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Crustal structure of the Volgo–Uralian subcraton revealed by inverse and forward gravity modelling

Abstract. 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.
