nLine 74: ‘Accelerated glacial erosion following climate change supported by isostasy’ is confusing.\nLine 76-78: Could be reworded for clarity.\nLines 96-98: It is not really ‘low-density eclogite’ or necessarily at ‘low metamorphic-grade’ but the proportion of eclogite\npresent.\nLine 145: Is ‘today’ necessary?\nLine 180: ‘observed topography and topography’\nLines 202-203: repetition of unusual\nLines 207-208: SPT/STP. What effect could temperature variations related to e.g. lithosphere thickness play?\nLine 221: This misrepresents Gabrielsen et al. They do not say ‘probably more than 8km’, rather ‘more that 5000m, but even\nelevations close to the maximum continental crust could sustain (8000-9000km) have been proposed.'\nLine 222: exhumation rather than uplift?\nLines 227-228: ‘complicates the tectonic scenario’ is a little vague. If there has been a fair amount of eclogite formation in the\nlower crust isn’t serpentinization in the upper mantle also possible?\nLines 297: What is the other 65%? I think you need some more discussion of lower crust vs mantle and what is meant by\nMoho here.\nLine 311: Subdued?\nLine 319-320: This is only supported by the velocities, and the density is assumed?\nLine 321-322: It seems possible that the variation is related to variations in the availability of fluid rather than necessarily\n‘metamorphic degree’ in the sense of PT conditions.\nLine 349: ‘could be underestimated globally’", "reviewer_2": "The authors present a new cross-section through the crust and upper mantle of the Scandes mountain range and the\nwestern part of the Proterozoic shield in Scandinavia. In its western part it crosses the northern “dome” of the Scandes\nmountains, which shows exceptionally high topography. The cross-section consists of a P-wave and density model, which\nwas derived from deep-seismic sounding data of the “Silver-Road” profile, Bouguer gravity data of the EGM2008 gravity\nmodel, and topographic height data.\nBased on seismic travel-time interpretation and ray-tracing and on gravimetric and topographic modelling the authors arrive\nat the following conclusion:\na. The Moho appears to be flat all along the continental part of the profile. Especially, the Scandes mountain range does not\nshow a crustal root.\nb. An exceptionally low Pn-wave velocity of 7.6 km/s occurs in the uppermost mantle beneath the Moho underneath the\nScandes.\nc. For isostatic compensation and support of the topography, up to 60 or even 120 km deep reaching density anomalies\nneed to be assumed, which are explained through varying degrees of serpentinization and eclogitization.\nThis would be clearly a very important result in the research of lithospheric and tectonic processes. However, when trying to\nunderstand the steps of data interpretation, I discovered some methodological deficits or even errors. I therefore suspect that\nthe results are not reliable. The critical points are the following:\n1. Conclusions “a” (flatness of the Moho) and “b” (Pn-wave velocity of 7.6 km/s) are not comprehensible from the descriptions\nand figures of the paper and the supplements. Moho dip and Pn-wave velocity can only be determined reliably if the\nconsidered interface section is sampled by refraction arrivals from “reverse shots” (or equivalent overlapping branches), in\nthis case arrivals shots from both W and E ends of the profile. However, the ray-tracing diagrams in the supplement show that this is not the case. Pn arrivals from shots in the W are observed only at surface coordinates >150 km, sampling Moho\ndip and Pn only in the E part of the profile. Conversely, the W part of the profile is sampled only through shots from the E.\nTherefore, neither Pn-velocity nor Moho dip can be determined accurately. Indeed, in the absence of reverse observation, an\napparent velocity of 7.6 km/s could be explained alternatively by a “true” (normal) Pn-velocity of 8.2 km/s in combination with\nabout 8° Moho dip to the west, similar to what is found underneath the southern dome of the Scandes (Moho dipping East\nin the West).\nOne might object that the Moho depth could be determined through PmP reflection, but this phase also does not provide\ncontinuous coverage, and travel time picks are often vague – at least as far as shown in the supplemental figures.\nAnother complication with respect to comprehensibility is that the deep seismic phases, especially Pn, seem to be hardly\nrecognizable in particular in the west of the profile. Zooming into the seismograms I could not verify many of the “key arrivals”\nand/or their picks. This may be because of the quality of the figure, lack of filtering or whatever, but it clearly questions the\nresults as such.\n2. The reliability of a density model derived from gravity modelling or inversion depends critically on constraints from other\ninformation sources because of the well-known methodical ambiguity. One of the constraints, the authors applied, is a\n“standard” relation relating the density and P-wave velocity of crustal rocks. It is used for computing the gravity effect of the\ncrust from the seismic model. One issue is that no formula is provided for this model, neither in the paper nor in the cited\nreference. More serious is that any such relation should not be applied without considering its uncertainty bounds. Data\ncollections usually scatter around the regression curves in a tube of ±0.1 g/cm^3 and ±0.2 km/s width, respectively. This\nneeds to be considered for assessing the uncertainty of the derived mantle density values. Since this aspect has been\nignored in the paper it seems to be quite likely that parts to the true crustal density variation was projected into the mantle.\nThis view is supported by that the residuals show a long-wavelength systematic that seems to correlate visibly with the\ngeometry of the crustal model. An uncertainty assessment can be easily performed using a random number approaches,\nwhich has become a widely accepted standard.\n3. The lack of a serious uncertainty assessment concerns also teleseismic constraints used for selecting the compensation\ndepths. These are assumed as given values adopted from receiver function studies. But RF have wave lengths of tens of km\norder of magnitude, and so is the depth uncertainty.\nI identified some more minor items, but I won’t list them for now.\nActually, I like the general idea of the article of combining seismic, gravity and topography data for improving the knowledge\non the upper mantle and its processes. But the lack in transparency and comprehensibility of the seismic model generation,\ndeficits in seismic coverage and seismic data presentation (or perhaps even quality), and lacking uncertainty assessment\nseriously question the reliability of the results, which would be very attractive otherwise. But with the deficits given, I cannot\nrecommend accepting the article. The study as such seems to require a fundamental reworking."}, "author_response": "Response to reviews:\nReviewer #1 (Remarks to the Author):\nThe paper presents seismic and gravity modelling evidence for the nature of the upper lithosphere\nbeneath the Scandes mountain range and argues that the topography is controlled by variations in\ndensity associated with variable amounts of eclogite at particular levels. The role of metamorphism\nin the support of topography is an interesting active area of research, and in my view this is an\nvaluable contribution to this field. I enjoyed reading it and I am confident it will be publishable. I\nhave two concerns in its present form. The first is presentational but requires a careful review of the\nway critical concepts are introduced, and the second relates to the justification of assumptions of\nthe gravity modelling.\nThank you for this succinct comment to the manuscript and for the following specific proposals for\nrevision, which certainly have helped us improve the manuscript. Further comments will follow the\nspecific recommendations.