Volgo–Uralia is a Neoarchaean easternmost part of the East European craton. Recent seismic studies of the Volgo–Uralian region provided new insights into the crustal structure of this area. In this study, we combine satellite gravity and seismic data in a common workflow to perform a complex study of Volgo–Uralian crustal structure, which is useful for further basin analysis of the area. In this light, a new crustal model of the Volgo–Uralian subcraton is presented from a step-wise approach: (1) inverse gravity modelling followed by (2) 3D forward gravity modelling.
First, inversion of the satellite gravity gradient data was applied to determine the Moho depth for the area. Density contrasts between crust and mantle were varied laterally according to the tectonic units present in the region, and the model is constrained by the available active seismic data.
The Moho discontinuity obtained from the gravity inversion was consequently modified and complemented in order to define a complete 3D crustal model by adding information on the sedimentary cover, upper crust, lower crust, and lithospheric mantle layers in the process of forward gravity modelling, where both seismic and gravity constraints were respected. The obtained model shows crustal thickness variations from 32 to more than 55 km in certain areas. The thinnest crust with a thickness below 40 km is found beneath the Precaspian basin, which is covered by a thick sedimentary layer. The thickest crust is located underneath the Ural Mountains as well as in the centre of the Volgo–Uralian subcraton. In both areas the crustal thickness exceeds 50 km. At the same time, initial forward gravity modelling has shown a gravity misfit of ca. 95 mGal between the measured Bouguer gravity anomaly and the forward calculated gravity field in the central area of the Volgo–Uralian subcraton. This misfit was interpreted and modelled as a high-density lower crust, which possibly represents underplated material.
Our preferred crustal model of the Volgo–Uralian subcraton respects the gravity and seismic constraints and reflects the main geological features of the region with Moho thickening in the cratons and under the Ural Mountains and thinning along the Palaeoproterozoic rifts, Precaspian sedimentary basin, and Pre-Urals foredeep.
Crustal thickness and thicknesses of individual layers of the Earth's crust play a determining role in estimating the thermal field due to the relative abundance of the radioactive heat-producing elements in the crust (Beardsmore and Cull, 2001; Bouman et al., 2015; Hantschel and Kauerauf, 2009). This fact is particularly important in the case of the Volgo–Uralian subcraton as it is located underneath the Volga–Ural oil- and gas-bearing province with several giant oil fields, where the maturity of the organic-rich rocks is considered to be tightly related to the temperature distribution in the crust (Khasanov et al., 2016; Khristoforova et al., 2008). Therefore, having the knowledge of the Volgo–Uralian crustal structure is the first major step into further basin analysis of the area.
Volgo–Uralia is a large easternmost segment of the East European craton (EEC). It has been regarded as a separate subcraton along with Sarmatia and Fennoscandia since the end of the 20th century (Gorbatschev and Bogdanova, 1993). The Volgo–Uralian part of the EEC is mostly embedded in the East European (Russian) platform, and like the rest of the platform, it does not show any significant topographic variations. It represents a flat area with absolute relief heights ranging from 50 to 250 m for most of the territory. Despite the unremarkable topography of Volgo–Uralia, the same does not hold for its lithospheric structure. Different crustal layers of the subcraton show thickness variations in the order of several tens of kilometres (Artemieva, 2007; Artemieva and Thybo, 2013; Mints et al., 2015).
Several recent crustal models which encompass Volgo–Uralia are based for the most part on regional seismic investigations (Artemieva and Thybo, 2013; Mints et al., 2015). Nevertheless, the gravitational field can also be an essential constraint for the Moho depth especially on the areas devoid of seismic data or with moderate seismic coverage (e.g. Aitken et al., 2013; Steffen et al., 2017). Nowadays, due to the advent of satellite gravimetry, it is possible to obtain gravity field maps with uniform coverage for almost any desired territory of the Earth with a resolution sufficient for regional Moho depth investigation (Bouman et al., 2015).
