CRUST AND MANTLE DECOUPLING IN SE TIBET

The Eastern Himalayan syntaxis is a superb area for studying relations between mantle and crustal deformation because of the large scale and rapid changes that have occurred to accommodate the Cenozoic indentation of India into Eurasia. Structural evolution of the upper crust indicates early to middle Cenozoic extrusion, deformation and complex clockwise rotation of crustal fragments around the syntaxis followed by similar, but spatially superposed late Cenozoic- Recent deformation. Our geological, geodetic and geodynamic work SE Tibet indicate that late Cenozoic to Recent flow of lower crust to the E and SE is driven largely by gravitational potential energy and is responsible for crustal thickening and the modern topography of eastern Tibet and its foreland. However, crustal flow is partially to wholly decoupled from the uppermost crust that undergoes clockwise rotation around the syntaxis and the upper crustal system may be moving toward the Pacific driven by processes in the mantle that are as yet unclear. Results from our broadband seismic study in SE Tibet indicate: 1. Presence of a low velocity zone within the crust, consistant with lower crustal flow. 2. Velocity differences in the upper mantle that have no correlation to upper crustal structure or surface topography. 3. A complex pattern of shear wave splitting generally unrelated to upper crustal structure. Modeling of shear wave splitting data shows anisotropy of deformational fabrics at two different levels of the mantle and crust, with a pattern consistant with our geodynamic interpretation. These geophysical data support decoupling within the lithosphere and between the lithosphere and deeper mantle