\nThe subject inevitably blurs some boundaries in terminology which are used casually in places. For\nexample, the eclogite containing bodies are interpreted to be of lower crustal origin but are below\nthe Moho discontinuity interpreted in the seismic data. This is difficult to explain, and the\nmanuscript only really addresses this in the discussion. As a result, for much of the paper it is unclear\nwhether the authors are discussing mantle eclogite, lower-crustal eclogite mixed with mantle or a\nlower crustal root rich in eclogite that is below the ‘seismological’ Moho. Clarifying this is not that\neasy, and it is clear the authors are aware of the problem and have tried to address it (e.g. lines 341-\n343), but I do not think they have been entirely successful in this version (or at least it could still be\nmade easier for the reader). I think more precise definitions of ‘Moho’, ‘mantle’, the nature of the\neclogite-rich body etc. in the introduction and throughout are needed. These issues are covered in\nthe discussion but need to be addressed earlier. I understand they may feel that this preempts the\ndiscussion too much, but I think this will greatly help fresh readers. Some other terms (e.g. ‘uplift’)\nare also used loosely and could be clarified. These references occur throughout the manuscript and I\nthink reconsidering how these terms are used and how to lead the reader through the concepts\nwould be beneficial.\nWe were indeed aware of this naming problem in the initial submission. Following the\nrecommendation of the reviewer we have introduced definitions of seismic and petrological Moho\nas well as eclogites as metamorphic mafic crustal rocks, and we have clarified that we mean the\nseismological Moho in the abstract.\nThe gravity modelling is non-unique and so the choices of depth ranges where variation is allowed\nwill make major differences to the results. The seismic data presented here allows them to constrain\nthe upper 40 km as well as the layer immediately below this. This layer is assumed to be 20 km thick\nbeneath both the Scandes and the shield on the basis that ‘a recent S-wave receiver function profile\nincludes a strong positive velocity anomaly below the seismic Moho’ at 60 km. It is not clear where\nthis profile is from, the reference is to a conference abstract so it is not possible to assess it, and it is\nnot clear to me why this layer should be the same thickness in both locations (the shield and the\nScandes). Would it be possible to show the receiver function and plot its location? Is it reasonable to\nassume a layer of constant thickness in both regions? Equally another layer is placed below this to\nexplain the rest of the variation in the gravity data in the interval 60-120 km. This is assumed\nbecause this is the base of a ‘teleseismic low velocity layer’ below the Scandes (incidentally it is the\nmethodology rather than then layer which is tele seismic). It is assumed that there is no variation\nbelow this. These decisions seem somewhat arbitrary (or at least require better documentation).\nThe association between low velocities and high densities beneath the Scandes in the 60-120 km\ndepth range is very surprising, but must also be somewhat dependent on the assumptions made.\nIt seems possible that some small component of dynamic uplift could also produce the gravity signal\nwithout requiring the high densities at this level?\nCertainly gravity modelling is non-unique, and we include substantial analysis of uncertainty in the\nsupplementary material. However, the reviewer is right that the validity of our model depends on\nthe choices of the mantle boundaries at 60 and 120 km depth. These boundaries are chosen from\nindependent seismological constraints as noticed by the reviewer, and the choices are not arbitrary:\n(1) The S-wave receiver function interface at 60 km does extend along our whole profile; A\nmanuscript covering the whole of Fennoscandia is in preparation, and we have extracted the profile\ncoinciding along the SilverRoad profile from the database. We have added this information to the\nmanuscript with reference to a presentation, and here we enclose a copy of both the P- and S-wave\nReceiver Function profiles coinciding with our profile (black boxes in profiles below). This data\ndemonstrates that there cannot be any substantial change in depth to the Moho and the 60 km\ninterface. We cannot reproduce this profile in the paper or in the supplementary material due to the\ninterests of the PhD student who calculated the receiver functions and still did not defend her PhD.\n(2) The selection of 120 km depth for the lower interface in the lithospheric mantle is based on a\npublished tomography study of Fennoscandia (Bulut et al., 2021). The teleseismic model shows a\nclear low-velocity interval below the Scandes down to ca. 120 km depth next to a high-velocity\nmantle in the shield proper further east; below this depth the two sides of our profile (the Scandes\nand the shield proper) show similar velocities. We agree with the reviewer that teleseismic\ntomography has significant uncertainties regarding depth estimates. We have therefore added a\nsensitivity assessment of the importance of the exact depth to the lower boundary of the low-\nvelocity layer to the Supplementary Information (new Suppl. Figs. S14-S15). The exact depth affects\nthe exact density of this layer, but it does not affect the relative differences in the layer densities\nthat are the important measures for our key conclusions.\nWe agree with the reviewer that “The association between low velocities and high densities beneath\nthe Scandes in the 60-120 km depth range is very surprising”. We spent long time on understanding\nthis surprising finding, and we think it is another key result of the paper, other than the conclusion\nregarding topography. Due to this surprising result we delayed the submission until we found that\nwe have a well constrained case that relies primarily on Figure 6, which probably will be considered\nvery important by the community. We are confident about the result, and we agree that it is against\nexpectation and common wisdom, which largely originates from density relations for crustal rocks –\nnot mantle rocks. However, we believe that we provide a plausible explanation, as summarized in\nFigure 6.\nAs pointed out by the reviewer, there could potentially be a small component of dynamic uplift, and\nwe assess this contribution in our isostatic calculations by taking the Free Air Gravity anomaly\nvariation into account.\nWe thank the reviewer for providing these constructive comments to the manuscript. They have\nhelped us to improve the quality and sharpness of our arguments in the manuscript and led us to\nadd additional figures to the supplementary material.\nMinor comments\nLine 29: Seismological Moho?\nGood point. Added.\nLines 47-51: When referring to uplift it Is important to specify what is uplifted relative to what\nbaseline: See England, P. and Molnar, P., 1990. Surface uplift, uplift of rocks, and exhumation of\nrocks. Geology, 18(12), pp.1173-1177.\nAgree. We added uplift of the mountain range compared to the shield and shelf and defined the\nassociated exhumation, such that the meaning should be clear in the rest of the paper.\nLine 53-54: The lack of a significant FA gravity anomaly is important and so this should include a\nreference to a figure and be quantified.\nWe quantify this statement in the later analysis.\nLine 68: ‘Deep’ is very vague: ‘at these depths'?.\nChanged to a more specific formulation.\nLine 74: ‘Accelerated glacial erosion following climate change supported by isostasy’ is confusing.\nChanged to an understandable formulation\nLine 76-78: Could be reworded for clarity.\nReformulated.\nLines 96-98: It is not really ‘low-density eclogite’ or necessarily at ‘low metamorphic-grade’ but the\nproportion of eclogite present.\nGood point. We have reformulated throughout the manuscript, although low metamorphic grade of\nthe eclogitic rock means low proportion.\nLine 145: Is ‘today’ necessary?\nRemoved\nLine 180: ‘observed topography and topography’\nTypo corrected\nLines 202-203: repetition of unusual\nCorrected\nLines 207-208: SPT/STP. What effect could temperature variations related to e.g. lithosphere\nthickness play?\nThe variation in temperature and lithosphere thickness along the profile is very minor (Artemieva,\n2019, Earth Science Reviews, Lithosphere structure in Europe from Thermal Isostasy) and, therefore,\ninsignificant. Text has been updated.