The Tectonic Evolution of the Tibetan Plateau: Insights from the Deformation and Erosion History of Northern Tibet and the Surrounding Region.

Abstract

The Tibetan Plateau-Himalaya system is the archetype of a continental collision thus, defining its tectonic and topographic evolution is of great importance to our understanding of continental tectonics. Ongoing India-Eurasia convergence since ~55 Ma is the widely accepted cause of great crustal thickness (>60 km) and high elevation (>5 km) within this region; however, the manner in which this convergence is accommodated is controversial. In this dissertation, I employ a multidisciplinary approach to constrain the structural and topographic evolution of the Tibetan Plateau at two spatial scales: a detailed study along the northeastern plateau margin and a broader-scope study across the width of the plateau interior. Results from geologic mapping, low-temperature thermochronometry, and 40Ar/39Ar geochronology indicate that pre-Miocene deformation within northern Tibet occurred primarily along contractional structures oriented favorably to accommodate NNE-SSW Indo-Eurasian plate convergence. Deformation patterns appear to have changed beginning in the post middle-Miocene, as both thrusting and strike-slip fault motion of variable orientation is observed in a zone of transpression bounded by the Kunlun and Haiyuan left-lateral faults. Results from the broad-scale, detrital low-temperature thermochronometry erosion study suggest that erosion rates increase by at least an order of magnitude between 11-4 Ma following a period of slow erosion across the entire east-central Tibetan Plateau. Taken together, the finding of an early to mid-Cenozoic deformation history in northern Tibet and the synchroneity of accelerated erosion across the entire eastern plateau challenges the widely accepted view that the orogen grew northward through time. Instead, widespread accelerated river incision during the mid-to-late Miocene is consistent with regional scale uplift that occurred in concert with eastern expansion of the orogen by lower crustal flow. Timing of proposed eastward crustal flow overlaps with the shift to a predominantly left-lateral strike-slip fault regime documented in northern Tibet and with the timing of onset of other major intracontinental strike-slip faults in southern and central Tibet. Here I propose that these crustal processes occur as a consequence of long-term plate convergence rather than resulting from a specific event, such as mantle delamination, or abrupt change in plate kinematics.