Oligocene termination of shortening and initiation of volcanism in the northern Tibetan Plateau: Early removal of Tibetan mantle lithosphere?

Abstract:

Deformed sedimentary rocks of the western Hoh Xil Basin lie within the northern Tibetan Plateau at elevations of ~5000 m. Attainment of high elevations in this region by middle Miocene lithospheric removal has been linked to the termination of shortening and initiation of volcanism and hypothesized to explain observations of low topographic relief and high surface heat flows. We provide new constraints on the timing of crustal shortening and duration of volcanism in the western Hoh Xil Basin from detrital zircon U-Pb minimum depositional age (MDA) analysis of a deformed red bed sequence and ⁴⁰Ar/³⁹Ar dates of undeformed lava flows that cap the red beds. Our work limits the timing of deformation on the >280 km-long, south-directed Dogai Coring thrust fault. At the western mapped end of the Dogai Coring Fault, 28 Ma lava flows overlie tilted red beds near an associated back thrust. Satellite imagery demonstrates that these lava flows form an extensive mesa that can be little deformed since eruption. Along fault strike, 180 km to the east, tilted red beds contain detrital zircons with U-Pb dates as young as 30 Ma, suggesting that they are no older than Oligocene. Undeformed lava flows in this area are dated from 9 to 7.5 Ma and overlie both deformed red beds and the Dogai Coring fault. These age constraints suggest that the fault was active by at least 30 Ma and terminated by 28 Ma, while volcanism was been persistent in this region since 28 Ma. The onset of volcanism at 28 Ma is coeval with the oldest known undeformed volcanic sequences from the eastern Hoh Xil Basin, which limit deformation there to older than 27 Ma. This temporal correspondence suggests that an Oligocene cessation of faulting was largely synchronous across at least 500 km of the Hoh Xil basin. The termination of shortening and initiation of volcanism is thus more compatible with regional lithospheric removal in Oligocene time, rather than in middle Miocene time, as has been previously proposed.