Termination of Mid-to-Lower Crustal Extrusion on the Eastern Flank of the Eastern Himalayan Syntaxis: Implied From Trans-Regional Ambient Noise Tomography

The Eastern Himalayan Syntaxis (EHS) serves as a natural laboratory for the study of intense continental collision and lateral extrusion tectonics. By aiming at the intricate tectonic dynamics south and southeast of the EHS, we integrate seismic data from new broadband stations in central Myanmar with permanent stations in southeastern Tibet to establish a high-resolution 3-D shear wave velocity model through ambient noise surface wave tomography. Our imaging results reveal distinct differences in crustal seismic velocity structures between the West Burma Block, Chuan-Dian Block, and the Shan Plateau, highlighting the extent of oblique subduction and restricted crustal extrusion. Notably, two north-south oriented low-velocity zones in the mid-to-lower crust of southeastern Tibet are mainly confined within the Chuan-Dian Block and terminate near the Red River Fault, with limited extension into the Shan Plateau. The Eastern Himalayan Syntaxis (EHS) marks the point where the Indian and Asian plates collide most intensely in the eastern Himalayas. Geoscientists have observed significant mountain-building activities around the EHS extending into southeastern Tibet (SE Tibet), driven by the intense collision of these continental plates. In SE Tibet, materials are squeezed sideways, a process known as lateral extrusion due to the collision of these plates. This is evident from seismic studies showing slower wave speeds in two distinct regions of the middle to lower crust. We analyzed seismic data from new stations in central Myanmar and permanent stations in SE Tibet to better understand the zone transitioning from intracontinental collision to lateral extrusion. Using a technique called ambient noise tomography, we developed a high-resolution 3-D model of the crust and uppermost mantle. Our findings reveal significant differences in the crust's structure between areas of subduction, extrusion, and transition, enhancing our understanding of the geodynamic processes beneath the EHS and its surrounding areas. Particularly, two slow-speed belts related to the lateral extrusion are confined within SE Tibet, supporting a model of limited crustal extrusion. We obtain an integrated 3-D Versus model by ambient noise tomography based on the data from both central Myanmar and southeastern Tibet Distinct crustal structures in the West Burma Block, southeastern Tibet, and the Shan Plateau coincide with distinct tectonic responses Two north-south low-velocity zones in mid-to-lower crust end near Red River Fault, indicating restricted crustal extrusion