KINEMATICS OF THRUSTING IN THE HIMALAYA AND ITS RELATION TO TIBET TECTONICS

The India-Asia collision zone is certainly an outstanding place to study continental deformation and mountain building processes in particular. A coherent model of the kinematic of crustal deformation in the Nepal Himalaya has now emerged from various geological, thermochronological and geodetic studies: the Indian crust is being thrust under the Himalayan front at a rate of ~15mm/yr, with about 20% of the crust being scrapped off and transferred to the Himalayan wedge by basal accretion; the eroding Himalaya wedge is thrust over the Main Himalayan Thrust fault at a rate of ~5mm/yr with little internal deformation. This has resulted in about 20mm/yr of shortening across the range. The current structure of the range and its exhumation history require that kinematics has gone on for at least the last 10 Myr. As a result a significant volume of Indian crust has been thrust beneath the plateau along a subhorizontal shear zone with presumably high temperature, as suggested by thermokinematic models, and hence potentially low viscosity. We discuss the effect of this low viscosity zone on Himalayan tectonics and south Tibet tectonics and the potential for channel flow extrusion. Based on simple mechanical modeling and comparison with surface strain within Tibet, we show that the Indian crust keeps its mechanical integrity as it is underthrust along a sub-horizontal zone of simple shear beneath southern Tibet.