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.