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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. [ABSTRACT FROM AUTHOR]

The incision history of the Three Rivers (Salween, Mekong, and Yangtze) region in the Southeast Tibetan Plateau has been linked to both tectonic and climatic controls. In this study, we report new apatite (U‐Th)/He and fission‐track thermochronology data from the >6,000‐m‐high Kawagebo massif, which forms the edge of the high plateau on the western flank of the steepened knickzone reach of the middle Mekong River valley. Thermal‐history modeling of a thermochronological age‐elevation profile shows rapid cooling since ~1.5 Ma and suggests a mean Quaternary exhumation rate of >1 km/Myr at the valley bottom. The amount of Quaternary exhumation is too high to be caused by fluvial incision alone and requires additional tectonic uplift. Comparing our data from the western flank of the Mekong River valley with published data from the eastern flank shows differential exhumation across the valley in the late Miocene, with the western flank undergoing more exhumation, but relatively uniform exhumation in the Quaternary. We relate rapid exhumation since the late Miocene on the western flank of the Mekong valley and the high topography of the Kawagebo massif to localized tectonic uplift associated with a restraining (left stepping) overstep between the still‐active right‐lateral Parlung and Zhongdian strike‐slip faults. The pattern of river steepness index across the knickzone also indicates that it results from locally focused uplift. Our results demonstrate the importance of detailed thermochronologic studies in this very active region to constrain the complex multiphase tectonic history before invoking any potential climatic forcing of river incision. Key Points: New low‐temperature thermochronology data show rapid Quaternary exhumation in the Mekong River knickzone, Southeast TibetBoth the amount of Quaternary exhumation and the pattern of river steepness point to a tectonic origin of the knickzoneThe exhumation history and topography around the knickzone are related to localized tectonic uplift associated with a restraining overstep between the Zhongdian and Parlung faults [ABSTRACT FROM AUTHOR]