UPLIFT AND EVOLUTION OF THE EASTERN TIBETAN PLATEAU

The Tibetan plateau is the highest region on earth, standing more than 5 km. Uplift of the plateau is the result of post-collisional convergence between India and Eurasia beginning at ~50 Ma. Within eastern Tibet uplift and associated crustal thickening have occurred without significant shortening of the upper crust. Instead, crustal material within deep crustal channels has flowed eastward for hundreds of kilometers from beneath the high plateau, doubling the thickness of the crust in eastern Tibet by lateral injection of middle to lower crustal material. Seismic data and the extremely low topographic gradient across the plateau also argue for a weak lower crust in this region. Eastward flow of deep crustal material is evidenced by the morphology of the eastern plateau margin, where flow is impeded by the strong lithosphere of the Sichuan basin. Here the plateau margin is steep and rimmed by an anomalously high-standing plateau edge, which reflects the elevated flow pressure within the crustal channel as material is diverted north and south around the Sichuan basin. The evolution and ongoing deformation of eastern Tibet attest to a lack of coupling between the upper crust and the underlying mantle lithosphere, with upper crustal, deep crustal and mantle rocks moving in different directions on length scales of hundreds to a thousand kilometers and displaying different orientations of seismic anisotropy. Thus in areas of widespread continental deformation, surface motions are not necessarily good indicators of the kinematics of the underlying mantle lithosphere or within the deeper asthenosphere.

The preexisting landscape of southeastern Tibet, which formed at low elevation prior to uplift, is still preserved over much of eastern Tibet and on the broad, 2000 km wide, southeastern margin of the plateau, where it forms a high-elevation mantle for the plateau. This older surface is currently being incised by headward erosion along the huge rivers that drain the eastern plateau where they cut gorges up to 3 km deep. Isotopic dating of the gorges indicates that rapid incision, and presumably plateau uplift, began at ~8-12 Ma, making uplift of the eastern plateau approximately coeval with onset of the southeast Asian monsoon and supporting the hypothesis that plateau uplift is genetically linked to the development of the monsoon.