RAPID EXHUMATION ALONG THE LONGMEN SHAN SUSTAINED FOR >30 MA: IMPLICATIONS FOR MOUNTAIN BUILDING ALONG THE EASTERN MARGIN OF TIBET

High topography along the eastern margin of the Tibetan Plateau is coincident with the Longmen Shan thrust belt (LMSTB), an east vergent system of Mesozoic thrust faults developed along the western margin of the Sichuan Basin and reactivated during the Cenozoic. Although abundant evidence for Miocene-present exhumation attests to growth of topography in the past ~15 Ma, deformation and mountain building during the Paleogene remains enigmatic. Here we present the results of low-temperature thermochronologic studies along a ~4 km relief transect in the Xuelongbao massif, spanning a section of Proterozoic granitoids and gneissic basement of the Yangtze craton that represents the deepest portions of the LMSTB. The results of 40Ar/39Ar biotite (BtAr), zircon fission-track (ZFT), apatite fission-track (AFT) and apatite (U-Th-Sm)/He (AHe) thermochronology provide evidence for rapid and sustained exhumation throughout the latter half of Cenozoic time. ZFT ages are ~25 Ma at the top of the massif (5500 m elevation) and ~15 Ma at the base of the transect. AFT ages range from 15 – 6 Ma, while AHe ages range from ~6 – 4 Ma. BtAr ages range from ~75 – 250 Ma, and are consistent with prior constraints on initial temperatures of 350 °C during the early Cenozoic (Robert et al., 2010). Modeling of these data with a 1D finite-difference approach to conductive and advective cooling during exhumation (TQTec) suggests that 1) rapid exhumation initiated by ~30 Ma, consistent with the findings of Wang et al. (2012) but that 2) total Cenozoic exhumation is 15-20 km. We are currently exploring details of thermal models to refine the exhumation history through time, but to first order, exhumation rates in excess of 500 m/Myr appear to have been sustained since the Late Oligocene. Our results imply slow, but sustained, mountain building along the Longmen Shan for at least 30 Ma.