Exhumation History Of The Xining Basin And Its Marginal Mountain Ranges Since The Late Mesozoic

The northeastern Qinghai-Tibetan plateau has deformed strongly since the Cenozoic,which has typical rhombic-shaped basin-mountain distributions. It is the naturallaboratory for studying the far-field effect of the collision between the India Plate andEurasian Plate. But there still remain debates about the responding time of thenortheastern Qinghai-Tibet Plateau to the collision, the process of the deformation andlater evolution. The key of these debates is the initial evolution time of thenortheastern Qinghai-Tibetan Plateau. So the exhumation histories of the geologicalunits located in the northeastern Qinghai-Tibetan Plateau during the Cenozoic arehelpful for understanding the above questions. The Xining basin is located in thenortheastern Qinghai-Tibet Plateau, which has continuous Cenozoic strata. So ittotally records the growth and evolution of the northeastern Qinghai-Tibet Plateau.The northern edge of the Xining basin is the Daban shan, The southern edge of basinis the Laji shan. Different authors have obvious opinions about the formation andevolution of Xining basin, and the uncertainty of the exhumation time of the basin andthe basin marginal mountains may result in this difference: the exhumation time ofLaji shan remains debate, and we know little about the exhumation history of Dabanshan and the basement of the Xining basin. So there is no well constraint about theevolution of Xining basin. In this study, we collect samples from the Xining basin andthe basin marginal mountains for the fission track dating to study the exhumationhistory of these units. We will also discuss the evolution of Xining basin combiningwith related previous studies. The responding time, model and process of thenortheastern Qinghai-Tibet Plateau to the India-Eurasia collision will be discussedbased on the above analysis. Aptite has low closure temperature range(110-60℃) forFission Track dating, this AFT can reveal the tectonic events of the shallow surfaceeffectively. Evidence from fission track has shown that there are multi-stageexhumation periods in the study area:1. the Mid-Late Jurrassic, Xining basinpredecessor experienced rapid exhumation period resulting from the collision between the Qiangtang Block and the Tarim Block.2. the late Early Cretaceous-early LateCretaceous, the predecessor of Xining basin experiences exhumation, which mayrepresent the transformation of the basin.3. the latest Cretaceous, the Xining basinand the Laji shan experienced exhumation resulting from the collision between theLhasa Block and the Qiangtang Block.4. the Early Cenozoic (ca.50-30Ma), theDaban shan experienced exhumation, the Laji shan was exhumed slowly. Both of thebasin marginal mountains were exhumed as the edge of the rift valley uplift. In thisperiod, the Central Qilian Block experience24°clockwise rotation, including theXining basin. The Guide Block did not experience the rotation. In this condition, therewould form an open to the west of the basin, which was the Xining basin. ManyCenozoic basins in the northeastern Qinghai-Tibet Plateau were formed in the latephase of this period, which is the important period of the plateau crustal thickeningand extending outward. The Daban shan exhumed rapidly, the Laji shan had beeninfluenced by this event. The sedimentary rate of the Xining basin also increased.5.the Late Miocene(17-8Ma), there were widespread deformation happened in thisperiod. The Laji shan strongly uplifted, whose direct reason was the clockwiserotation of the Guide basin. The sedimentary rate of Xining basin increasedsignificantly.