Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

摘要

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (similar to 4200-4800m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16Ma, AHe ages from 2 to 6Ma, and one zircon fission track age is similar to 99Ma. These data imply a tectonic quiet period sustained since at least 100Ma with a slow denudation rate of similar to 0.03km/Ma, interrupted at 7 to 5Ma by exhumation at a rate between 0.59 and 0.99km/Ma. We relate that faster exhumation to the onset of motion along the left-lateralormal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an hidden plate-tectonic model rather than with a lower channel flow model.