\nLine 221: This misrepresents Gabrielsen et al. They do not say ‘probably more than 8km’, rather\n‘more that 5000m, but even elevations close to the maximum continental crust could sustain (8000-\n9000km) have been proposed.'\nGabrielsen et al. discuss possible values of minimum 5000 m and up-to 9000 m and they adopt “a\nfigure of 8000 m” in their calculations, which is the figure that we refer to as a “probable” value.\nOur conclusion will not be significantly affected if the actual height was only 5000 m.\nLine 222: exhumation rather than uplift?\nCorrected\nLine 227-228: ‘complicates the tectonic scenario’ is a little vague. If there has been a fair amount of\neclogite formation in the lower crust isn’t serpentinization in the upper mantle also possible?\nOur argument is primarily that a 150 km long serpentinite body is unlikely because serpentinization\nrequires huge amounts of water, which probably only can be achieved in the immediate vicinity of\nthe subducting slab. Eclogitization requires less water which makes this process more likely. Further,\nthe reference to Schiffer et al. indicates that the subduction was in the opposite direction of what\nwould be needed for substantial serpentinization. We deliberately used a vague formulation about\nthe subduction direction because it may be relatively poorly constrained.\nLines 297: What is the other 65%? I think you need some more discussion of lower crust vs mantle\nand what is meant by Moho here.\nImplemented by the addition to the introduction. Here we added the underlined part of “in\nagreement with the estimated eclogite fraction of ~35% in the lower crustal material, which now is\nlocated below the seismic Moho”\nLine 311: Subdued?\nChanged to “depressed”.\nLine 319-320: This is only supported by the velocities, and the density is assumed?\nIn the model, unit S2 has well defined velocity and density. The density of S2 is assumed to\ncorrespond to a “normal” peridotite” from which the other densities are constrained. We changed\n“parameters” to “velocity”.\nLine 321-322: It seems possible that the variation is related to variations in the availability of fluid\nrather than necessarily ‘metamorphic degree’ in the sense of PT conditions.\nWe agree that variability in eclogite content can be essentially caused by fluid availability during\nmetamorphism. Modified to (underlined): “the interpretation of eclogitic material with various\neclogite content in the lower crustal material in the sub-Moho depth interval down to 60 km”\nLine 349: ‘could be underestimated globally’\nDone\nReviewer #2 (Remarks to the Author):\nThe authors present a new cross-section through the crust and upper mantle of the Scandes\nmountain range and the western part of the Proterozoic shield in Scandinavia. In its western part it\ncrosses the northern “dome” of the Scandes mountains, which shows exceptionally high topography.\nThe cross-section consists of a P-wave and density model, which was derived from deep-seismic\nsounding data of the “Silver-Road” profile, Bouguer gravity data of the EGM2008 gravity model, and\ntopographic height data.\nBased on seismic travel-time interpretation and ray-tracing and on gravimetric and topographic\nmodelling the authors arrive at the following conclusion:\na. The Moho appears to be flat all along the continental part of the profile. Especially, the Scandes\nmountain range does not show a crustal root.\nb. An exceptionally low Pn-wave velocity of 7.6 km/s occurs in the uppermost mantle beneath the\nMoho underneath the Scandes.\nc. For isostatic compensation and support of the topography, up to 60 or even 120 km deep reaching\ndensity anomalies need to be assumed, which are explained through varying degrees of\nserpentinization and eclogitization.\nThis would be clearly a very important result in the research of lithospheric and tectonic processes.\nThank you for this succinct comment to the manuscript and for the following specific comments to\nthe data analysis.\nHowever, when trying to understand the steps of data interpretation, I discovered some\nmethodological deficits or even errors. I therefore suspect that the results are not reliable. The\ncritical points are the following:\nWe do not find any “methodological deficits or even errors in our manuscript”, but rather we find\nthat the reviewer may not be familiar with the methodology applied in our paper or they may have\nmisunderstood certain parts of the description. Due to the uncertainty about the documentation\narising from the reviewer’s comemnts, we have added extra illustration to document the unclear\npoints of our data analysis for the benefit of non-specialist readers.\n1. Conclusions “a” (flatness of the Moho) and “b” (Pn-wave velocity of 7.6 km/s) are not\ncomprehensible from the descriptions and figures of the paper and the supplements. Moho dip and\nPn-wave velocity can only be determined reliably if the considered interface section is sampled by\nrefraction arrivals from “reverse shots” (or equivalent overlapping branches), in this case arrivals\nshots from both W and E ends of the profile. However, the ray-tracing diagrams in the supplement\nshow that this is not the case.\nWe have provided standard documentation for this type of profile, and it appears that the reviewer\nmay have misunderstood certain parts of our documentation. The reviewer’s statement here is\nincorrect. Figure 2 shows that there is reverse coverage by Pn in the distance interval km 50-250 by\nSP1 and SP5. This is further illustrated in Supplementary Figures S2-S8 for all shot points. Further,\nthe Moho depth is constrained by reflections (PmP) throughout the whole interval from km 50 to km\n250 which would enable us to confidently constrain the Pn velocity based on one-way recordings\nonly, even if that were the case. Therefore, the Pn velocity is well constrained in the distance\ninterval km 50-250, and the Moho cannot be dipping. We have added a direct comment on this\naspect in the revised manus to simplify the reading for non-specialists, and we have marked the\ncoverage of the Moho by these two phases in Fig. 2 as well as documented this coverage in a new\nSupplementary Fig. S9.\nPn arrivals from shots in the W are observed only at surface coordinates >150 km, sampling Moho\ndip and Pn only in the E part of the profile.\nThe reviewer may not have studied Supplementary Figure S2 in detail. It shows that the Pn is\nsampled from km 100 by upcoming rays from air gun shots and also covered by Pn from SP1 from\nkm 60, not only for “surface coordinates >150 km”.\nConversely, the W part of the profile is sampled only through shots from the E. Therefore, neither\nPn-velocity nor Moho dip can be determined accurately.\nThe coverage is better than assumed by the reviewer. With the airgun stacks and the W shot points,\nwe constrain the Pn velocity from km 20 to km 250, and with the eastern shots we constrain the Pn\nvelocity from km 250 to km 60. The unresolved parts at the ends of the profile are clearly illustrated\nin Fig. 2, which also shows ray coverage. Further, the Moho is constrained by P P wide-angle\nm\nreflections along the profile from km 0 to 250, such that one-way (non-reversed) Pn-coverage would\nbe sufficient to obtain reliable estimates of the Pn velocity, cf. Supplementary Figures S2-S8, which\npresent further documentation for all land shots and two of the airgun stacks.\nIndeed, in the absence of reverse observation, an apparent velocity of 7.6 km/s could be explained\nalternatively by a “true” (normal) Pn-velocity of 8.2 km/s in combination with about 8° Moho dip to\nthe west, similar to what is found underneath the southern dome of the Scandes (Moho dipping East\nin the West).\nFirst, as explained above, it is a reversed profile as documented in the figures. Second, this\ntheoretical consideration could in principle be right if we only had data from one shot point, but it\ndoes not apply to our data coverage which is far better than one single shot point in one direction as\ndescribed above.