No abstract

Exhumation of the southern Tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the Himalaya-Tibet orogen. We report thermochronometric data from a 1.2-km elevation transect within granitoids of the eastern Lhasa terrane, southern Tibet, which indicate rapid exhumation exceeding 1 km/Ma from 17-16 to 12-11 Ma followed by very slow exhumation to the present. We hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the Lhasa terrane to their current positions within the Himalaya. At ∼17 Ma, steep erosive drainage networks would have flowed across the Himalaya and greater amounts of moisture would have advected into the Lhasa terrane to drive large-scale erosional exhumation. As convergence thickened and widened the Himalaya, the orographic barrier to precipitation in southern Tibet terrane would have strengthened. Previously documented midcrustal duplexing around 10 Ma generated a zone of high rock uplift within the Himalaya. We use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks, resulting in substantial drainage reorganization. When combined with a strengthening orographic barrier to precipitation, this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern Tibet.Tibet-Himalaya | thermochronometry | landscape evolution T he Himalaya-Tibet orogenic system, formed by collision between India and Asia beginning ca. 50 Ma, is the most salient topographic feature on Earth and is considered the archetype for understanding continental collision. Geophysical and geologic research has illuminated the modern structure and dynamics of the orogen (1). Nonetheless, how the relatively low relief and high elevation Tibetan plateau grew spatially and temporally and what underlying mechanism(s) drove the patterns of plateau growth remain outstanding questions.In the internally drained central Tibetan plateau, evidence from carbonate stable isotopes suggest that high elevations persisted since at least 25-35 Ma (2, 3). Sustained high elevations since shortly after collision commenced have also been used to explain low long-term erosion rates in the internally drained plateau interior (4-6). In contrast to the central plateau, the externally drained Tibetan plateau margins serve as the headwaters for many major river systems in Asia. Because externally drained rivers provide an erosive mechanism to destroy uplifted terrane, understanding why these rivers have not incised further and more deeply into the Tibetan plateau is essential to decipher how the plateau grew. Recent research in the eastern (7, 8) and northern (9) Tibetan plateau indicates that erosion rates have increased significantly since ∼10 Ma. These increases suggest that rock uplift rates have also increased and that the plateau has expanded to the e...