Preliminary Results of Low-temperature Thermochronology in the Three Rivers Region, SE Tibetan Plateau

Abstract:

The SE margin of the Tibetan Plateau is characterized by a long-wavelength, low-gradient topography over 1500 km distance. Most representative of the diffuse margin is the Three Rivers region, where the Mekong, Salween and Yangtze flow nearly in parallel for hundreds of kilometers. Low relief surfaces perch high and between deeply incised river valleys, a unique geomorphic feature on Earth. The process and mechanism of its genesis are debated. Models of plateau deformation make contrasting predictions in regional surface uplift and the corresponding response of river incision. we utilize apatite and zircon U-Th/He and apatite fission track thermochronology to map regional spatial-temporal pattern of exhumation and river incision. We sample five vertical transects at different locations along the margin, in order to determine whether there is a difference in the initiation time of the latest stage of denudation, which may provide a test to conceptual models of Tibetan plateau margin development. Preliminary results show that there is large north-south difference in timing of the phase of fast exhumation. The northernmost transect (~29.5°N) shows a sustained fast exhumation of ~250 m/Myr since ca. 40 Ma, middle transect (~28.5°N) records a phase of fast exhumation (120 m/Myr) at 50±10 Ma, followed by a slight slowing-down exhumation since 40Ma. The southern profiles (~26.5°N) is characterized by a pulse of very fast exhumation at ca. 15±5 Ma, followed by a slowing-down in exhumation to ~140 m/Myr. The profile on a tributary of Jinsha (27.5°N ) is an exception with a sustain slow exhumation (<30 m/Myr) since Late Cretaceous. Common to all locations is a slowing-down in exhumation rate since late Miocene, albeit a north-south difference in timing of commencement. Our data also suggest an eastward decreasing gradient in latest-stage exhumation rate: samples on tributaries of Salween suggest a larger exhumation rate than those on Mekong, which in turn larger than on Yangtze. Since Salween is incising deeper than Mekong, which in turn deeper than Jinsha at the same latitude, we suggest that trunk river incision may drive hillslope denudation, and that the pattern has maintained over Myr- time scale. We will also discuss the implications of our data to tectonic models of SE Tibetan plateau margin development.