\nWe further refer to Supplementary Figure S1, which illustrates the uncertainties for both depths and\nvelocities by a standard resolution test. It shows that all parameters within the ray coverage area are\nwell constrained within uncertainties of ±2 km for Moho depth and ±0.2 km/s for the Pn velocity, i.e.\nthat the Moho is flat within an uncertainty of ±2 km, and the Pn velocity difference of 0.6 km/s\nbetween the two parts of the model also is well constrained. We emphasise that the reviewer’s\ntheoretical calculation assumes only one shot point, which is not the case here.\nWe agree with the reviewer that documentation of the coverage is important. To facilitate the\nunderstanding by non-specialists, we provide a new Supplementary Figure S9 which shows an\nextract of the information already provided in Figure 2 and Suppl. Figures S1-S8 regarding the\nimportant coverage by PmP and Pn. This figure further documents our conclusion that Moho depth\nand Pn velocity are well constrained between km 50 and km 250 in the model. We have also added\nthis information in Fig. 2.\nOne might object that the Moho depth could be determined through PmP reflection, but this phase\nalso does not provide continuous coverage, and travel time picks are often vague – at least as far as\nshown in the supplemental figures.\nAgain, it appears like a misunderstanding by the reviewer who may base their comment on the\nassumption of only one seismic section. The Moho reflection points do not provide fully continuous\ncoverage as is normal for this type of profile, but they do provide a very good coverage along the\nwhole profile when information from all sources are combined, cf. the new Supplementary figure S9\non coverage by PmP and Pn. The data coverage demonstrates that the Moho is flat and there is a Pn\nvelocity difference of ~0.6 km/s between the western and the eastern ends of the profile. The ray\ntracing results demonstrate that the PN originates from just below the Moho as also seen in the\nseismic sections where Pn originates from the critical PmP reflection. The new Supplementary Figure\nS9 clarifies and summarises this information.\nAnother complication with respect to comprehensibility is that the deep seismic phases, especially\nPn, seem to be hardly recognizable in particular in the west of the profile. Zooming into the\nseismograms I could not verify many of the “key arrivals” and/or their picks. This may be because of\nthe quality of the figure, lack of filtering or whatever, but it clearly questions the results as such.\nThe seismic phases have been carefully picked according to standard methodology, correlating\ncontinuous seismic phases, checking reciprocity and the relation between PmP and Pn arrivals and\nusing frequency change in the seismograms. It appears that the relatively low resolution of the\nfigures in the pdf provided to the reviewer may have prevented them from identifying the Pn phase\nfor the SP5 section. It is noisy, but the phase is clear, in particular because we can use the two\nreciprocal arrivals (shown by large circle with a cross in Supplementary Figure S8) for its\nidentification. As the reviewer had problems with identifying the phases, we now provide zoom-ins\nof the relevant phases in Supplementary Figures S7 and S8.\n2. The reliability of a density model derived from gravity modelling or inversion depends critically on\nconstraints from other information sources because of the well-known methodical ambiguity. One of\nthe constraints, the authors applied, is a “standard” relation relating the density and P-wave velocity\nof crustal rocks. It is used for computing the gravity effect of the crust from the seismic model. One\nissue is that no formula is provided for this model, neither in the paper nor in the cited reference.\nMore serious is that any such relation should not be applied without considering its uncertainty\nbounds. Data collections usually scatter around the regression curves in a tube of ±0.1 g/cm^3 and\n±0.2 km/s width, respectively. This needs to be considered for assessing the uncertainty of the\nderived mantle density values. Since this aspect has been ignored in the paper it seems to be quite\nlikely that parts to the true crustal density variation was projected into the mantle. This view is\nsupported by that the residuals show a long-wavelength systematic that seems to correlate visibly\nwith the geometry of the crustal model. An uncertainty assessment can be easily performed using a\nrandom number approaches, which has become a widely accepted standard.\nWe do use a standard relation between velocity and density, which is documented by a figure in\nreference 30 (Herceg et al., 2016); we now have added a reference with the formula used (Brocher,\n2005). Reference 30 provides an in-depth discussion of the uncertainties related to such velocity\nto density conversion.\nWe apply similar methodology for our sensitivity tests as illustrated in Supplementary Figures S10-\nS15 (original Suppl. Figs. S9-S12), exactly in the manner proposed by the reviewer. We do not ignore\nthis aspect, rather on the contrary as eseen in Suppl. Figures S9-S12 of the original submission. It\nshould not be surprising that the residuals show correlation with the model – the gravity and\ntopography curves are calculated from the model. Use of random numbers for testing model\nsensitivity is not a standard approach for testing sensitivity and may represent mainly an academic\nexercise without relation to real data and nature.\n3. The lack of a serious uncertainty assessment concerns also teleseismic constraints used for\nselecting the compensation depths. These are assumed as given values adopted from receiver\nfunction studies. But RF have wave lengths of tens of km order of magnitude, and so is the depth\nuncertainty.\nWe respectfully disagree with the reviewer here. As detailed above, the manuscript includes the\nuncertainty analysis. The depth uncertainty for crustal Receiver Functions (RF) is not “of tens of km\norder of magnitude” – that only may apply to the wavelength of the teleseismic signals in the mantle\n– not the crust as here. The wavelength for a dominating 1Hz signal is around 5 km for a crustal S-\nwave, but the picking uncertainty is much smaller – of the order of 2-3 km at maximum. The\nabsolute uncertainty may be considerable for comparison between different regions due to variation\nin Vp/Vs (Poisson’s ratio), but the relative uncertainty in a local area as here is much smaller unless\nVp/Vs shows strong short-wavelength variation, such as if local occurrences of partial melts are\npresent, which cannot be the case here. Therefore, the relative depth in the S-RF image used for\nidentifying the 60 km deep interface is well constrained within an uncertainty of ca. 3 km.\nThe RF images identifies the Moho at the same depth as in our high-resolution refraction model,\nwhich indicates that the Vp/Vs used for the depth conversion is correct. Therefore, we can assume\nthat the 60 km deep interface also is well constrained within the assumed 2 km uncertainty.\nHowever, we agree that the uncertainty of the depth to the base of the model at 120 km in Figure 5\nis larger than for the 60 km deep interface, because this depth is inferred from a seismic teleseismic\nvelocity model, which has large vertical uncertainty. We refer to our reply to reviewer 1 for this\nuncertainty and the new Supplementary Figures S14-S15 which illustrate the sensitivity of this model\nparameter.\nI identified some more minor items, but I won’t list them for now.\nActually, I like the general idea of the article of combining seismic, gravity and topography data for\nimproving the knowledge on the upper mantle and its processes. But the lack in transparency and\ncomprehensibility of the seismic model generation, deficits in seismic coverage and seismic data\npresentation (or perhaps even quality), and lacking uncertainty assessment seriously question the\nreliability of the results, which would be very attractive otherwise. But with the deficits given, I\ncannot recommend accepting the article. The study as such seems to require a fundamental\nreworking.