Cenozoic Crustal Shortening and Plateau Uplift within the Hoh Xil Basin, North-central Tibetan Plateau: Implications for Causal Mechanisms of Plateau Evolution

Lydia Staisch, University of Michigan
Nathan Niemi, University of Michigan
Marin Clark, University of Michigan
Hong Chang, Institute of Earth Environment

The timing and magnitude of deformation within the Hoh Xil Basin, located in the northern Tibetan Plateau, provides important constraints on the growth and uplift of orogenic plateaux. We present new 40Ar/39Ar fault gouge ages, low-temperature thermochronologic data, and a balanced cross section from the Fenghuoshan Fold and Thrust Belt (FFTB) to constrain the shortening history of the central Hoh Xil Basin. Fault gouge for 40Ar/39Ar dating suggests that thrust fault motion initiated in the mid-Eocene. Apatite fission-track data and apatite (U-Th)/He ages were modeled using HeFTy software. The modeled cooling history of the FFTB is consistent with 40Ar/39Ar fault gouge ages and suggests that deformation of the thrust belt took place from mid-Eocene to early Oligocene time. Geochronologic constraints indicate that shortening within the Hoh Xil Basin ceased by ~27 Ma. A geologic cross section across the FFTB, based on field observations and new and existing geologic mapping, was line and area balanced and uncertainties in hanging wall cut-offs, stratigraphic thicknesses, and depth to décollement were propagated to restore the cross section to an original width of 142.77 ± 10.02 km. Given the current length of this section of 103 km, we derive a shortening estimate of 40.26 ± 10.03 km (28.0 ±- 7.2%). When combined with previous constraints on the timing of crustal shortening within the northern Tibetan Plateau, our work suggests that deformation initiated throughout the northern plateau near the onset of Indo-Asian collision.

We test whether Eocene – Oligocene shortening of the Hoh Xil Basin can account for published Miocene paleoelevations (3400-4200 m) by calculating isostatic uplift in response to shortening and thickening of the lithosphere. We assume that the initial elevation of the Hoh Xil Basin was similar to modern retroarc foreland basins, and thus less than 1 km. Our results indicate that 28.0 ± 7.2% crustal shortening and associated thickening produced surface uplift between 0.6 and 1.8 km and crustal thickness values of 40.2 ± 14.8 km. These results cannot account for Miocene paleoelevations or modern crustal thicknesses (65-70 km). Attainment of high elevations and modern crustal thickness require further mechanisms of crustal thickening and surface uplift, such as lower crustal flow, distributed thickening of the lower crust, and/or removal of the mantle lithosphere. These results indicate that attainment of high elevations in the Hoh Xil Basin occurred after the Oligocene, and in the absence of upper crustal shortening.