Crustal structure of the Tian Shan Orogen and its adjacent areas inferred from EIGEN-6C4 gravity field data

Abstract

The study of crustal structure plays a crucial role in understanding the tectonic development and seismicity of orogens. In this contribution, we analyzed the gravitational responses caused by the subsurface structure of the Tian Shan orogen using global EIGEN-6C4 gravity field data. The quality of the EIGEN-6C4 gravity data in the eastern Tian Shan area was evaluated, and we employed the wavelet multi-scale analysis method in combination with an existing P-wave velocity model to reveal the structures at different scales. The gravity Moho of the study area that best fits the seismological Moho was obtained from the filtered third-order wavelet approach component after several tests using different reference depths and density contrasts. The results reveal significant lateral discontinuities in the lower crust and upper mantle structures across the eastern and western Tian Shan orogens. The Moho beneath the western Tian Shan is about 9 km deeper compared to the eastern Tian Shan. Towards the Tarim and Junggar basins, the Moho decreases to about 45 km in depth. The analysis of the isostatic anomaly suggests that the eastern and western Tian Shan possess varying capabilities for isostatic adjustment. Furthermore, it indicates a gradual weakening of mantle upwelling in the Tian Shan orogen from west to east, implying that different stages of mantle dynamics may have occurred in the western and eastern Tian Shan. The north Tarim fault, Nikolaev-North Nalati fault, and Kazakh fault serve as channels that control magma upwelling in the Tian Shan orogen. The distribution of earthquakes differs between the eastern and western Tian Shan, with the western Tian Shan exhibiting greater gravitational potential energy. These variations between the eastern and western Tian Shan may be attributed to the clockwise rotation of the Tarim craton.