Constraints on late cenozoic tectonics in the Southern Longmen Shan: evidence from low-temperature thermochronology

Abstract

ABSTRACT The NE-trending Longmen Shan (LMS) thrust belt, located between the Songpan-Garze terrane and the Yangtze craton, is one of the steepest topographic margins around the Tibetan Plateau. However, the exhumation history and the fault activity of the southern segment of the LMS have remained poorly constrained, particularly in the range front, where little thermochronology ages have been reported. Herein, we report 17 apatites (U-Th)/He (AHe) ages from the southern LMS along the Qingyi River across the Dachuan–Shuangshi Fault (DSF), the Yanjing–Wulong Fault (YWF), and the Gengda–Longdong Fault (GLF). The AHe ages in the hanging-wall of the DSF are ca. 6–11 Ma, which increase to ca.19–23 Ma in the footwall of the DSF. New AHe ages reveal that exhumation rates across the DSF increased significantly from 0.05 to 0.1 km/Myr in the footwall to 0.2–0.4 km/Myr in the hanging-wall since the Late Miocene. The width of the high exhumation rate (0.4–0.6 km/Myr) belt concentrated between the DSF and the GLF, and towards DSF eastward and GLF westward, the exhumation rates decrease rapidly. The results show that continuous crustal shortening along the DSF. Our results support the upper crustal thrusting model that emphasizes crustal shortening deformation along the eastern Tibetan Plateau margin.