NEW INSIGHTS INTO THE BASEMENT STRUCTURE OF THE WEST

Abstract

The oil- and gas-rich West Siberian Basin is underlain by a layer of flood basalts of late Permian-Triassic age that are coeval with the Siberian traps. The extent and thickness of the basalts is unknown, but knowing their thickness is important for discussions on the end- Permian mass extinction because basalt volume constrains estimates of emitted volatiles. We have used GRACE satellite and terrestrial gravity data to study the structure of the crust and basalt distribution. Published seismic sections are used to constrain the sediment isopachs and to estimate a depth-density function. We use published models of crustal thickness and basement depth to reduce the observed gravity field to the basement level. The resulting 3D density model gives information on density anomalies in the lower crust and upper mantle and on the basalt thickness. We identify several rift-graben structures which are presumably filled with basalt. The lower crust below the West Siberian Basin shows considerable density variations and these variations allow the region to be divided into four major blocks. The eastern part of the basin, towards the Siberian platform, shows an arch-shaped density increase in the lower crust that is accompanied by a linear high-density anomaly at shallower depths. Our work demonstrates the way in which the GRACE-gravity field can be applied to map geological structures like buried rifts and large basins. The same techniques can be used for other large, remote basins such as those in cratonic South America.