In this paper, we present a novel model of the Volgo–Uralian subcraton's crustal structure based on inverse and forward 3D gravity modelling with seismic constraints. The main objective of the study is to build a regional crustal model of Volgo–Uralia which in turn can serve as a basis for the further geothermal modelling of the area. In this paper, Sect. 2 is devoted to a brief overview of the tectonic setting and history of the region. Section 3 gives an outlook on the methods and datasets that were used in the study. All the used datasets are outlined in Sect. 3.1. Applied gravity inversion methods are discussed in Sect. 3.2, which is followed by Sect. 3.3, where the process of forward gravity modelling is described. The main results of the inverse and forward gravity modelling as well as the final crustal model of Volgo–Uralia and its comparison to other existing models are presented and discussed in Sect. 4.
The present-day tectonic setting of the Volgo–Uralian region has formed throughout the assembly of the EEC. It started with the collision of Volgo–Uralia and Sarmatia at 2.1–2.05 Ga which led to the creation of a megacontinent Volgo–Sarmatia with Volga–Don collisional orogen developed on the junction zone between the two segments (Bogdanova et al., 2016). Later, during Meso- and Neoproterozoic times, the Pachelma aulacogen was formed along the Volgo–Uralia–Sarmatia border, which, in combination with the Precaspian sedimentary basin, now delineates the south-western border of the Volgo–Uralian subcraton (Fig. 1). After several hundred million years, at 1.8 Ga, the collision between Volgo–Sarmatia and Fennoscandia commenced. It ended during the formation of the Rodinia supercontinent at 1.0 Ga. The suture intervening Fennoscandia and Volgo–Sarmatia was the place of Central Russian orogeny growth which then was reworked into Central Russian and Volyn–Orcha rifts. At present, the Central Russia rift system represents the north-western border of the Volgo–Uralian subcraton (Bogdanova et al., 2016). On the east, Volgo–Uralia is separated from the West Siberian basin by the young Late Palaeozoic Uralide orogen and Late Proterozoic Timanide orogen (Artemieva, 2007).
Main tectonic elements of Volgo–Uralian subcraton (redrawn after Bogdanova et al., 2016).
In contrast to Sarmatian and Fennoscandian segments of the EEC, Volgo–Uralia, except for the Taratash complex, is completely covered by Neoproterozoic to Phanerozoic sediments which prevent direct studies of the rocks from the outcrops. Nonetheless, extensive drilling activity due to the high hydrocarbon potential of the region has provided numerous core samples of the basement which are telling the composition and the age of the Volgo–Uralian segment (e.g. Bogdanova et al., 2010).
For the most part, Volgo–Uralia is comprised of Archaean continental crust, which is concentrated in large blocks surrounded by Palaeoproterozoic mobile belts. The two most prominent blocks of Archaean crust are the Meso- to Neoarchaean Tokmovo megablock and Palaeo- to Neoarchaean Middle Volga megablock, which in the literature are often associated with the so-called “ovoid” patterns of geophysical anomalies (Bogdanova et al., 2016; Mints et al., 2010). These blocks are dismembered by Elabuga and Chusovaya deformation belts and correspond to relative crystalline basement highs. The sedimentary thickness of the Archaean part of Volgo–Uralia rarely exceeds 2 km. Local increases in thicknesses of sedimentary cover are observed in Palaeoproterozoic aulacogenic, and graben-like structures and can reach 5–10 km (Shargorodskiy et al., 2004). A regional trend of a considerable increase of sedimentary cover thickness is observed towards the Ural Mountains in the system of Kama–Belsk rifts (Fig. S1 in the Supplement). Especially thick sedimentary sequences are located to the south of the Volgo–Uralian subcraton where it reaches the Precaspian depression. There sediments have accumulated in successions with a thickness of more than 20 km (Artemieva and Thybo, 2013).
