Denudational response to surface uplift in east Tibet: Evidence from apatite fission-track thermochronology

Abstract

East Tibet preserves broad areas of low relief that occur at elevations >3000 m and are dissected by major strike-slip faults that have been repeatedly reactivated since the late Mesozoic. Apatite fission-track samples from these low-relief, high-elevation surfaces indicate Cretaceous cooling (ca. 90 Ma). Fast rock uplift of the 110 °C isotherm and a fossilized partial annealing zone began in the plateau interior of east Tibet in the Miocene. Low denudation rates in the plateau interior during the Cenozoic, which are constrained by thermal history modeling and a consistency in ages from apatite fission tracks, suggest that dip-slip displacement on faults has been minor. Some localized areas have experienced the effects of structurally enhanced rock uplift and denudation or fault-related hydrothermal reheating. Cooling ages in Eocene sediments deposited on the older plateau surface suggest that rapid denudation occurred from the Miocene to Holocene. Other evidence of significant Cenozoic denudation comes from river valleys, in particular, the Yalong River valley, where incision was initiated in the Oligocene to Miocene (28–12 Ma). Estimated mean denudation in east Tibet during the Cenozoic was ~1–2 km in the low-relief, high-elevation plateau interior and at least 5 km at the high-relief plateau margin in the Longmen Shan. The region south of the Longmen Shan and the Sichuan Basin was not uplifted with the main Tibetan Plateau in the Miocene, but it has undergone enhanced denudation since the Eocene. Due to an orographic effect, most of the denudation in the east Tibetan Plateau interior has occurred in major river valleys. Incision in the major rivers of the Longman Shan occurred simultaneously and has kept pace with surface uplift that began in the middle–late Miocene (12 Ma), with acceleration of denudation rates in the late Miocene (>5 Ma).