Transpression along North Altyn Fault and Northwestern Margin of Tibetan Plateau Indicated by Structural Observations and Fission Track Thermochronology

Abstract:

Because the North Altyn fault (NAF) within the Altyn Tagh fault system defines the topographic boundary along the northwestern edge of the Tibetan Plateau, determining its structure and evolution is essential for understanding the growth of this plateau margin. Here we investigate the structure, exhumation depth, and magnitude of shortening along the westernmost 120 km of NAF known as the Jianglisai reach. Using seismic reflection and well-log data, together with satellite imagery from SE Tarim, we have identified a new frontal fault in the subsurface. North of this fault, the crystalline basement of SE Tarim is overlain by a flat-lying, Jurassic-Quaternary sequence 3.3-4.8 km thick. South of the frontal fault, the NAF is a system of basement-cored, fault-bounded slivers that are elongated against Altun Shan along the Jianglisai fault. Our data show the NAF continues to the northeast along the Jianglisai fault within the Altun Shan, rather than at the range front or in the subsurface of SE Tarim. Our fission-track results, combined with published data, document Early Cenozoic burial reheating both north and south of the Jianglisai fault, with samples adjacent to the fault indicating ca. 35 Ma onset of deformation. The thermochronologic data further attest to post-20 Ma shortening across the NAF. Exhumation is deepest south of the Jianglisai fault, where samples show fully reset ages of 15 to 10 Ma. Exhumation is intermediate in the fault-bounded slivers, where samples were exhumed from varying depths within the apatite PAZ starting 20-5 Ma; while to the north, burial in the basin is ongoing. However, shortening across the frontal fault is very low (1.5 to 3 km). Post-20 Ma strike-slip deformation along the Luo Jianglisai fault is suggested by our fission track data that indicate west-side-up tilting of the fault-bounded sliver, which we infer may result from migration through a local restraining bend in the Jianglisai fault. Thus, we conclude that the Jianglisai reach comprises a transpressional system in which reverse and strike-slip deformation happened simultaneously, with shortening occurring as Laramide-type, thick-skinned deformation focused along the frontal fault and within a narrow set of fault-bounded slivers, rather than a thin-skinned thrust system in SE Tarim.