Stratigraphically, the oldest sediments that have accumulated on the Volgo–Uralian territory are of the late Proterozoic age. They can be found sporadically in deep aulacogenic structures within the cratonic area or the deepest zones of large depressions like the Precaspian basin (Postnikov, 2002; Muslimov et al., 2007). The most extensive sedimentation started in the Middle Devonian and was present throughout the Carboniferous and Early Permian periods. Mesozoic sequences were developed in the north-western and southern peripheries of Volgo–Uralia. Cenozoic sediments are present only in the southern part of the region (Postnikov, 2002). Active Palaeozoic sedimentation in concert with subsequent vertical tectonic movements during the Alpine orogenic phase led to the formation of large arch-like structures surrounded by various troughs. These structures now shape the geometry of sedimentary successions on the Volgo–Uralian subcraton (Bogdanova, 1986; Muslimov et al., 2007). Some of the most prominent structures are the North and South Tatar arches (Fig. S2 in the Supplement). These are the gently sloping uplifts of the sedimentary sequences which started to form in the Middle-to-Late Devonian (Bogdanova, 1986). During the Palaeozoic time, they were mostly the place for marine carbonate sedimentation which was rarely interrupted by upward tectonic movements with marine regression and terrigenous rocks formation. Both North and South Tatar arches are on the relatively uplifted crystalline basement and are reflected in the topography (Burov et al., 2003). Moreover, the South Tatar arch, despite being similar to its northern counterpart, is a very peculiar structure itself. Not only it is an outstanding petroleum-bearing region that holds the giant Romashkino oil field, but it is also a place of active fluid circulation in-between the sediments and the crystalline crust as indicated by Plotnikova (2008). That there is fluid circulation is supported by several phenomena, one of which is decreased density of oil within the South Tatar arch that could result from outgassing of the crystalline basement (Plotnikova, 2008). Overall, it can be said that sedimentary structures of Volgo–Uralia are linked to the fault block structure of the crystalline basement which is partly inherited from the old Precambrian crustal complexes (Postnikov, 2002).
In terms of the crustal structure, Volgo–Uralia is generally a region of thick and dense crust principally in its Archaean part (Bogdanova et al., 2016). Locally, crustal thickness can reach depths up to 60 km in the centre of the craton. The evidence of such thick crust in Volgo–Uralia is found in the recent seismic survey of Tatarstan, where several crustal roots plunging to depths of more than 55 km were disclosed by the TATSEIS-2003 reflection profile (Artemieva and Thybo, 2013; Trofimov, 2006). Relatively shallow Moho was observed seismically within the Central Russian and Pachelma Palaeoproterozoic rifts representing suture zones between individual segments of the EEC. Another region with documented thin crust is the Precaspian sedimentary basin where the crust is thinning down to 32–36 km (Artemieva, 2007). The recent seismic model EUNAseis suggests that Volgo–Uralia has a thick upper crust (with thickness of more than 30 km in some places) which is associated with the above mentioned crustal roots (Artemieva and Thybo, 2013). Earlier findings reveal the correlation between the thicknesses of crustal layers and the tectonic history of the region. That is to say, there is thickening of the upper crust along the Central Russia Palaeoproterozoic rift system and thickening of the lower crust beneath the Archaean blocks of the subcraton (Artemieva, 2007).
The work was subdivided into two main steps to build a crustal model of
Volgo–Uralia:
Gravity field inversion where a preliminary estimate of the Moho depth
boundary is obtained (see a detailed description in Sect. 3.2). 3D forward gravity modelling where an extensive crustal model of Volgo–Uralia
is built. The model incorporates sedimentary, crustal, lithospheric mantle,
and asthenospheric layers along with the previously obtained Moho interface
(see a detailed description in Sect. 3.3).
Before the inversion, the gravity data was preprocessed by calculating and
subtracting the sedimentary cover effect from the topographically corrected
vertical gravity gradient anomaly. The schematic workflow of the study is
shown in Fig. 2.
