WHEN DID THE EASTERN TIBETAN PLATEAU BECOME ELEVATED?

The onset of rapid exhumation in major river canyons along the eastern margin of the Tibetan Plateau during the Late Miocene (~10-15 Ma) is argued to herald the growth of high topography in this region. One key aspect of this argument relies on the similarity in the timing of exhumation along the Longmen Shan, where deep exhumation immediately adjacent to the Sichuan Basin precludes any significant lag time between uplift and exhumation, and in southeastern Tibet. Here, we present a detailed study of the cooling history of crystalline basement rocks in the Longmen Shan that reveals a complex and protracted history of Cenozoic exhuamtion. Multiple thermochronmeters, including fission track and (U-Th)/He from both zircon and apatite, derived from 20 samples collected along a >3km age-elevation transect in the immediate hanging wall of the Yingxiu-Beichuan fault provide constraints on the Cenozoic cooling history of the Longmen Shan. In contrast to previous studies, our results suggest that topographic relief was present in the early Tertiary. Rocks presently exposed at the surface were being exhumed between 50 – 30 Ma at rates ~100 m/Myr. Thermal histories inferred from forward models appear to require an acceleration in cooling rate during the Late Oligocene – Early Miocene (approximately 30 – 20 Ma). Low-temperature themochronometers, however, suggest that this rate could not have been sustained until present. Rather, our data appear to require a period of relatively slow cooling during the Miocene, followed by renewed exhumation in the past 10 – 12 Ma. Although complex, our data clearly attest that topography along the Longmen Shan began to develop in the early to middle Tertiary. Whether this range growth was associated with development of the plateau margin in eastern Tibet, or whether expansion of the plateau subsumed a pre-existing highland, remains an outstanding question.