Lithospheric deformation and corresponding deep geodynamic process of the SE Tibetan Plateau

As a crucial segment of the oblique Indian-Eurasian convergence zone, the southeast (SE) Tibetan Plateau exhibits intricate crustal deformation and frequent seismic activity. The complex lithospheric deformation characteristics and associated dynamic mechanisms have been subjects of intense debate. By integrating geophysical data, active tectonics, and geodetic observations, we analyze the lithospheric deformation features and geodynamic processes in SE Tibetan Plateau. Our analysis reveals that the upper crust in SE Tibetan Plateau undergoes clockwise rotation around the Eastern Himalayan syntaxis, indicating distributed deformation between the Sagaing and Xianshuihe-Xiaojiang faults. In the lower crust, deformation direction significantly differs from striking of surface structures. Approximately bounded by 26 degrees N, the seismic anisotropy observations in the northern part of the study area mainly originates from the crust, while that in the southern part stems from the asthenospheric mantle. Additionally, significant variations in crustal and lithospheric thickness and topography are observed along this boundary. The northern region features a crustal thickness of 60-70 km, lithospheric thickness of 140-180 km, and average elevations exceeding 4000 m, whereas the southern region shows a crustal thickness of about 30 km, lithospheric thickness of 80-100 km, and average elevations decreasing to 2000 m. The lithosphere in SE Tibetan Plateau is mechanically weak, characterized by a thin equivalent elastic thickness. Seismogenic layers are present in both the crust and upper mantle. The existence of two middle-to-lower crustal weak zones suggests potential material flow over geological timescales driven by gravitational (topography) variations. We argue that complex lithospheric deformation in SE Tibetan Plateau results from multiple geodynamic processes. North of approximately 26 degrees N, lithospheric deformation can be attributed to gravitational collapse and induced middle-to-lower crustal flow, and extrusion of upper crustal blocks. South of this boundary, in addition to block extrusion and gravitational collapse, tractions from mantle flow may dominate, possibly influenced by the retreat/rollback of the Burma and Sunda plates or mantle upwelling from the Hainan mantle plume.