TIMESCALES OF UPLIFT OF THE TIBETAN PLATEAU

The role of Cenozoic mountain uplift in accelerating global weathering rates and hence the downpull of atmospheric CO₂ has received widespread discussion. More specifically the uplift of the Tibetan plateau has been linked to monsoonal intensity, but such propositions can only be tested if the elevation history of the plateau is properly understood.

The initiation of east-west extension in southern Tibet, a consequence of dissipation of potential energy following plateau uplift, has been placed at ~19 Ma from Ar-Ar studies of north-trending graben and dykes. We suggest that the plateau of southern Tibet has not significantly increased in altitude since the Early Miocene. Cosmogenic exposure ages from southern Tibet indicate slow exhumation rates of 2-10 mm/ka over the past 145 ka compared with rates determined from apatite fission tracks, of 80-100 mm/ka over the past 50 Ma, consistent with stabilisation of this part of the plateau.

Foliar analysis of Neogene flora recovered from the Namling basin (Southern Tibet) indicates that 15 Ma ago the basin floor was at an altitude of 2900±100m. Assuming a minimum plateau elevation of 5000m during the Miocene, the elevation differential between basin floor and plateau surface has since decreased from ~2100m to 600m today. The implied minimum exhumation rate (100 mm/ka) is indistinguishable from rates deduced from fission-track studies.

In northern Tibet the recent tectonic history of the plateau is much more active as indicated by high erosion rates (up to 730 mm/ka during the Holocene), Quaternary volcanism, and Pliocene normal faults. We conclude that uplift of the Tibetan Plateau was strongly diachronous with elevation of the south during the Early Miocene considerably predating uplift in the north.