From Left Slip to Transpression: Cenozoic Tectonic Evolution of the North Altyn Fault, NW Margin of the Tibetan Plateau

The Cenozoic North Altyn Fault (NAF) is a major splay of the Altyn Tagh Fault along the northwestern margin of the Tibetan Plateau, but its role in the development of this plateau margin in response to the India-Eurasia collision is highly debated. Here, we investigate fault geometry, kinematics, and shortening magnitude along the westernmost 120 km of the NAF. Seismic surveys reveal minimal Cenozoic shortening in the subsurface of the Southeast Tarim Basin and support for large-scale (>120 km) left slip on the NAF. Based on the satellite imagery, two new faults are identified to define the northern boundary of the NAF system, which together with the NAF to the south constitute a narrow transpressional shear zone comprised of three basement-cored, fault-bound slivers. Fission track data and thermal modeling indicate that the NAF zone experienced broad reburial during early Cenozoic that was locally interrupted by similar to 40 to 35 Ma exhumation proximal to the NAF, followed by widespread but heterogeneous exhumation since similar to 17 to 15 Ma associated with similar to 9 to 11 km of total shortening across the NAF system via thick-skinned faulting. We conclude that the NAF initiated as a left-slip fault at similar to 40 to 35 Ma and then became transpressional at similar to 17 to 15 Ma during the middle-Miocene reorganization of the Altyn Tagh Fault system. We find no evidence to support prior inferences of large-scale (similar to 100 km) underthrusting of the Tarim Basin beneath the Tibetan Plateau along the NAF.