Lydia M. Staisch

Cretaceous - early Eocene depositional age of the Fenghuoshan Group (Hoh Xil Basin), and tectonic implications for the Tibetan Plateau

 

This project is based on refining our knowledge of the sedimentary history of the Northern Tibetan Plateau, specifically in the Hoh Xil Basin. The Fenghuoshan Group marks the initiation of terrestrial deposition in the Hoh Xil Basin and preserves the first evidence of uplift above sea level of northern Tibet. The depositional age of the Fenghuoshan Group is debated, as are the stratigraphic relationships between the Fenghuoshan Group and other terrestrial sedimentary units in the Hoh Xil Basin. The goal of this project is to reassess the depositional age of the Fenghuoshan Group and clarity stratigraphic relationships within the Hoh Xil Basin. To do so, we radiometrically a volcanic tuff interbedded within the Fenghuoshan Group and volcanic units that are variably deformed. This is combined with a compilation of published biostratigraphic data which are used to reinterpret existing magnetostratigraphic data from the Fenghuoshan Group We find that the Fenghuoshan Group was deposited from late Cretaceous to earliest Eocene time, rather than the previous estimate of Eocene - Oligocene time.

 

The provenance of the Fenghuoshan Group is also an important aspect to study, because it suggests which regions in Tibet were uplifted and eroded to create the thick sequence of sediments found in Hoh Xil Basin. We use detrital age spectra from the Fenghuoshan Group and Tibetan terranes and Mesozoic sedimentary units. We propose that these strata were sourced from the central Qiangtang Terrane, and may share a sediment source with Cretaceous sedimentary rocks in Nima Basin. 

 

Together, our age constraints and field relationships imply exhumation of the central Qiangtang Terrane from the late Cretaceous – earliest Eocene, followed by Eocene – Oligocene deformation and shortening of the northern Qiangtang and southern Songpan-Ganzi terranes. Crustal shortening within the Hoh Xil Basin ceased by late Oligocene time, as is evident from flat-lying basaltic rocks.