Rift propagation in south Tibet controlled by under-thrusting of India: a case study of the Tangra Yumco graben (south Tibet)

Active graben systems in south Tibet and the Himalaya are the surface expression of ongoing east-west extension, although the cause and spatiotemporal evolution of normal faulting is a still a matter of debate. We reconstructed the exhumation history driven by normal faulting in the southern Tangra Yumco graben using new thermochronological data. The Miocene cooling history of the footwall of the main graben-bounding fault is constrained by zircon (U-Th)/He ages (16.7 +/- 1.0 to 13.3 +/- 0.6 Ma), apatite fission track ages (15.9 +/- 2.1 to 13.0 +/- 2.1 Ma) and apatite (U-Th)/He ages (7.9 +/- 0.4 to 5.3 +/- 0.3 Ma). Thermo-kinematic modelling of the data indicates that normal faulting began 19.0 +/- 1.1 Ma at a rate of c. 0.2 km myr(-1) and accelerated to c. 0.4 km myr(- 1) at c. 5 Ma. In the northern Tangra Yumco rift, remodelling of published data shows that faulting started c. 5 myr later at 13.9 +/- 0.8 Ma. The age difference and the distance of 130 km between these two sites indicates that rifting and normal faulting propagated northward at an average rate of c. 25 km myr(-1). As this rate is similar to the Miocene convergence rate between India and south Tibet, we argue that the under-thrusting of India beneath Tibet exerted an important control on the propagation of rifts in south Tibet.