Crustal deformation across the southeastern flank of the eastern Himalayan syntaxis from 3D velocity and anisotropic structures

The southeastern flank of the Eastern Himalayan Syntaxis (EHS) hosts both Tibetan Plateau extrusion and Indian Plate subduction and is a key location for understanding the tectonic transition from continental collision to plateau uplift and lateral extrusion. In this study, we used the densest broadband seismic arrays ever to extract dispersion curves for group and phase velocities with periods ranging from 3 to 40 s. The high-resolution 3-D radially and azimuthally anisotropic model obtained from ambient noise tomography provides valuable insights into interpreting crustal deformation. Our results reveal the differences in deformation mechanisms between tectonic plates on both sides of the Jinshajiang Fault-Red River Fault (JSJF-RRF) system. The SichuanYunnan block on the eastern side exhibits positive radial anisotropy in the low-velocity weak layers of the mid-lower crust. Large-scale continuous crustal flow does not exist under the blocking of multiple fractures and high-velocity bodies, but rather localized material flow occurs due to partial melting. On the western side, the Indo-China Block (ICB) and the Yunnan-Myanmar-Thailand Block (YMTB) are distributed in complex depthvariant anisotropy patterns. We consider this to be the combined result of clockwise rotation and extension, followed by asthenospheric upwelling and lateral crustal shortening during oblique eastward subduction of the Indian lithosphere, which further heated the lower crust. Additionally, the extension and deformation of midcrustal melt on southeastern EHS significantly influence earthquake preparation and occurrence.