\nContrary to the reviewer’s conclusion, we do provide uncertainty assessments in the Supplementary\nMaterial (which may not have been noticed by the reviewer), we document reliable data coverage,\nour model is constrained within the uncertainties provided in the manuscript, and the techniques\nhave been applied at state-of-the-art. We refer to our comments above as well as our comments to\nthe other review."}, {"round": 2, "reviewer_comments": {"reviewer_1": "In my previous review my comments principally related to: 1) Issues of clarity around terminology when describing the\nproposed model and 2) Questions around the constraints from seismic data being brought into the gravity modelling.\nThe authors have adequately addressed these.\n1) The subject is complex, and further revisions could perhaps be made, but the addition of clearer definitions in the\nintroduction means that careful readers should be able to follow the arguments more easily. I think that it would be\nreasonable to publish in its current form.\n2) The diagram included in their response helps me to better understand their choices of interface depth for the gravity\nmodelling and, with the additional uncertainty analysis, I think that it is reasonable to present their simplified model to the\ncommunity for further consideration and debate.", "reviewer_2": "The authors made efforts to improve the article and could rule out some of the concerns included in my previous review.\nHowever, there are still unanswered items that question the central claims of the article from my perspective. Therefore, I\ncannot recommend publication at the present state. These are the critical points:\nMoho dip and decreased P-wave velocity beneath the Moho in the western half of the profile:\nThanks for adding the new figure S9, which makes it easier to get an overview of refraction and reflection observations\nconstraining Moho contour and mantle velocity. I checked it with Figures S2 to S8 and found one mistake, which should be\ncorrected: reverse refraction coverage is given only for distances >100 km (not 70 km) (the ray entry points at the Moho from\nthe westernmost shot points do not contribute). Anyway, this shows that the Moho contour is only constrained by PmP\nreflections in the essential westernmost 100 km of the profile. Deficits of the corresponding data analysis to be fixed are:\n- Reflection data for distances 10 to 40 km are missing in the data documentation (readers have to “believe”)\n- Shot gathers AGS105 and S3: modelled PmP travel times locate at the temporal top of an 0.5 to 1 s wide reflection band. In\nmany continental settings Moho is the bottom of a laminated reflection band. If choosing the temporal bottom of this band\none ends up with a about 5 km deeper Moho, which would be compatible with the 7° or so dip I mentioned earlier, and which\nin turn would question the claimed decreased P-wave velocity in the mantle. So, the authors should present an explanation\nfor the reflection band and make it plausible why they picked the start time of the reflection band as PmP rather than its\nmiddle or end time. Or even better present alternative models for the different cases resulting from this uncertainty of the\n“true” PmP travel time.\n- In this context the apparent Pn velocities observed for SP3 and SP5 are of special relevance. Showing the corresponding\nseismograms in Figs. S7 and S8 in enlarged form is principally very helpful, but the thick dots and lines cover the essential\nparts of the seismograms and should be replaced by smaller or thinner ones. The blow-up of the seismograms makes clear that\npicking the arrivals in presence of the given noise level is problematic, not obviously reproducible and therefore\nconnected with high uncertainty. Since an accurate determination of the apparent Pn velocity is essential for the study the\nmanual picking should be replaced or complemented by slant stacking, enabling an uncertainty assessment in addition.\nWith regard to the uncertainty assessment of the density model I requested in the previous review I was probably not clear\nenough as to what belongs to such an assessment. Of course had I seen, and understood, the “Explanatory Note: Gravity\nModel Sensitivity Test”. But, sorry, this is not an uncertainty assessment, but just what it is called in the heading: a sensitivity\ntest with respect to variation of single variables. The assessment needs to consider (at least in my understanding):\n- The uncertainty of the P-wave velocity-density relation applied to compute the effect of the crust. This relation is empirical\nand shows wide error bars as I mentioned earlier. Most adequate would be a Bayesian approach.\n- The uncertainty of the 60 km boundary. The figure of the unpublished receiver functions provided by the authors in\nresponse to review 1 does not show a clear unquestionable indication of a 1st order discontinuity at 60 km depth. The 60 km\nline runs at least 50% through “white” (meaning probably zero amplitude), and the neighboring “half-waves” are up to 10 km\nwide, possibly interference patterns. Interpreting this is clearly a study for itself. But I find it problematic to state a\ndiscontinuity at 60 km, without error bars, based on this unpublished and undiscussed material.\n- The mutual ambiguity, i.e. uncertainty relations in between the involved variables.", "reviewer_3": "This document presents a combined study of wide-angle seismic reflection/refraction, gravity and topography data modeling\nalong the Scandes mountain range to provide a model of the structure of the crust and uppermost mantle. I find this work\nvery challenging and interesting, that could be worth publishing. I also enjoyed reading it as I am not very familiar with the\narea studied and found it very complex, therefore I congratulate the authors for their ambition.\nIn the following I will provide some comments that I in my opinion could improve the manuscript to make it publishable.\nI have identified major issues regarding the seismic data and its modeling.\nRegarding the seismic phases identified, the authors state that they have identified 3 crustal refraction phases: Pg, P1,\nP2 (Line 420), but they have not marked P1 in none of the figures of the supplementary, although they say they have\nobserved P1 in almost all sections (Lines 425-426). Please clarify this point.\nIn addition, with the images provided I cannot properly identify seismic phases at a depth corresponding to the upper crust.\nCould all the model just be explained with Pg, PcP, Pn and PmP (phases that are very clear in the seismic data)?\nIn the model of fig. 2b, there is no significant Vp contrast in the upper crust interface (e.g., 6.2 to 6.4 km/s in the shelf which\nalso have less ray coverage, and 6-6.2 to 6.3 km/s in the Caledonian). Furthermore, the uppermost layer definied in the\nseismic profile is not used for the final Vp and density model (fig. 5d) as it all falls into the category of “typical baltic shield\ncrust”. Therefore, how is that layer defined in the Vp model? Is this interface supported by the data/model? I would\nappreciate to have the seismic images without the picking and with lables indicating the phases modeled.\nI find the “travel time fit of seismic phases” section, more a description of the Vp model itself (fig. 2b). The authors describe\nthe seismic phases in terms of Vp and offset, however, phases identified in the shot records would be better described in\ntime and offset. Please, could you clarify this? I have made some comments on the way it is written in some places in the\nmanuscript but please, check it all over the text.\nOther comments regarding the rest of the manuscript:\n- In the introduction I miss the proper objective and aim of the whole study. It needs to be clearly defined here.\n- The geographical names and geological units need to be unified all along the text. I find the terminology used sometimes\nconfusing. For example: “shield”, “shield proper”, “Svecofenian shield”, “Baltic shield”. Sometimes Svecofenian and\nsometimes “craton proper”. Indicate clearly in the figures and in the text where is the shield (or shield proper, please unify)\nlocated. Is it the Caledonian unit? Futhermore, in fig. 1a, check that the color of the Caledonian in the legend is the same\nthan in the map.