Tenfold lower late quaternary throw rates in central Tibet compared to southern Tibet reflect different extensional deformation mechanisms

Abstract

Present-day tectonic deformation in central Tibet is characterized by a series of ∼NS-trending grabens which accommodate EW extension. Quantifying the geometry and kinematics of these grabens is essential to understand Cenozoic tectonic deformation and Tibetan Plateau evolution. Here, we focus on the Norma Co graben (NCG), i.e., the southern segment of the Shuanghu-Norma Co graben (SH-NCG) system, which is the most prominent graben system within the Qiangtang terrane in central Tibet. We study its tectonic and geomorphologic characteristics to determine the late Quaternary throw rates of the normal faults bounding the graben, based on high-resolution satellite images interpretation, field investigation, and cosmogenic 10Be dating (n = 23). Using terrestrial LiDAR, UAV, and kinematic GPS, we precisely measure vertical offsets (up to 15 m) of ∼90–120 ka-old alluvial surfaces, yielding a throw rate of 0.10(+0.04/-0.03) mm/yr. This rate is ten times lower than those published along other NS-trending grabens in southern Tibet, reflecting different deformation mechanisms, as previously suggested: grabens in eastern Qiangtang formed by rapid eastward block extrusion, those in western Qiangtang formed by distributed extension on numerous scattered normal faults, and those in southern Tibet result from divergent orthogonal thrusting along the curved Himalayan arc.