Schematic workflow of the study. The initial step is to prepare
the gravity data for the inversion by subtracting the sedimentary cover
effect from the topographically corrected vertical gravity gradient. Then it
can be followed by a subsequent gravity inversion with laterally variable
crust–mantle density contrast (Haas et al., 2020). The inverted Moho depth is
incorporated along with other lithospheric interfaces as well as seismic and
gravity constraints in IGMAS
For successful crustal model construction, four main groups of data were
utilized:
Seismic data used to constrain the Moho during the inverse and forward
gravity modelling; Gravity data used as a main source of information for gravity inversion and
one of the constraints in the forward modelling; Structural data, used for inverse and forward gravity modelling; Petrophysical data, which were implemented in the forward gravity modelling
process.
A summary of the used datasets with their sources is given in Table 1.
Summary of datasets used in the modelling.
Seismic estimations of crustal thickness play a crucial role in gravity modelling as they are the main constraint on the crustal structure. We used seismic data within the studied region from the USGS global seismic catalogue (Chulick et al., 2013), which has information on crustal thickness from the main reflection and refraction surveys performed on the Russian platform mostly during the Soviet period. We also added data from recent regional seismic surveys made at the end of the 20th and beginning of the 21st century on the Volgo–Uralian subcraton which were not originally included in the catalogue. These are TATSEIS-2003 geotraverse (Trofimov, 2006) going through the centre of Volgo–Uralia, and URSEIS-95, ESRU, and UWARS profiles which mark the crustal structure on the eastern border of Volgo–Uralia crossing the Ural Mountains (Brown et al., 2002; Thouvenot et al., 1995; Tryggvason et al., 2001). Moho depth estimations from seismic databases used in the study are shown in Fig. 3.
Framework of the studied region with the seismic constraints on Moho depth. Relief is taken from the ETOPO1 model (Amante and Eakins, 2009). Seismic estimates of depth to Moho are used according to USGS seismic catalogue (Chulick et al., 2013), TATSEIS-2003 (Trofimov, 2006), URSEIS-95 (Puchkov, 2010; Tryggvason et al., 2001), ESRU (Brown et al., 2002; Rybalka et al., 2006), and UWARS profiles (Thouvenot et al., 1995).
In the present workflow, the gravity field is the main source of information used for crustal thickness estimation in the area devoid of seismic constraints. It was shown that GOCE gravity gradients on satellite height are sensitive to interfaces with large density contrasts like Moho (Bouman et al., 2015). That is why we utilized topographically corrected GOCE vertical gravity gradient grids on a satellite height of 225 km altitude in the process of gravity inversion (Bouman et al., 2016). In addition, the same topographically corrected GOCE vertical gravity gradient was utilized as a constraint for forward gravity modelling along with the surface simple Bouguer gravity anomaly from the global gravitational model XGM2019e (Zingerle et al., 2019).
Several complementary structural datasets were used in the modelling. Surface relief and sedimentary cover thickness are necessary to subtract the gravitational effect of sediments from the topographically corrected vertical gravity gradient field and prepare the gravity data for the inversion (Sect. 3.2). For that purpose, we took ETOPO 1 topographic model (Amante and Eakins, 2009) and sedimentary cover structure inferred from the EUNAseis seismic model for Moho and crustal structure in Europe, Greenland, and the North Atlantic region (Artemieva and Thybo, 2013).
Knowing the structure of the Earth's lithosphere can also be useful in the forward gravity modelling process as the lithosphere–asthenosphere boundary (LAB) is an interface with a density contrast that affects the gravity field. Here, we added the LAB boundary calculated from the concept of thermal isostasy by Artemieva (2019). Being an isothermal boundary, it not only serves as additional density contrast but also provides information about the thermal state of the lithospheric mantle.
The main petrophysical parameter which is involved in operations with gravity field is density. The density model used in the study is given in Table 2. Densities of sediments were described by the function of exponential growth of density with depth obtained for the EEC (Artemieva, 2007). Densities of the upper and lower crust were taken based on the seismic estimates of the densities from the CRUST 1.0 model (Laske et al., 2013).
