LANDSCAPE EVOLUTION OF SE TIBET: COMBINING SEDIMENTOLOGICAL AND THERMOCHRONOLOGICAL APPROACHES (Invited Presentation)

River networks are a key feature for addressing the geomorphic and geodynamic evolution of the Tibetan plateau. Because relief in this region is partly related to the coupling of tectonic uplift and fluvial incision, sediments have recorded the deformation and paleoenvironmental evolution of Tibet.

We focus on two zones of Cenozoic sedimentation in the Yangtze River drainage area: the Jianchuan basin (Yunnan) located between the Yangtze first bend and the Red River, and further north the Xiangcheng area (Sichuan) incised by a tributary of the upper Yangtze. We combine a sedimentology approach with an estimation of river incision based on low-temperature thermochronometry.

The Jianchuan basin has been used for paleoaltimetry and paleoenvironmental reconstructions, which makes it necessary to establish an accurate stratigraphy of the sediments. U/Pb dating of volcanites and volcanodetritus, along with paleontological evidence, indicate that fluvial deposits of the Paleo-Yangtze River and lacustrine sediments, thought to be Miocene in age, were in fact deposited before 34 Ma. The potential connection between the paleo-Yangtze River and the Red River was probably disrupted at the end of the Eocene, possibly because of the initiation of the strike-slip Ailao Shan-Red River fault or onset of magmatic activity.

In the Xiangcheng area, unsorted conglomerates and sandstones are exposed on the flank of deep valleys and fill a paleorelief. They do not correspond to a large riverbed like in the Jianchuan basin, but rather correspond to an intense sedimentation episode, followed by an incision varying between 0.5 and 1.2 km. Granitic samples from a pluton located in the same area yield ZHe ages ranging from 70 to 50 Ma. AHe ages range from 15 to 8 Ma (samples at elevation between 3300 and 3870 m) and 40 to 30 Ma (~3900 m). Thermal and exhumational modeling of the thermochronometric data indicates rapid cooling during middle Miocene that likely relates to fluvial incision.

Our findings are consistent with a pre-middle Miocene establishment of the regional drainage configuration of the upper Yangtze River. Our data, along with the increasing number of thermochronometric studies in the area, suggest apparent heterogeneity in space and time of the Cenozoic exhumation. We will discuss the implications on crustal deformation in SE Tibet.