Miocene cooling in the northern Qilian Shan, northeastern margin of the Tibetan Plateau, revealed by apatite fission-track and vitrinite-reflectance analysis

Apatite fission-track and vitrinite-reflectance data from Phanerozoic rocks of the northern Qilian Shan and Jiuxi basin, at the northeastern margin of the Tibetan Plateau, show a complex thermal history with multiple paleothe rmal events. Peak paleotemperatures, reached in the middle Cretaceous, sugg est that hydrocarbon generation occurred in the basin at that time. Paleozo ic-Mesozoic samples show evidence of a middle Tertiary cooling episode betw een 20 and 10 Ma. This thermal fingerprint is significant because stratigra phic evidence for Miocene exhumation is overwhelmed by spectacular Pliocene -Pleistocene deformation and erosion. We interpret the Miocene cooling as r ecording initial exhumation of the northern Qilian Shan-earlier than other models have proposed (i.e., after 6-5 Ma). Miocene cooling is broadly coinc ident with Cenozoic uplift and exhumation recorded elsewhere in the Tibetan -Tarim region.