Mesozoic and Cenozoic tectonics of the northeastern edge of the Tibetan plateau: Evidence from modern river detrital apatite fission-track age constraints

Thermochronometry of modern river sands exploits the natural sampling of detritus in a river catchment to infer plausible regional exhumation patterns, when combined with available bedrock thermochronometric information. Apatite fission track age distributions for six detrital samples are reported in this study to resolve the exhumation history of the upper reaches of the Heihe River, draining North Qilian Shan and Tuole Shan, the northeastern margin of the Tibetan Plateau. The apatite fission track (AFT) ages include four major populations: late Paleozoic (300 +/- 55-293 +/- 43 Ma), early Mesozoic (223 +/- 15-205 +/- 17 Ma), late Jurassic-Cretaceous (112 +/- 6.2-84.8 +/- 5.6 Ma), and Cenozoic (63.8 +/- 5.9-19.3 +/- 1.7 Ma). The ages of these populations correspond well with the deformation and cooling history of the study area. The late Paleozoic and early Mesozoic AFT age populations record cooling events resulting from continental collision between the Qiantang and Kunlun blocks. The late Jurassic-Cretaceous AFT age populations are interpreted to record cooling events associated with exhumation of North Qilian Shan and Tuole Shan due to the collision of the Lhasa block with Asian continent. The Cenozoic AFT age populations record several cooling events. A cooling event is recorded corresponding to exhumation of North Qilian Shan and Tuole Shan due to late stress from the Lhasa-Asia collision, causing deformation nearly synchronous with the India-Asia collision. Another cooling event is recorded corresponding to uplift and exhumation in Oligocene time due to ongoing India-Asia convergence. North Qilian Shan and Tuole Shan were constructed as a distal effect of India-Asia convergence in the early Miocene while approaching their present mountain-river-basin distributions. Thermochronometry applied to Heihe River sediments provide convinced evidence of multi-stage exhumation of most northeastern margin of the Tibetan Plateau in the late Paleozoic-Cenozoic.