Lydia M. Staisch

Eocene - Oligocene crustal shortening in Hoh Xil Basin, north-central Tibetan Plateau

 

This project is aimed to assess the timing and magnitude of deformation in the Hoh Xil Basin and shed light on the various mechanisms of plateau formation. During the deposition of the Fenghuoshan Group (85-51 Ma), 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. 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. Fault gouge ages and deformed rhyo-dacitic flows suggest deformation progressed southward during this interval. 40Ar/39Ar dating of undeformed basalts and the widespread occurance of flat-lying Miocene lacustrine strata indicate that crustal shortening ceased by the late Oligocene. This, and previous work throughout the Tibetan Plateau, indicates that convergence between India and Eurasia was not accommodated via crustal shortening within the Tibetan Plateau proper, but rather by shortening outboard of the plateau, by eastward lateral extrusion, and, possibly, by subduction of Indian continental lithosphere.

 

A balanced cross section constructed across the Fenghuoshan Range indicates crustal shortening magnitudes of ~24% (34 km). This amount of crustal shortening cannot account for modern crustal thicknesses (65 km) unless the initial crust was ~52 km thick (which is unlikely). Additionally, isostatic calculations indicate that crustal shortening cannot produce the modern high elevations of the northern Tibetan Plateau, which suggests that other mechanisms of surface uplift must have taken place.

 

On the basis of modern geophysical observations and new isostatic calculations, we suggest that lower crustal flow may have contributed to crustal thickening and elevation gain in northern Tibet. Miocene and younger volcanism in Hoh Xil Basin suggests that mantle root loss may have contributed to surface uplift, in addition to lower crustal thickening.