\n- The CDF is only mentioned in the figures captions but it represents the main boundary between C1+C2 and S1+S2 units.\nWhat geological meaning does it have? Does it really represent a fault down to 120 km depth? Is this a major suture?\nI would like to read about it in the introduction and/or geological context.\n-The TIB could also have some more geological context in the introduction. It has apparently no imprint in the Vp model. An\nigneous intrusion as wide as it is represented in fig. 1b (labled “TIB zone”), would not have yielded to a significant\nmodification of the Vp in the model? Or would that mean that the data presented by the authors allow to interpret that the\nTIB zone towards the N gets thinner?\nFor instance, large granite intrusions in Proterozoic-Paleozoic basement in the Iberian Massif did show a significant lateral\nvariation of the Vp distribution of the upper crust (DeFelipe et al., 2022, doi 10.1029/2021TC007143). I am aware that the\ngeological context is very different, and that is why I miss more geological background of the Scandes.\nFigures:\n- Fig 1b: I would rather see a more detailed geological map, at least indicating the faults that separate the different domains.\nIt will help readers that are not familiar with the Scandes (see my comments above related to the CDF and the TIB).\n- Fig 2: TIB, can it be coloured according to map in fig. 1b?\n- Fig 3i: “Topography versus Moho plot in Scandes” add Moho Depth.\nIn the legend, what unit does the authors refer to with “Proterozoic”?\nSome remarks regarding the terminology used:\n- I would rather use a consistent system. E.g., always Scandes instead of sometimes Fenonscandia… Sometimes “shield”\nand sometimes “shield proper”, unifiy please.\n- Consider substitute “onshore” por “on land”. Mind expressions such as “onshore topography”.\nMinor comments are indicated along the PDF.\nLast, I liked the general idea of the manuscript, as it is a clear example of multidisciplinarity in a complex geological setting. I\nam confident that the manuscript can be published after some modifications."}, "author_response": "Response to reviews\nReviewer #1 (Remarks to the Author):\nIn my previous review my comments principally related to: 1) Issues of clarity around terminology\nwhen describing the proposed model and 2) Questions around the constraints from seismic data\nbeing brought into the gravity modelling.\nThe authors have adequately addressed these.\n1) The subject is complex, and further revisions could perhaps be made, but the addition of clearer\ndefinitions in the introduction means that careful readers should be able to follow the arguments\nmore easily. I think that it would be reasonable to publish in its current form.\n2) The diagram included in their response helps me to better understand their choices of interface\ndepth for the gravity modelling and, with the additional uncertainty analysis, I think that it is\nreasonable to present their simplified model to the community for further consideration and debate.\nThank you for these encouraging remarks.\nReviewer #2 (Remarks to the Author):\nThe authors made efforts to improve the article and could rule out some of the concerns included in\nmy previous review. However, there are still unanswered items that question the central claims of\nthe article from my perspective. Therefore, I cannot recommend publication at the present state.\nThese are the critical points:\nMoho dip and decreased P-wave velocity beneath the Moho in the western half of the profile:\nThanks for adding the new figure S9, which makes it easier to get an overview of refraction and\nreflection observations constraining Moho contour and mantle velocity. I checked it with Figures S2\nto S8 and found one mistake, which should be corrected: reverse refraction coverage is given only\nfor distances >100 km (not 70 km) (the ray entry points at the Moho from the westernmost shot\npoints do not contribute). Anyway, this shows that the Moho contour is only constrained by PmP\nreflections in the essential westernmost 100 km of the profile.\nThe comment from the reviewer is incorrect. It is correct that the Pn rays from the airgun shots in\nthe west cross the Moho upward to the stations at distances km >100, but the incident rays cross\nthe Moho from around km 0, and the Pn from these shots travels in the sub-Moho part between km\n0 and km 250. Therefore, the velocity is constrained in the eastern direction by these rays from km 0\nto km 250. The arrivals would be much too early if the velocity would be higher than in the model\nbetween km 0 and km 100 as suggested by the reviewer, cf. illustrated new ray tracing calculations\n(Fig. A at the end of this rebut). It is not possible to move the Moho deeper because it is constrained\nby the PmP along the whole profile. Therefore, the model does have coverage for the Pn velocity\nfrom km 0 to km 250 (although reversed only from km 65) as we illustrated it in Fig. S9, now\nSupplementary Fig. 10. In any case the Pn velocity is constrained with reverse coverage from km 65\nto km 250, where the western part shows a well-constrained Pn velocity of 7.6 km/s from km 65 to\nca. km 130, from where the velocity increases to 8.2 km/s at around km 200 which continues as\nconstrained by reverse coverage to km 250\nDeficits of the corresponding data analysis to be fixed are:\n- Reflection data for distances 10 to 40 km are missing in the data documentation (readers have to\n“believe”)\nThank you. The reviewer is right that we did not illustrate this part. We have now added yet another\nnew Supplementary Figure 2 with an example of an airgun stack showing the requested reflection.\n- Shot gathers AGS105 and S3: modelled PmP travel times locate at the temporal top of an 0.5 to 1 s\nwide reflection band. In many continental settings Moho is the bottom of a laminated reflection\nband. If choosing the temporal bottom of this band one ends up with a about 5 km deeper Moho,\nwhich would be compatible with the 7° or so dip I mentioned earlier, and which in turn would\nquestion the claimed decreased P-wave velocity in the mantle. So, the authors should present an\nexplanation for the reflection band and make it plausible why they picked the start time of the\nreflection band as PmP rather than its middle or end time. Or even better present alternative models\nfor the different cases resulting from this uncertainty of the “true” PmP travel time.\nFirst of all, the AGS05 and SP3 ringing PmP arrivals, referred to by the reviewer, originate from km\n60-80 and km 210-230, respectively, i.e. at very different locations along the profile, not from the\nsame location as supposed by the reviewer. Therefore, the reviewer’s speculation cannot introduce\na dipping Moho, but instead a generally deeper Moho between at least km 80 and km 210. Further,\nmost shot and AGS sections do not show similar reflectivity. It is common to observe variable PmP\nwaveform along controlled source profiles in cratonic areas.\nOne cannot rule out that the reflection band observed in the SP3 section may represent lower\ncrustal reflectivity based on this section alone, because the section is too short to include the Pn\narrivals needed for identification. Therefore this section, viewed in isolation, cannot identify the PmP\nin itself, similar to normal-incidence reflection sections from where the reviewer apparently has this\nidea. However, the PmP from SP3 eastward is reversed by the PmP from SP5 westward, and the\nsection for SP5 includes the Pn wave starting from the critical point of the PmP reflection. As the Pn\narrival time and the critical PmP arrival times match, it is impossible that the Moho can be deeper\nthan modelled. If Moho were deeper, the Pn arrival would not fit the observations. Therefore the\nPmP is at the beginning of the reflective band in the section for SP3. Please also notice that the Pn\nfrom SP5 is further reversed by SP1 and SP2, which both also match their PmP with Pn. These\naspects makes it impossible to fit data with a deeper or dipping Moho. We have calculated the\nrelevant traveltimes for a model as suggested by the reviewer to illustrate that almost all arrivals are\ntoo late for the reviewer’s model (Fig. B at end of this rebut). We have added a sentence to explain\nthis aspect.