Eocene - late Oligocene crustal shortening within the Hoh Xil Basin, north-central Tibetan Plateau

Recent work within the Hoh Xil Basin, located in the north-central Tibetan Plateau, documents late Cretaceous to earliest Eocene deposition of the Fenghuoshan Group, which is presumed to derive from uplift and erosion of proximal regions to the south (likely the present-day Tanggula Shan). Combined with geologic observations from the Lhasa and southern Qiangtang terranes, we surmise that an elevated region of deformation that pre-dated the Indo-Asian collision may have extended from the southern continental margin of Asia northward to the geographic center of the Tibetan Plateau. During this time, the Hoh Xil Basin must have been relatively lower in elevation than central and southern Tibet. Presently, the Hoh Xil Basin lies at an average elevation of ~4500 meters, similar to southern Tibet, and must have been uplifted during or following the onset of Indo-Asian collision. In order to better constrain the timing of deformation within north-central Tibet, we collected and analyzed samples from the Fenghuoshan Range, located within the Hoh Xil Basin, for low-temperature thermochronology, fault gouge dating, and geochronology on variably deformed volcanic rocks. Apatite fission-track lengths were obtained from deeply exhumed sandstone samples and modeled using HeFTy software. Both modeled thermochronologic data and 40Ar/39Ar fault gouge ages independently suggest that the Fenghuoshan Range underwent deformation and exhumation during the Eocene and Oligocene. Moderate deformation of a 33.5 Ma rhyo-dacitic lava flow indicates crustal shortening continued at least until this time. Sub-horizontal basaltic flows cap the deformed Fenghuoshan strata and limit the timing of deformation to before 27.3 Ma. A balanced cross section constructed across the Fenghuoshan Range indicates crustal shortening magnitudes of ~33% (18 km). An unresolved question is the extent to which Eocene - Oligocene crustal shortening contributed to thickening and uplift of the northern Tibetan lithosphere. One possible scenario is that the crustal thickening of northern Tibet during Eocene - Oligocene time is the main mechanism to attain a modern crustal thickness of 65 km. This would require an initial crustal thickness of ~44 km. However, such an initial thickness requires the removal of a significant amount of Asian lithosphere during collision to accommodate the 2500 - 3000 km of convergence between India and Eurasia since early Eocene time. Alternatively, if the 33% shortening in the Fenghuoshan region represents only upper crustal strain, then the modern crustal thickness of 65 km could be attained from a 30-km-thick pre-collisional crust by preferential thickening of the lower crust, either by bulk strains of 65% or through other mechanisms, such as lower crustal flow.