Mass transfer between eastern Tien Shan and adjacent basins (central Asia): constraints on regional tectonics and topography

Abstract

SUMMARY Using depths and ages derived from isopachs, drill-holes or cross-sections, it is possible to reconstruct the space-time depositional history of a sedimentary basin. Drill holes and cross-sections give the local sedimentation history, while isopachs allow the definition of the spatial distribution of the sediments. Assuming several simple hypotheses, such as similarity of the strata and regional applicability of results derived from local analyses, one can reconstruct balanced maps of the solid (or grain) volumes, and hence the mass of sediments deposited during several time intervals since the Palaeogene. Applying this method to the Tarim and Dzungar basins (NW China), we estimate the total Cenozoic solid-phase volume and mass of sediments stored to be 1358 f 520 x lo3 km3 (36.7 f 14 x 1017 kg) and 172 f 56 x lo3 km3 (4.6 f 1.5 x 1017 kg) respectively. The reconstruction also enables us to detect two main pulses in the sedimentation. The first, around 17 Ma, affected only the northern part of the Tarim Basin (also known as the Kucha or Kuche Depression) at the foot of the Tien Shan Mountains and supports the idea that the presently active shortening regime in that range started at that time. The second, 5 to 6 Ma, affected most of the depositional areas of the region and may have an even greater geographical extent. Assuming local isostasy, we estimate the volume of shortening induced by the rotation of the Tarim block relative to Siberia and stored in the range and adjacent basins to be between 1.15 x lo6 and 4.23 x lo6 km3. This correspbnds to a clockwise rotation of between 2.5 and 8.7. We use these results in two simple models of self-similar growth of pyramidal topographies that approximately fit the eastern Tien Shan.