Density model used in the study.
Upper mantle density was calculated taking into account the contribution of
thermal expansion to the density variations in the subcrustal lithosphere
assuming that the average lithospheric mantle temperature is a mean
temperature between the temperature at the Moho and temperature at the LAB:
In this study, we consider that the Archaean upper mantle is depleted in
mafic components which lowers its density (Kaban et al., 2003). We take the
density of the lithospheric mantle of EEC at room conditions of 3340 kg m
As the temperature at the Moho boundary does not contribute to the thermal
expansion of the asthenosphere, we can slightly modify Eq. (1) to get in
situ density of the asthenosphere by taking asthenosphere temperature as
equal to LAB temperature:
Asthenosphere density is equal to 3225 kg m
Gravity field inversion requires initial gravity data to be refined to leave
only the gravity signal of interest. In our case, the desired crustal
interface is the Moho boundary. In order to obtain the signal that is
produced primarily by the Moho undulations, several corrections to the
gravity field must be applied. These necessarily would include correction
for the latitude, free-air correction, and topographic correction. All the
listed corrections are taken into account in the topographically corrected
gravity gradient anomaly. We use topographically corrected vertical gravity
gradient for the region with 2670 kg m
Another important interface with high-density contrast that causes anomalies
on the satellite gravity field of the same wavelength as Moho is the
sediments–upper crust boundary (Steffen et
al., 2017). Volgo–Uralia, despite not having a large variation in sedimentary
thickness in its cratonic part, is neighboured by Pre-Uralian trough and
Precaspian basin where sedimentary successions can locally reach 10–20 km thickness (Artemieva and
Thybo, 2013; Neprochnov et al., 1970). Therefore, it is essential to
subtract the gravity effect of sediments from the topographically corrected
gravity gradient to get the refined gravity gradient signal produced by the
Moho interface:
As the modelled area is considerably large, we utilized tesseroids to account
for the sphericity of the Earth (Uieda et
al., 2016). First, the depth of the sediments–upper crust interface was
calculated on
Lastly, the gravity effect of sediments was calculated using Tesseroids Python package and it was consequently subtracted from the topographically corrected gravity gradient (Fig. 4).
For the gravity field inversion, we followed the novel approach of Haas et al. (2020) which allows laterally variable
crust–mantle density contrasts according to the tectonic regions present in
the area of study. This approach solves the inverse problem with the
Gauss–Newton algorithm uses second-order Tikhonov regularization to ensure
the stability of the solution, and requires two hyperparameters for the
inversion: reference Moho depth
Although one can use any gravitational component for the inversion in the abovementioned algorithm, we stuck to the vertical gravity gradient as it is shown to be more sensitive to the Moho undulations than the other components (Bouman et al., 2016). Here, for the purpose of tectonic regionalization, we take the main crustal provinces of Volgo–Uralia from Bogdanova et al. (2016) which include the Archaean cratonic continental crust and Palaeoproterozoic mobile belts. We also distinguished Uralide orogen in a separate tectonic region because of its relatively young age and distinct crustal composition. Another tectonic region that deserves our attention is the Precaspian sedimentary basin. The sedimentary strata in its central part reach 20 km of total thickness and include layers of Permian salt with thickness reaching ca. 4–5 km (Volozh et al., 2003; Brunet et al., 1999), which is unique for the EEC. We distinguished this region as a fourth tectonic unit used for gravity inversion.