\n- In this context the apparent Pn velocities observed for SP3 and SP5 are of special relevance.\nShowing the corresponding seismograms in Figs. S7 and S8 in enlarged form is principally very\nhelpful, but the thick dots and lines cover the essential parts of the seismograms and should be\nreplaced by smaller or thinner ones. The blow-up of the seismograms makes clear that picking the\narrivals in presence of the given noise level is problematic, not obviously reproducible and therefore\nconnected with high uncertainty. Since an accurate determination of the apparent Pn velocity is\nessential for the study the manual picking should be replaced or complemented by slant stacking,\nenabling an uncertainty assessment in addition.\nDone: We have reduced the line thickness and size of picking dots. We assume that the reviewer\nrefers to SP4 and SP5, not SP3. The picks are very clear. We suggest that reviewer 3 assesses this\naspect. The proposed slant stacking method will in our opinion not contribute positively to the\npicking, and it is not in common use in wide-angle controlled source seismics as here.\nWith regard to the uncertainty assessment of the density model I requested in the previous review I\nwas probably not clear enough as to what belongs to such an assessment. Of course had I seen, and\nunderstood, the “Explanatory Note: Gravity Model Sensitivity Test”. But, sorry, this is not an\nuncertainty assessment, but just what it is called in the heading: a sensitivity test with respect to\nvariation of single variables. The assessment needs to consider (at least in my understanding):\n- The uncertainty of the P-wave velocity-density relation applied to compute the effect of the crust.\nThis relation is empirical and shows wide error bars as I mentioned earlier. Most adequate would be\na Bayesian approach.\nOne can apply many approaches. We use a well-established methodology that has been applied\nsince the 1980’ies at least and we use a well-established empirical velocity-density relation which\nhas been tested in many papers as referenced in our manuscript (refs. 31,32). Indeed, there is\nuncertainty in the relation and in Suppl. Figs. S11- S16 we are testing the sensitivity of the modelling\nresults to variation of the densities within and beyond the uncertainties found in the literature (cf.\nrefs.) for the velocity-density relation. This test follows a standard procedure applied in several\npapers, also recent papers. It has the advantage that the reader can make their own judgment of the\nsensitivity – and thereby the uncertainty – in the results, whereas more sophisticated automatic\napproaches do not offer such possibility. We do not agree that “a Bayesian approach” will do better.\n- The uncertainty of the 60 km boundary. The figure of the unpublished receiver functions provided\nby the authors in response to review 1 does not show a clear unquestionable indication of a 1st\norder discontinuity at 60 km depth. The 60 km line runs at least 50% through “white” (meaning\nprobably zero amplitude), and the neighboring “half-waves” are up to 10 km wide, possibly\ninterference patterns. Interpreting this is clearly a study for itself. But I find it problematic to state a\ndiscontinuity at 60 km, without error bars, based on this unpublished and undiscussed material.\nWe have now included a statement on the uncertainty of the receiver function depth. Reviewer 1\noriginally questioned the existence of the 60 km deep boundary, and we notice that they now have\naccepted our analysis of the receiver function results. We remind that the 60 km converter is seen in\nthe S-RF only, not in the P-RF; maybe this causes the disagreement?\n- The mutual ambiguity, i.e. uncertainty relations in between the involved variables.\nOur experience is that it is basically impossible to make such assessment. We have provided the\nevidence for the sensitivity to key parameters in the model according to standard procedures. To\nprovide any estimate of the interrelation between the variables would require the use of a Monte\nCarlo inversion algorithm, which nevertheless in any case would provide uncertainties for each\nindividual parameter and not their interrelation. We emphasise that we are using a standard\napproach for such studies.\nIt appears that the reviewer is proposing us to carry out a completely different study than the one\nwe have submitted. We report a model constrained by integration of multiple data types and we\nhave applied standard approaches for the interpretation. We have avoided the use of automatic,\n“black-box type” computer algorithms (which do not provide knowledge but instead provide\nundistinguishable statistics) in order to maintain knowledge of the significance of the various\nparameters used for constraining the model. The reviewer proposes us to carry out a completely\ndifferent computer based stochastic study, which in our opinion does not improve the knowledge of\nthe uncertainties. We therefore maintain the sensitivity analysis, as also reviewer 1 approves.\nReviewer #3 (Remarks to the Author):\nIn my previous review of this manuscript I pointed out mainly the phases interpretation of the wide-\nangle seismic refraction and reflection data, geographical remarks and general geology comments\non the study area, and more clarifications on the purpose and aims of this research.\nAfter a second review of this document and the answers of the authors to my comments, my\nconcerns about the geological background and geographical locations stated at the document as\nwell as those regarding the focus of the research, are solved. The interpretation of the wide-angle\nphases and their description is adquate to me.\nThank you. The reviewer finds that we have adequately responded to her suggestions.\nI would point out minor changes to the manuscript:\n- The sentece added in lines88-90 of the tracked changes manuscript is pasted a bit out of context. I\nwould place it after “The surface geology across Fennoscandia is exposed in the Precambrian part of\nthe Baltic Shield, which generally lacks a sedimentary cover.”\nDone\n- In the figures of the supplementary material 5d, 6d, 7d, 8d and 9d, remove the traveltime path\nwhere the phase is not identified in the seismic image. E.g., fig. 6d from distance 0 to -60km.\nThe reviewer may not have noticed that there actually are rays arriving between -60 and 0 km\ndistance in these figures. We have inserted the following text in the caption for Fig. S5: “Please\nnotice that the gradient in the upper layer is small such that the refracted rays stay at the top of the\nlayer”. We only show traveltimes that correspond to actual traveltime picks in Figs. S5c,S6c, S7c, S8c,\nS9c. These c-sections provide the illustration that the reviewer asks for. In the d-sections we show\nmore information, i.e. all possible arrival times for the model. This demonstrates that no phases\narrive earlier than the first arrival in the sections. If we only showed the arrivals to actual picks, we\ncould not guarantee that no arrival may be too early. This requires showing all possible arrival times\nas we do. Further, including the arrival times in the interval -60 km to 0 km makes it easier to the\nreader to identify the various seismic phases. Therefore, we maintain the figure outline for all shot\npoints.\n- Provide data of elevation of the Scandes range, as in the introduction it is only said that is “high\ntopography”.\nThe introduction already includes: “It is a NNE-SSW striking mountain range in the Caledonian\ndeformed part of western Fennoscandia (Baltic Shield), consisting of two domes with elevation up-to\n2500 m separated by a lower relief section (Fig. 1a)”. We have added that the lower relief section\nhas topography less than 1000 m.