Mid-Miocene (17–11 Ma) fast exhumation in the upper Jinsha River and its implication for regional faulting in Southeast Tibet

Abstract

The rugged southeast (SE) Tibet hosts a network of evolving faults, many of which are poorly understood due to a lack of sedimentary records and poor accessibility. The Jinsha River fault zone (JRFZ) is vaguely considered as a major fault bounding the Chuan-Dian Block in the northwest, yet little is known about its tectonic evolution history, which hinders our clear understanding of the tectonic deformation pattern, process and geomorphic response of SE Tibet. Here, we collected 10 samples from the hanging wall of the Xiongsong reverse fault, one strand of the JRFZ, near Markam area and on the trunk of the upper Jinsha River, to reveal its exhumation and kinematic history through low-temperature thermochronology. The ZHe ages (~17–13 Ma) of the samples from the hanging wall are much younger than the published ones in the footwall, but the AFT (~11–9 Ma) and AHe ages (~14–6 Ma) are only slightly younger than those in the footwall. The QTQt and 3-D thermo-kinematic Pecube modeling suggest consistently that the Xiongsong fault experienced strong activity during the mid-Miocene (~17–11 Ma) and then weakened. Post-11 Ma decreases in exhumation here on the trunk of the Jinsha river, if interpreted as river incision, is opposite to previously inferred river response to late Miocene surface uplift driven by the lower crust flow. Significant differences in thermochronological data and modeling results on both sides of the Jinsha River indicate the importance of local faulting in the development of large rivers. In addition, late Miocene fault activity shifts have also been observed on other major structures in SE Tibet, such as the left-lateral faulting of the Xianshuihe-Xiaojiang fault, the enhanced thrusting of the Longmen Shan-Yalong thrust belt and the kinematic reversal of the Ailao Shan-Red River shear zone. We infer that the near synchronized change in regional fault activity may be caused by the clockwise rotation of SE Tibet, while its being extruded southeastward from the pushing front of the Indian collision.