For the density contrasts, we chose a range of 350 to 550 kg m
Gravity inversion was followed by forward gravity modelling which, was
done with IGMAS
At the beginning of the modelling, the study area was laterally extended by
2500 km to minimize edge effects. This has been done by extending the
thicknesses of the modelled layers from the edges of the study area. The
dimensions of the modelled study area are 2672 km E–W We imported seismic, structural, and gravity data in IGMAS We adjusted the structure of gravity inverted Moho boundary where seismic
data exposed different depths and when it led to the enhancement of the
gravity fit or when the seismic data showed consistently different Moho
depths on one of the digitized profiles. We forward calculated gravity and gravity gradient fields from the current
model and observed a significant gravity misfit of ca. 95 mGal in the centre
of the Volgo–Uralian subcraton. This misfit was attributed to the
underplated body with a relatively higher density located in the lower crust
(see Sect. 4.2). We estimated mass imbalance (surplus and deficit) in the area by isostatic
calculations following the approach of Ebbing (2007) for the
Scandinavian mountain chain: The density of the underplated body was set to 3100 kg m The last step was to modify the geometry of the layers to reach a good fit
to the gravity data. Here Moho boundary and upper–lower crust interface were
subjected to further modifications. The upper–lower crust interface was
modified in order to both provide better gravity fit and resemble the
patterns of the bottom of the “felsic-intermediate” crust from the
EUNAseis model. The Moho was modified in areas of no seismic constraints to
enhance the gravity fit.
As a result, a new crustal model of the Volgo–Uralian subcraton was obtained throughout the gravity field inversion and forward gravity modelling.
In the gravity inversion two hyperparameters, the reference depth and the density contrast, were estimated such that the resulting gravity-inverted Moho showed the minimum RMSE with the seismic Moho depth estimates.
The reference depth which gave the best-fitted Moho to the seismic data was
equal to 45 km. Such a relatively deep estimate was obtained due to the fact
that TATSEIS-2003 and URSEIS-95 seismic profiles provided a considerable
fraction of Moho depths' measurements of more than 50 km. In terms of the
density contrast, Archaean cratonic crust and Uralide orogen resulted in a
density contrast of 550 kg m
The obtained gravity-inverted Moho depth map generally respects the main known structural features of the crust in the region: Moho thickens in the cratons and Uralides, and thins along the Palaeoproterozoic rifts, Pre-Urals foredeep, and Precaspian sedimentary basin (Fig. 5b).
The final product of the forward gravity modelling is the IGMAS
Comparison between measured and calculated gravity fields.
Comparison between measured and calculated gravity gradient
fields.
A 3D lithospheric model of Volgo–Uralia developed in IGMAS
Prior to reaching the aforementioned gravity fit, a considerable misfit of measured and calculated gravity revealed at the initial stage of forward modelling was interpreted and modelled as an underplated material (Sect. 3.3). This misfit arose after fitting the inverted Moho depth to the seismic data in the north-western portion of TATSEIS-2003 seismic profile. The depth difference between the seismic and inverted Moho depth is shown in Fig. 9a. Figure 9b shows the difference between the Moho depth obtained in the forward modelling and the inverted Moho depth. From Fig. 9 one can see that Moho is much deeper in the centre of the Volgo–Uralia in the area of interpreted underplating. This feature is not initially seen on the gravity inverted Moho depth map.
The hypothesis of underplating in the area is not new. It was previously suggested by Thybo and Artemieva (2013) and is generally mentioned in the literature (Bogdanova et al., 2016, 2010; Mints et al., 2010). The recovered underplated body appears to be located on the north of the Tokmovo megablock under the Oka block (Fig. 1). This body is defined on a TATSEIS-2003 seismic profile as an acoustically transparent region (Trofimov, 2006). Mints et al. (2010) interpreted this feature as a domain of homogeneous mafic rocks partially metamorphosed into high-density granulites or eclogites at the basis of the so-called Vetluga synform. The isostatic calculations from Eq. (5) also show the high-density body with an average thickness of ca. 10 km which is clearly outlined by the area of isostatic imbalance in the centre of Volgo–Uralia (Fig. 10). Other regions with the major mass deficits are located on the south-east of the map and are related to the Precaspian depression and South Ural orogen. However, they do not correspond to any significant gravity misfit and are produced simply by the high deviation of the sedimentary and crustal thicknesses from the average values on the territory yielding higher values of mass imbalance.
Thickness of the high-density lower crustal layer from the isostatic calculations.