\nReviewer #4 (Remarks to the Author):\nThe main result of this contribution according to the authors is a crustal model that results from an\nintegrated interpretation of: control-source wide-angle seismic reflection/refraction data, and\npotential field modeling (modeling of the gravity anomalies). The current topographic variation\nalong the profile alog this model can be explained by physical features within the crust. The\ngeophysical observations are consistent with an increase in the metamorphic eclogitic grade from\n35% below the high-topography Scandes to 70% below the low-topography shield. The contrast\nbetween the low-grade (low density) and the high-grade (high-density -- eclogitic rocks) below the\nrelatively horizontal seismological Moho can account for the topography of the mountain range\nwithout a crustal root.\nThank you for this precise résumé.\nUnder my point of view, the authors successfully answer and discuss the reviewers concerns.\nNevertheless, recent published literature has not been taken into account when presenting the\nproposed model which might help support the authors points (for example: Shiddiqi et al., 2022,\nGeophys. J. Int. https://doi.org/10.1093/gji/ggac086) Further complementary data and models are\nprovided by Mauerberger et al., 2022 Tectonophysics https://doi.org/10.1016/j.tecto.2022.229507\nand Schiffer et al., 2023 JGR, Solid Earthhttps://doi.org/10.1029/2022JB025983. The consistency of\nthese models with the proposed hypothesis should be addressed to further increase the value of this\ncontribution.\nThank you for the comment and the suggestions for additional references. We are aware of these\npapers, but we also find that they do not add new relevant information. The paper by Shidiqqi et al,\n2022 only use a minor subset of the data and is based on older technology than the referenced Bulut\net al., 2023. The paper by Mauerberger et al., 2024 will not add fundamentally new information to\nour paper. The paper by Schiffer et al., 2023 discusses isostasy under very simple assumptions. Given\nthat we are already at the maximum number of references permitted, we have not added these\nreferences.\nRESPONSE TO REVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nThe authors improved some of the figures and applied some text modifications in response to my\ncriticisms. Through this the observational basis of the seismic model is now better to understand\nthan beforehand. Some more explanation in the text regarding the crucial aspects of the seismic\ninterpretation, especially regarding alternative models and why and how they can be ruled out,\nwould still be helpful for the readers.\nThe authors did not improve the uncertainty assessment of the density model, what I see as a major\nshort-coming of the study with regard to the petrological interpretation of the seismic findings. I\nagree with the authors’ opinion that there are different ways to perform such an assessment. But\nthe applied method doesn’t do the job. It considers neither the uncertainty of the applied Vp-density\nrelation nor the multi-fold ambiguity of gravity interpretation in combination with the uncertainty of\nthe Vp-density relation. The fact that the method used was introduced back in the 1980s does not\nmake things any better.\nThe reviewer expresses a difference in opinion between the authors and them. We notice that the\nreviewer does not consider our reference to the paper by Herceg et al., GJI, 2016, which in detail\nanalyses and discusses the uncertainty in velocity-density conversion. The review does not provide\nsuggestions for improvement.\nReviewer #3 (Remarks to the Author):\nIn my previous review of this manuscript I pointed out mainly the phases interpretation of the wide-\nangle seismic refraction and reflection data, geographical remarks and general geology comments\non the study area, and more clarifications on the purpose and aims of this research.\nAfter a second review of this document and the answers of the authors to my comments, my\nconcerns about the geological background and geographical locations stated at the document as\nwell as those regarding the focus of the research, are solved. The interpretation of the wide-angle\nphases and their description is adquate to me.\nThank you. The reviewer finds that we have adequately responded to her suggestions.\nI would point out minor changes to the manuscript:\n- The sentece added in lines88-90 of the tracked changes manuscript is pasted a bit out of context. I\nwould place it after “The surface geology across Fennoscandia is exposed in the Precambrian part of\nthe Baltic Shield, which generally lacks a sedimentary cover.”\nDone\n- In the figures of the supplementary material 5d, 6d, 7d, 8d and 9d, remove the traveltime path\nwhere the phase is not identified in the seismic image. E.g., fig. 6d from distance 0 to -60km.\nThe reviewer may not have noticed that there actually are rays arriving between -60 and 0 km\ndistance in these figures. We have inserted the following text in the caption for Fig. S5: “Please\nnotice that the gradient in the upper layer is small such that the refracted rays stay at the top of the\nlayer”. We only show traveltimes that correspond to actual traveltime picks in Figs. S5c,S6c, S7c, S8c,\nS9c. These c-sections provide the illustration that the reviewer asks for. In the d-sections we show\nmore information, i.e. all possible arrival times for the model. This demonstrates that no phases\narrive earlier than the first arrival in the sections. If we only showed the arrivals to actual picks, we\ncould not guarantee that no arrival may be too early. This requires showing all possible arrival times\nas we do. Further, including the arrival times in the interval -60 km to 0 km makes it easier to the\nreader to identify the various seismic phases. Therefore, we maintain the figure outline for all shot\npoints.\n- Provide data of elevation of the Scandes range, as in the introduction it is only said that is “high\ntopography”.\nThe introduction already includes: “It is a NNE-SSW striking mountain range in the Caledonian\ndeformed part of western Fennoscandia (Baltic Shield), consisting of two domes with elevation up-to\n2500 m separated by a lower relief section (Fig. 1a)”. We have added that the lower relief section\nhas topography less than 1000 m.\nReviewer #4 (Remarks to the Author):\nThe main result of this contribution according to the authors is a crustal model that results from an\nintegrated interpretation of: control-source wide-angle seismic reflection/refraction data, and\npotential field modeling (modeling of the gravity anomalies). The current topographic variation\nalong the profile alog this model can be explained by physical features within the crust. The\ngeophysical observations are consistent with an increase in the metamorphic eclogitic grade from\n35% below the high-topography Scandes to 70% below the low-topography shield. The contrast\nbetween the low-grade (low density) and the high-grade (high-density -- eclogitic rocks) below the\nrelatively horizontal seismological Moho can account for the topography of the mountain range\nwithout a crustal root.\nThank you for this precise résumé.\nUnder my point of view, the authors successfully answer and discuss the reviewers concerns.\nNevertheless, recent published literature has not been taken into account when presenting the\nproposed model which might help support the authors points (for example: Shiddiqi et al., 2022,\nGeophys. J. Int. https://doi.org/10.1093/gji/ggac086) Further complementary data and models are\nprovided by Mauerberger et al., 2022 Tectonophysics https://doi.org/10.1016/j.tecto.2022.229507\nand Schiffer et al., 2023 JGR, Solid Earthhttps://doi.org/10.1029/2022JB025983. The consistency of\nthese models with the proposed hypothesis should be addressed to further increase the value of this\ncontribution.\nThank you for the comment and the suggestions for additional references. We are aware of these\npapers, but we also find that they do not add new relevant information. The paper by Shidiqqi et al,\n2022 only use a minor subset of the data and is based on older technology than the referenced Bulut\net al., 2023. The paper by Mauerberger et al., 2024 will not add fundamentally new information to\nour paper. The paper by Schiffer et al., 2023 discusses isostasy under very simple assumptions. Given\nthat we are already at the maximum number of references permitted, we have not added these\nreferences."}]

Decision letter