A DETRITAL RECORD OF EROSION PATTERNS FROM APATITE HELIUM THERMOCHRONOMETRY ALONG A NE-SW TRANSECT ACROSS THE TIBETAN PLATEAU

Whereas most thermochronometric studies document recent acceleration in erosion rates from around the perimeter of Tibet, relatively little is known about the erosion history of the plateau interior. Such studies require sample suites with broad spatial distribution from remote areas with limited accessibility that render traditional bedrock-sampling approaches impractical, if not impossible. In contrast, detrital studies are better suited for orogen-scale work because a single sample contains rock materials integrated from a large spatial area. Here we employ a systematic study of the Tibetan Plateau interior by sampling from sandbars along major modern rivers across a NE-SW transect of the east-central plateau that spans > 1500 km and covers a region where almost no thermochronometric ages have been reported previously. Apatite grains from seven catchments were analyzed for (U-Th)/He concentrations (closure temperature ~60 – 70°C). Corrected helium ages from these catchments show a wide distribution that in most cases spans the Cenozoic and Late Mesozoic eras. We apply a recently developed Bayesian methodology that employs the Markov chain Monte Carlo algorithm (MCMC) to interpret detrital apatite helium data through inversion of erosional models. Using detrital age and watershed elevation distributions, we model predicted probability density functions (PDFs) for various erosion scenarios. Mesozoic ages preserved in all but one catchment require limited and/or slow erosion in early to mid Cenozoic time, which is inconsistent with a propagating, steep plateau front initiating with Indo-Asian collision at 45 – 50 Ma. Furthermore, modeling results suggest that rates must abruptly increase between the mid-to-late Miocene and early Pliocene for all of the catchments studied. Synchronicity in accelerated erosion rather than a spatial and temporal progression from catchments that span the entire expanse of the Tibetan Plateau implies a broad, relatively abrupt, and recent event dominates the erosional signal across the east-central plateau.