Dip-Slip Rate Determined by Cosmogenic Surface Dating on a Holocene Scarp of the Chem Co Graben, Western Tibet

The east-west thinning of the upper crust of the Tibetan Plateau is expressed by a series of N-S trending normal faults associated with regional E-W extensional grabens, which is used to be attributed to either gravitational collapse of elevated crust or continuous deformation driven by a low-viscosity mid-crustal channel or geodynamic processes that may be unrelated to plateau formation. Among these grabens, the Chem Co graben is the westernmost one in the central Tibet, which is characterized by a prominent Latest Holocene scarp on its boundary fault. Using cosmogenic surface exposure dating with ¹⁰Be and ²⁶Al, we have measured bedrock exposure ages as a function of elevation along an exposed bedrock scarp of the Chem Co graben. Both ¹⁰Be and ²⁶Al exposure ages increase monotonically with elevation ranging from 3.67 ka to 14.06 ka and from 3.47 ka to 41.76 ka respectively over the 112.6 m scarp length giving an average dip-slip rate of 2.9-3.1 m/ka during the Holocene period and a vertical uplift rate of 1.7-1.8 m/ka. This result from the westernmost area of the central Tibet is distinctly higher than the rate of 0.3 m/ka derived from U-series dating of pedogenic carbonate along the main boundary fault of Shuang Hu graben in the middle-east area of the central Tibet, but consistent with the rate of 1.9±0.6 mm/yr inferred for normal faults bounding the Yadong-Gulu graben system in southern Tibet. We therefore infer that most of the normal faulting in the central and southern Tibet is largely controlled by a united process, and either the material migration in the central Tibet is diachronous from west to east, or some of the normal faulting in the graben system in the central Tibet is controlled by local processes.