Late Mesozoic-Cenozoic Tectonic Evolution Of The Northern Hexi Corridor

The Qinghai-Tibetan Plateau was formed by the collision between the India and Eurasian plates, and with the persistent compression the Qinghai-Tibetan Plateau is in constant growth. Especially the northeastern region of the Qinghai-Tibetan Plateau is still in activities. But the time of the initial deformation and the present boundary of the northeastern margin of the Qinghai-Tibetan Plateau are still debatable.Hexi Corridor is located in the northeastern margin of the Qinghai-Tibetan Plateau. There are a series of mountains located in the north side of it, and the Longshoushan is one of them. Mainly composed of the Pre-Cambrian metamorphic rocks, the Paleozoic, Mesozoic and Cenozoic sedimentary rocks and the Paleozoic granite, these mounta ins are the suitable objects for research of deformation process of the northeastern margin of the Qinghai-Tibetan Plateau and the determination of its boundary location. With the research of the apatite fission track geochronology of Longshoushan and the investigation of the Late Mesozoic to Cenozoic deformation along the northern Hexi Corridor, we can restore the tectonic evolution of the northern Hexi Corridor during late Mesozoic and Cenozoic.The apatite fission track(AFT) ages and their thermal modeling of the Longshoushan and deformation along the northern Hexi Corridor on the north side of the Qinghai-Tibetan Plateau show that the Longshoushan of the northern corridor had experienced important multi-stage exhumations during the Late Mesozoic and Cenozoic. The apatite fission track(AFT) ages of 6 samples range from 38.4 Ma to 111.8Ma. The thermal modeling of the AFT ages of the samples showed that the Longshoushan experienced a significant exhumation during the Late Mesozoic to the Early Cenozoic(?130-25 Ma). The Late Mesozoic exhumation of the Longshous ha n may result from the continuous compression between the Lhasa and Qiangtang blocks and the flat slab subduction of Neotethys oceanic plate, this compression affected wide regions across the Qinghai-Tibetan Plateau. During the Early Cenozoic, the Longshoushan was still experienced exhumation, but it was caused by the IndianEurasian collision. Since then the Longshoushan had been in the stable stage for about 20 Ma and undergone erosion. Since ?5Ma, obviously tectonic deformation occurred along the whole northern Hexi Corridor, which was also reported from the periphery regions of the Qinghai- Tibetan Plateau, especially in the Qilianshan and northeastern margin of the plateau. The AFT ages and the Late Cenozoic deformation of the northern Hexi Corridor all indicate that the present northern boundary of the Qinghai-Tibe ta n Plateau has been situated along the northern Hexi Corridor.