The Tectonic Evolution of the Hoh Xil Basin and Kunlun Shan: Implications for the Uplift History of the Northern Tibetan Plateau.

Abstract

The deformation history of the northern Tibetan Plateau has significant and wide-ranging implications for understanding the timing and mechanisms involved in the topographic development of the orogen, yet remains controversial due to limited geologic observations. In this thesis, I focus on constraining the tectonic evolution of the Hoh Xil Basin and Kunlun Shan, located in north-central Tibet. I use a multi-faceted approach, including field mapping, magnetostratigraphic and biostratigraphic interpretation, geochronology, provenance analysis, thermochronology, 40Ar/39Ar fault gouge dating, balanced cross section construction, and isostatic uplift calculations. In Chapters II and III, I revise the sedimentary and structural history of the Hoh Xil Basin. Results indicate that late Cretaceous – early Eocene sedimentation within the Hoh Xil Basin records the deformation of the Tanggula Shan, located in central Tibet, and that deformation did not progress into northern Tibet until after 51 Ma. This marks the Tanggula Shan as the northern limit of deformation prior to the ~50 Ma Indo-Asian collision, considerably farther north than previously understood. I find that the Hoh Xil Basin underwent 28% north-south oriented crustal shortening from Eocene – late Oligocene time. Based on isostatic calculations, this amount of shortening is insufficient to produce the modern crustal thickness and elevation of northern Tibet, which implies that other mechanisms of thickening and surface uplift are required. In Chapter IV, I resolve that crustal shortening occurred in the Kunlun Shan between 47 and 24 Ma. Left-lateral strike-slip faulting initiated subsequently, possibly as early as 20 Ma. The crustal shortening history of the Kunlun Shan and Hoh Xil Basin and suggests that deformation of northern Tibet initiated soon after the onset of the Indo-Asian collision was distributed over a broad region. The mid-Miocene transition from crustal shortening to left-lateral shear is indicative of a shift in primary stress orientations and relates to the attainment of high elevation in northern Tibet. The data and interpretations from this thesis places much-needed quantitative constraints on the tectonic and topographic history of the northern Tibetan Plateau. These new constraints require reconsideration for the timing and tectonic processes involved in the deformation and uplift of northern Tibet.