Geodynamics of the southeastern Tibetan Plateau from seismic anisotropy and geodesy

Abstract

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.