Cenozoic Sedimentary Evolution And Exhumation In Jiuxi Basin And Its Implications For The Growth Of Northeastern Tibetan Plateau

The Tibetan Plateau,formed during the Cenozoic,is an ideal place to study the continental collision and subsequent intracontinental deformation processes.Cenozoic sedimentary basins on the northeastern margin of the Tibetan Plateau could record the regional uplift history and tectonic deformation during the collision and subsequent convergent processes.Studies on the tectonic-sedimentary evolution in these Cenozoic sedimentary basins are significant for understanding the Cenozoic growth and deformation mechanism of the Tibetan Plateau.The Jiuxi Basin,which constitutes a portion of the northeastern margin of the present Tibetan Plateau,should be sensitive to the Cenozoic growth of the northeastern Tibetan Plateau.Therefore,an integrated study on the Jiuxi Basin,including the Cenozoic sedimentary evolution,provenance analyses,and low-temperature thermochronology,has been performed in this paper to reveal the Cenozoic tectonic and sedimentary evolution in the northeastern Tibetan Plateau.By combining previous magnetostrati graphic analyses and exhumation studies,the Cenozoic tectonic evolution in the Jiuxi Basin has been well reconstructed in this study,which provides constraints for the Cenozoic growth of the northeastern Tibetan Plateau.Together with previous studies about the Cenozoic tectonic deformation in the northeastern Tibetan Plateau,the following conclusions are drawn:(1)The stratigraphic investigation on the Cenozoic strata in the Jiuxi Basin suggests the Cenozoic sedimentary faices are alluvial fan to fluvial and floodplain faices during the deposition of the Huoshaogou Formation(-40-33 Ma),fluvial and floodplain to delta-shallow lake faices during the deposition of the Baiyanghe Formation(?24-17 Ma),and fluvial and floodplain to distal fan faices during the deposition of the Shulehe Formation(?17-5 Ma).Shifting of the sedimentary faices indicates the Cenozoic deposition in the basin is a generally upward-thinning sequence followed by an upward-coarsening sequence,and experienced a major reorganization of the basin infill process at the period between depositions of the Baiyanghe and Shulehe formations at?17 Ma.(2)Provenance analyses based on detrital zircon U-Pb dating of the three Cenozoic formations in the Jiuxi Basin,reveal a shifting of the provenance from the Bei Shan-Kuantan Shan-Hei Shan area in the north to the North Qilian Shan in the south.This provenance shift is also documented by the paleocurrent directions and occurred at the period between depositions of the Baiyanghe and Shulehe formations at?17 Ma.Provenance analyses,together with Cenozoic stratigraphic investigation in the Jiuxi Basin,indicate the basin experienced a two-stage Cenozoic sedimentary evolution:(a)during the Eocene to the middle Miocene(?40-17 Ma),the Kuantan Shan-Hei Shan-Bei Shan area was experiencing a gradually weakened erosion,and provided sediments to form the generally upward-thinning sequence from the Huoshaogou to Baiyanghe formations in the Jiuxi Basin.(b)during the middle Miocene to present(?17-0 Ma),the North Qilian Shan rapidly uplifted with gradually enhanced erosion,and provided sediments for the basin to form the upward-coarsening sequence from the Shulehe Formation to younger strata.(3)Apatite fission track(AFT)analyses and corresponding thermal history modeling in the Kuantan Shan and Hei Shan area reveal two Cenozoic exhumation events:the first one occurred during the Eocene(?49-32 Ma)and the second one occurred since the middle-late Miocene(?14-10 Ma).Combined with previous studies about the early Cenozoic tectonic deformation in the northeastern Tibetan Plateau,this paper proposes that the KHHL(Kuantan Shan-Hei Shan-Heli Shan-Longshou Shan)fault system in the northern margin of the Hexi Corridor Basin,was an existing weak zone before the India-Eurasia collision,and formed the present northeastern margin of the Tibetan Plateau.(4)Integration of previous studies on thermochronology,sedimentology and other methods,together with results documented in the Jiuxi Basin in this study,suggests a two-stage Cenozoic evolution for the growth of the northeastern Tibetan Plateau,as the following:(a)during the Paleocene to Eocene(-65-35 Ma)at the initial India-Eurasian collision,the northeastern margin of the present Tibetan Plateau,the KHHL(Kuantan Shan-Hei Shan-Heli Shan-Longshou Shan)as an pre-existing weak zone,was reactivated attributed to an instantaneous and simultaneous far-field response to the collision;(b)during the late Eocene to present(?35-0 Ma),the significant surface uplift gradually propagated towards the northeast and arrived in the North Qilian Shan at?17 Ma due to continuous mountain-building processes from south to north in the northeastern Tibetan Plateau.