Cenozoic Tectonic Deformation Process Of The Qilian Shan(Northeastern Tibetan Plateau)Recorded By Detrital Apatite Fission-track Thermochronology

The collision and compression of Indian-Asian plates result in the uplift of the Tibetan Plateau,which is one of the most important geological event on the earth since 100 Ma.The formation of the Tibetan Plateau has profound impacts on the tectonics,topography,climate,and environment of the Asia.Therefore,the formation and evolution of the Tibetan Plateau have always been the focus in studies of international earth science.In particular,the uplift and expansion process of the Tibetan Plateau is crucial in understanding the process and dynamic mechanism of continental collision,plateau's growth,and lithospheric deformation,as well as the coupling relationship between tectonics and climate.The Qilian Shan,serving as the northeastern margin of the Tibetan Plateau,is a Caledonian orogenic belt and was tectonic activation and uplifted to a mountain under the deformational stress of the collision and compression of the Indian-Asian plates during the Cenozoic.The reactivation,deformation,and uplift of the Qilian Shan is a significant event in the evolution of continent in the northeastern Tibetan Plateau.The evolution of the Qilian Shan not only controls the formation,adjustment,and transformation of oil and gas resources and ore in basins flanking the Qilian Shan,but also serves as the current frontier of the NE-trend growth of the plateau thus the tectonic deformation process during the Cenozoic has important significance in understanding the remotely response of the collision of Indian-Asian plates and the process and dynamic mechanism of the growth of the Tibetan Plateau.Although predecessors have done numerous researches in the Qilian Shan region,there are dimness and divergence in the understanding in initiated time and developed process of tectonic deformation in this region during the Cenozoic.Therefore,it is imperative to constrain the tectonic deformation and evolution process of the Qilian Shan during the Cenozoic by more definitive evidences.Fission-track thermochronology can record the times that minerals cooling from deeper crust through specific crustal depth,which corresponds to specific closure temperature,as their cooling ages.Apatite fission-track thermochronology has a lower closure temperature so that it records cooling and exhumation history of host rocks in the shallow crust.While sediments in basin is the most direct and specific geological record to the tectonic evolution of orogenic belt.Therefore,detrital apatite fission-track thermochronology of synorogenic sediments in basin had became a key method to study problems such as tectonic deformation and uplift process of orogenic belt.This study seclect Cenozoic sections constrained by magnetstratigraphic chronology in the Jiuquan Basin and the Qaidam Basin that flanking the Qilian Shan to the north and south respectively,as well as modern fluvial sediments of streams draining from the Qilian Shan to these two basins,as study target.The key point of this study is the systemic analysis of detrital apatite fission-track thermochronology of 33 samples from Cenozoic strata and 2 samples from modern fluvial sands in the Jiuquan Basin,and 13 samples from Cenozoic strata and 2 samples from modern fluvial sands in the northern Qaidam Basin.Combining with provenance analysis and predecessor's research results,this study reveals the process of tectonic deformation and uplift of the Qilian Shan during the Cenozoic,and discusses the deformational mechanism of the Qilian Shan and the uplift and extension mechanism of the Tibetan Plateau.The following cognitions and conclusions are achieved:1.The detrital apatite fission-track modeled components of Cenozoic sediments from the Jiuquan Basin and the Qaidam Basin have “static peaks” with age of ~60-50 Ma and ~60-54 Ma in respective,indicating the Qilian Shan experienced rapid rock exhumation during the late Paleoceneearly Eocene(~60-50 Ma).This result suggests the leading edge of tectonic deformation was arrived at the Qilian Shan region in the northeastern Tibetan Plateau since the initial stage(~60-50 Ma)of the collision between Indian-Asian plates during the Cenozoic,and resulted in the uplift and exhumation of the Qilian Shan in the early Cenozoic.2.The detrital apatite fission-track modeled age components of Cenozoic sediments from the Jiuquan Basin have “static peaks” with age of ~45-37 Ma,~34-30 Ma,and ~26-21 Ma in respective.The detrital apatite fission-track modeled age components of Cenozoic sediments from Qaidam Basin have “static peaks” with age of ~45-38 Ma and ~34-30 Ma in respective,and a modeled age component with age of ~26.2 Ma.These results jointly indicate the Qilian Shan experienced rapid tectonic exhumation events during the middle-late Eocene(~45-37 Ma),early Oligocene(~34-30 Ma),and late Oligocene-early Miocene(~26-21 Ma)in respective.3.The detrital apatite fission-track modeled age components of Cenozoic sediments from the Jiuquan Basin have a “static peak” with age of ~16-10 Ma,and that from the Qaidam Basin has a component age of ~14.5 Ma,suggesting the Qilian Shan experienced rapid rock exhumation again during the middle-late Miocene(~16-10 Ma).Moreover,with the developing of deposition,detrital apatite fission-track modeled peak-ages of the Qaidam Basin suddenly changed older at depositional age of ~12 Ma,providing futher evidence for a tectonic deformation event occurred in the Qilian Shan at the middle-late Miocene.4.With the developing of deposition,detrital apatite fission-track modeled peak-ages of the Qaidam Basin suddenly changed again at depositional age of ~2.1 Ma,which changes back to the peak-age assemblages that before ~12 Ma.This change indicates the Qilian Shan experienced tectonic deformation again at ~2.1 Ma.5.Combining with the existing study results,we suggest there is no directed S-N transmission in deformation within the Qilian Shan during the Cenozoic,and the chronological order of Cenozoic tectonic activation in different parts of the Qilian Shan is based on the locations of preexisting tectonic weak belts in the upper crust of the Qilian Shan.Moreover,the episodic Cenozoic deformational process of the Qilian Shan(which tectonic deformation events occurred at ~60-50 Ma,~45-37 Ma,~34-30 Ma,~26-21 Ma,~16-10 Ma and ~2.1 Ma)represents the deformational process of the whole northeastern Tibetan Plateau.Among these events,the structure and topography pattern of the Qilian Shan and the northeastern Tibetan Plateau similar to nowadays were basically formed since the event at middle-late Miocene(~16-10 Ma)period.