Seismic anisotropy across the Kunlun fault and their implications for northward transforming lithospheric deformation in northeastern Tibet

Abstract

The northeastern Tibet is produced by the far field effects of the India–Eurasia collision in Late Cenozoic. As the southern boundary of the northeastern Tibet the Kunlun fault is a key to understand how the Tibetan Plateau transforms lithospheric deformation outward. Using a northeast–southwest trending seismic array deployed in Songpan–Ganzi, Qaidam–Kunlun, and Qilian blocks, we measured SKS/SKKS wave splitting parameters at 15 broadband seismic stations to study the variation in anisotropy across the Kunlun fault and among these blocks. The average splitting parameters of the southern segment in Songpan–Ganzi block are N132.3°E and 1.1s respectively, and those of the northern segment in Qilian block are N103.8°E and 0.98s, interfered with the middle segment in Qaidam–Kunlun block with a smaller average delay time 0.63s. A schematic geodynamic model is used to explain the distinct features among these blocks. Weak anisotropy suggests the strong lithosphere of Qaidam–Kunlun block with negligible deformation. A counterclockwise rotation of the fast polarization direction of about 30°from Songpan–Ganzi block to Qilian block is produced by a ladder-shaped strong Qaidam–Kunlun block.