Active tectonics of Tibet

Abstract

From an interpretation of Landsat imagery of Tibet the most recent structures appear to be normal faults that trend approximately north‐south. Fault plane solutions of 14 earthquakes in the central part of the Tibetan plateau indicate large components of normal faulting. The solutions are not well constrained, but for the most reliable ones the T axes are oriented approximately east‐west. Only on the margins of the high plateau, where elevations are lower, do fault plane solutions show active thrust faulting. These observations imply an east‐west extension of most of Tibet at the present time. We relate this pattern to the collision of India and Eurasia and to deformation of an especially weak Tibetan crust and upper mantle. India applies a pressure to Eurasia that maintains Tibet at a high uniform altitude, and the hydrostatic head caused by this elevation transmits the pressure northward. Accordingly, Tibet is the pressure gauge of Asia. At the same time a small east‐west strain (or flow) of the lower crust and upper mantle of Tibet would stretch the surface layer. Tibet's position relative to India at the present time is analogous to that of a ‘dead’ triangle in a plastic material indented by a rigid indenter (India).