It transitions northward to the low-relief areas of the Tibetan Plateau and southwards into the lower-elevation reaches of Yunnan ( Liu-Zeng et al., 2008). The landscape on the southeastern margin of Tibet is characterized by narrow and deeply incised fluvial valleys ( Fielding et al., 1994) with sharp bends close to faults. Our modeling also supports the idea that the exhumation pattern during the Cenozoic in the southeastern margin of the Tibetan Plateau is spatially and temporally heterogeneous. Our findings suggest that the upper Yangtze River and its tributary (Shuoqu) were connected by the middle Miocene. Thermal and exhumational modeling of the granite thermochronometric data indicates rapid cooling during the middle Miocene that was likely related to fluvial incision. The sediments were deposited during an episode of rapid sedimentation, followed by incision that varies between 0.5 and 1.2 km. They consist of unsorted conglomerates and sandstones that partly fill a paleotopography.
In the same region, Cenozoic continental sediments are exposed on the flanks of deep valleys. Samples located above 3870 m yield mean apatite (U-Th)/He ages ranging from 30.6 ± 1.4 Ma to 40.6 ± 2.7 Ma, whereas samples at lower elevations range from 9.8 ± 1.3 Ma to 14.6 ± 2.7 Ma. Mean ZHe cooling ages range from 49.5 ± 2.2 Ma to 68.6 ± 6.0 Ma. We performed apatite and zircon (U-Th)/He dating on a granitic pluton that has been offset by ∼10 km by motion on the sinistral strike-slip Xiangcheng fault in SW Sichuan, SE Tibetan plateau, where the Shuoqu River incises a deep valley before joining the upper Yangtze River.