Normal faulting in central Tibet since at least 13.5 Myr ago

Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust(1) or geodynamic processes that may be unrelated to plateau formation(2-6). A key piece of information needed to e valuate these models is the timing of deformation within the plateau. The o nset of normal faulting has been estimated to have commenced in southern Ti bet between about 14 Myr ago(7) and about 8 Myr ago(8) and, in central Tibe t, about 4 Myr ago(9). Here, however, we report a minimum age of approximat ely 13.5 Myr for the onset of graben formation in central Tibet, based on m ineralization ages determined with Rb-Sr and Ar-40-Ar-39 data that post-dat e a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, s upport models that relate normal faulting to processes occurring beneath th e plateau. Thinning of the upper crust is most plausibly the result of pote ntial-energy increases resulting from spatially and temporally heterogeneou s changes in thermal structure and density distribution within the crust an d upper mantle beneath Tibet. This is supported by recent geophysical and g eological data(10-17), which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.