Cenozoic Expansion Of The Ne Tibetan Plateau

The northeastern(NE)Tibetan Plateau marks obvious topographic and climatic transition zone in the inner Asia.The development history of high topography in the NE Tibetan Plateau is essential to test various plateau growth models,understand plateau construction and explore relationships between the plateau growth and climatic and environmental evolution in the inner Asia,but still is highly debated.In this study,we study the timing and pattern of the Cenozoic deformation in the East Kunlun Shan and Qilian Shan using apatite(U-Th)/He thermothronology and provenance anslysis.New apatite(U-Th)/He and integrated provenance data presented here can well refine the Cenozoic deformation timing and pattern of the East Kunlun Shan and timing of the topographic growth of the Qilian Shan,providing new constraints on the growth of the NE Tibetan Plateau.1.Revised timing of Cenozoic deformation in the East Kunlun Shan(Nuomuhong region)Published apatite(U-Th)/He vertical transects in the northern margin of the East Kunlun Shan suggest an Eocene(35-40 Ma)rapid exhumation,which is interpreted as initiation of the Cenozoic deformation.However,re-analysis of existing apatite(U-Th)/He data doubt that Eocene rapid exhumation may be result of inappropriate data organization.In order to refine timing of Cenozoic deformation in the East Kunlun Shan,seven apatite(U-Th)/He samples were collected around a tilted erosion surface in the Nuomuhong region,the middle segment of the East Kunlun Shan.In this study,paleodepth,defined as the perpendicular distance between samples and erosion surface,was measured and plotted against apatite(U-Th)/He ages,including both new and published data in the Nuomuhong region.New age-paleo-depth profile identifies a break in slope at?25 Ma,from?0.03 km/Myr to 0.4-0.5 km/Myr,which we interpret to represent the onset of thrust faulting at northern margins of the East Kunlun Shan.2.Timing and pattern of Cenozoic deformation in the East Kunlun Shan(Xiangride region)Previous studies observed several deformation styles(northward thrust faulting,southward thrust faulting and strike-slip motion along the Kunlun strike-slip fault)in the East Kunlun Shan and thinned Cenozoic strata toward the East Kunlun Shan in the Qaidam and Hoh Xil basins.Available models,such as northward thrust model, southward thrust model and two-stage tectonic evolution,could not reconcile above observations in the East Kunlun Shan and adjacent basins.Here,we hypothesize that deformation in the East Kunlun Shan is related to the flower structure on the Kunlun strike-slip fault and collected apatite(U-Th)/He samples across the East Kunlun Shan in the N-S direction in the Xiangride region,including three vertical transects and one horizontal transect.Dense apatite(U-Th)/He ages across the East Kunlun Shan defines a series of isochrones,which are nearly parallel with new identified erosion surface.This geometry relationship marks a northward tiling of the East Kunlun Shan.Horizontal transect across the East Kunlun Shan suggest southward increase on exhumation and that locus with the most exhumation is near the Kunlun strike-slip fault,consistent assumed flower structure model.Vertical transects jointly reveal that rapid exhumation in the East Kunlun Shan initiated at the late Oligocene(26-27 Ma)and rapid exhumation in the compressional structures along the Kunlun strike-slip fault(Buqing Shan)is prior?23 Ma.Thus,we infer that strike-slip motion along the Kunlun fault initiated at the late Oligocene and transpression along the strike-slip fault leads to northward tilting of the East Kunlun Shan.Taking observed late Oligocene thrusting faulting at the northern and southern East Kunlun Shan,the Cenozoic depositional models of the adjacent basins,geomorphic characters and crustal structures beneath the East Kunlun Shan into consideration,we proposed a new geometry model that one large-scale flower structure on the Kunlun strike-slip fault in the upper crust is accompanied by a lower crustal wedge tectonics.New model can not only account for observed thrust faulting,strike-slip faulting and regional tiling in the East Kunlun Shan,but also reconcile thrust faults at fronts of the East Kunlun Shan with the Cenozoic depositional pattern in the adjacent basins.3.Timing and pattern of Cenozoic deformation in the East Kunlun Shan(Golmud region)Based on the flower structure geometry model,we also collected apatite(U-Th)/He samples across the East Kunlun Shan in the Golmud region,including four vertical transects and one horizontal transect.North-south elevation profile is characterized by southward decrease on apatite(U-Th)/He ages,suggesting southward increase on exhumation and that locus with the most exhumation is near the Kunlun strike-slip fault.Four vertical profiles in the southern and central East Kunlun Shan record rapid exhumation during the late Oligocene to early Miocene(26-18 Ma)and one vertical profile record a low exhumation rate during 56-27 Ma and a possible break in slope at?27 Ma,which indicate rapid exhumation initiated at 26-27 Ma.Timing of the rapid exhumation and exhumation pattern in the Golmud region are similar with that in the Xiangride region,further supporting new geometry model that one large-scale flower structure on the Kunlun strike-slip fault in the upper crust is accompanied by a wedge structure in the lower crust.4.Topographic growth of the Qilian Shan since the middle MioceneThe Qaidam Basin is filled with as much as?14 km of the Cenozoic clastic sediments,which preserve an exceptional records on the topographic growth of the NE Tibetan Plateau.However,uncertainty on chronology of the Cenozoic strata in the Qaidam Basin causes a great debate on topographic growth history of the Qilian Shan.To avoid this issue,we focus on the Huaitoutala section in the NE Qaidam Basin,whose ages are well constrained to 15.7 to 1.8 Ma by magnetostratigraphy and a series of in situ fossils,and present integrated provenance data.Results show an increase in carbonate lithics(from<7%to>20%),an increase in Al₂O₃/Si O₂ ratios(from 20%to 29%),a negative shift in?_Nd values(from-9.9 to-12.4)and an appearance of large amounts(60%)of Precambrian zircon grains in the period of?13to?8 Ma,arguing that the sediment sources of the NE Qaidam Basin may have shifted from the East Kunlun Shan to the Qilian Shan during this time interval.Thus,we infer that significant topographic growth of the southern Qilian Shan occurred during the middle-late Miocene.Along with widespread middle to late Miocene deformation records across the Qilian Shan and abruptly shifts on provenance,sedimentary facies and climate indexes in its surrounding basins,present high topography of the Qilian Shan may have been established since the middle Miocene and shaped the modern climate pattern of the NE Tibetan Plateau.In summary,this study provide several new insights into the growth of the NE Tibetan Plateau:(1)Thrust faulting at northern margins of the East Kunlun Shan is revised to the late Oliogocene(?25 Ma);(2)Late Oligocene northward tilting of the entire East Kunlun Shan is identified,which is synchronous with initiation of the Kunlun strike-slip fault;(3)New model,that one large-scale flower structure on the Kunlun strike-slip fault in the upper crust is accompanied by a wedge structure in the lower crust,can reconcile previous incompatible geological and geomorphological observations in the East Kunlun Shan and surrounding basins;(4)widespread middle-late Miocene deformation may build the modern high topography of the Qilian Shan and shape modern climate of the NE Tibetan Plateau;(5)The NE Tibetan Plateau has experienced outward expansion from the East Kunlun Shan to the northern Qilian Shan since the late Oligocene.