The Moho depth of the developed IGMAS
Moho model of Volgo–Uralian subcraton obtained through the
gravity inversion with laterally variable density contrasts (Haas et al.,
2020) and subsequent forward gravity modelling with seismic and gravity
constraints in IGMAS
Most of the differences between seismic data and the Moho model developed in
IGMAS
Measured and calculated Bouguer gravity anomalies and the
topographically corrected vertical gravity gradient anomalies from the
crustal model
A similar trend can be observed when looking at the north–south intersection of the eastern border of Volgo–Uralia (Fig. 13). The crust is the thickest in the northern cratonic region which is away from the palaeorifts. Whereas under the rift-like structures and in the cratonic area adjacent to them, the crust is thinning down. Particularly thin crust is observed in the south of the section under the Precaspian sedimentary basin.
Measured and calculated Bouguer gravity anomalies and the
topographically corrected vertical gravity gradient anomalies from the
crustal model
The IGMAS
The resulting Moho model developed in IGMAS
Difference in Moho depths between
When comparing our model to EUNAseis and CRUST 1.0, it becomes obvious that the obtained model is relatively deeper on the north-western part of the territory, which corresponds to Fennoscandia. One possible explanation for this feature is that the south-western part of Fennoscandia has relatively sparse coverage with seismic stations. This could have led to the discrepancy of the Moho depth on this zone estimated by gravity and seismic-based methods. As a result, the model developed during this study and GEMMA gravity-based model show 5–10 km deeper Moho for south-western Fennoscandia compared to CRUST 1.0 and EUNAseis.
Another significant difference that is seen between our model and CRUST 1.0 is the thicker crust in the centre of Volgo–Uralia in our model where the underplated body is recovered. Most probably, this difference has been revealed because the most recent seismic investigations on the Russian platform including the TATSEIS profile were not used in the compilation of CRUST 1.0. One can see that the EUNAseis model which has an extensive seismic database for the Russian platform is closer to our model in the centre of Volgo–Uralia where the underplated body is located.
The last conspicuous feature worth mentioning is the shallower Moho of the obtained model on the south-east of Volgo–Uralia as opposed to the EUNAseis model. Such anomaly arises because USGS seismic catalogue and EUNAseis seismic database have been built independently and have certain differences in seismic Moho estimations in this region. Our model respects more the seismic estimates of Moho depth given by the USGS catalogue on the south-east of Volgo–Uralia (Fig. 11) but diverges from EUNAseis Moho estimations showing 3–9 km shallower Moho in the south-eastern part of Volgo–Uralia and south of Ural Mountains.
We presented a new crustal model of the Volgo–Uralian subcraton obtained through gravity inversion and thorough forward gravity modelling with seismic constraints.
The gravity inversion was performed using laterally variable crust–mantle
density contrasts. Three different density contrasts were estimated: 350 kg m
Gravity field inversion was followed by 3D forward gravity modelling
performed in IGMAS
The final crustal model respects all the main geological features of the Volgo–Uralian subcraton and its surroundings with Moho thickening in the cratons and under the Ural Mountains and thinning along the Palaeoproterozoic rifts, Precaspian sedimentary basin, and Pre-Urals foredeep. The obtained crustal model will serve as a basis for further basin analysis and geothermal modelling.
The code of Haas et al. (2020) for the gravity
inversion with laterally variable density contrasts is available at
The supplement related to this article is available online at:
IO and JE designed the study. IO collected and processed the data,
performed gravity field inversion, built the crustal model in IGMAS
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank Susanne Buiter for carefully managing the review process and two anonymous reviewers whose comments allowed us to greatly improve our paper.
This research has been supported by the Deutscher Akademischer Austauschdienst (grant no. 57507870) and the Ministry of Science and Higher Education of the Russian Federation (grant no. 075-15-2020-931) within the framework of the development programme for a world-class Research Centre “Efficient development of the global liquid hydrocarbon reserves”.
This paper was edited by Susanne Buiter and reviewed by two anonymous referees.