Cenozoic Sedimentary Evolution And Subsidence Processes In The Southwestern Tarim Basin And Its Implications For The Growth Of Western Kunlun,northwestern Tibetan Plateau

The collision between Indian and Eurasian plates in the early Cenozoic has led to the construction of the Tibetan Plateau,which is known as the third pole of the world.The India-Eurasia collision and continuous convergence have created a broad intracontinental deformation domain around the Tibetan Plateau lying thousands of kilometers from the collision front,and this deformation domain is termed as the Circum-Tibetan Plateau Basin and Orogen System(CTPBOS).The Cenozoic sedimentary basins in the CTPBOS supply a key to decipher the regional tectonic deformation and uplift history of the Tibetan Plateau.Understanding the tectonicsedimentary evolution of these Cenozoic basins is not only significant to enhance knowledge of the Cenozoic growth and deformation mechanism of the Tibetan Plateau,but also important for guiding oil and gas exploration.In this study,we focus on the Cenozoic sedimentary evolution in the southwestern Tarim basin through an integrated study of detailed field stratigraphic investigation,provenance analyses through detrital zircon U-Pb chronology,and subsidence process based on the thickness statistics of the Cenozoic strata.This study,in combination with previous magnetostratigraphic analyses,tectonic deformation analyses and thermochronological studies,has established the Cenozoic sedimentary evolution and subsidence process of the southwestern Tarim Basin and sheds new light on the Cenozoic uplift and growth processes and related geodynamics of western Kunlun in the northwestern Tibetan Plateau.The following conclusions are drawn.1.Based on field stratigraphic investigation of three Cenozoic sedimentary sections in the southwestern Tarim Basin,the variation of the sedimentary facies was established.The sedimentary facies are shallow marine-lagoon during the deposition of the Aertashi to Wulagen Formations,littoral delta during the deposition of the Bashibulake Formation,fluvial-distal alluvial fan during the deposition of the Keziluoyi Formation,lacustrine delta-lacustrine during the deposition of the Anjuan Formation,fluvial-distal alluvial fan during the deposition of the Pakabulake Formation,middle alluvial fan during the deposition of the Artux Formation and proximal alluvial fan during the deposition of the Xiyu Formation.These results show that the southwestern Tarim basin underwent a transition from marine to continental deposition,showing a general trend from low-energy to high-energy deposition,with a secondary reverse cycle from the Keziluoyi Formation to the Anjuan Formation.The continental sedimentary suquence displays a typical foreland basin deposition.2.Provenance analyses based on detrital U-Pb dating of the Cenozoic sediments from the Keliyang and Kashitash sections in the southwestern Tarim Basin reveal a significant shift of the basin provenance during the deposition of the Artux Formation.The sedimentary materials from the Bashibulake,Keziluoyi,Anjuan and Pakabulake Formations are mainly from Songpan-Ganzi and South Kunlun terranes,in contrast to those from the Artux Formation which are mainly originated from the South Kunlun and North Kunlun terranes.3.Based on the Cenozoic stratigraphic thickness statistics,the ratios of thickness to distance(RTD,ratio of the stratigraphic thickness to the distance from the frontal fault)reveal the subsidence variation of the southwestern Tarim Basin.The results show that the thickness of the Aertashi to Bashibulake Formations slightly thickens southwards,with a RTD value of ?-1.71 m/km,which shows insignificant weak tectonic subsidence at this stage.The thicknesses of The Keziluoyi,Anjuan and Pakablak Formations increase southwards significantly.The RTD values are three times as large as those of the Aertashi to Bashibulake Formations,which suggests significant tectonic subsidence during this time.The thickness of the Artux Formation increases and then decreases southwards,which indicates the northward migration of the depocenter of the basin by ?22 km during this period.The thickness of the Xiyu Formation continues to increase and then decrease southwards,which suggests another northward migration of the depocenter by ?34 km during this period.These results reveal that the depocenter of the southwestern Tarim foreland basin has been moving northwards since the Artux Formation period.4.Based on the aforementioned analyses,this study synthesises seven sedimentary sections in the southwestern Tarim Basin and determines the sedimentary characteristics and the stratigraphic division of the Cenozoic strata.These results,in combination with the Cenozoic stratigraphic age framework,reveal the sedimentary evolution and subsidence processes of the southwestern Tarim Basin,which has been divided into four major stages.(1)During the Paleocene to Eocene(?65-36.5 Ma),due to the transgression and regression of the Paratethys Sea and the tectonic activities in the Songpan-Ganzi and South Kunlun terranes,the southwestern Tarim Basin underwent weak tectonic subsidence and shallow marine to littoral sedimentary sequences were deposited in the basin.(2)During the Oligocene(?33-22.6 Ma),the basin began to experience significant tectonic subsidence and the sedimentary environment changed into continental sedimentary environment,which indicate the onset of foreland basin along the southwestern Tarim Basin.The sequnences form a generally upward-coarsening sequence interrupted by a subordinate upward-thinning sequence from the Keziluoyi to the Anjuan Formations,which is interpreted as a result of gradual northward approach of the foredeep of the foreland basin system.During this period,the sediments were sourced from the Songpan-Ganzi and South Kunlun terranes.(3)During the early to middle Miocene(? 22.6-15 Ma),the provenance of the basin shifted from the Songpan-Ganzi and South Kunlun terranes to the South Kunlun and North Kunlun terranes,with coarse-grained sediments deposited in the southwestern Tarim Basin.The depocenter of the basin migrated northwards by ?22 km at this time.(4)Since the middle Miocene(?15 Ma),the sediments in southwestern Tarim Basin changed into massive conglomerates,which indicates a high-energy proximal alluvial fan environment.The depocenter of the basin has migrated northwards by ?34 km and construct the the basin architecture nowadays.5.This study,in combination with previous tectonic deformation analyses and thermochronlogical studies,proposes a co-evolution model of Cenozoic western Kunlun and the southwestern Tarim Basin,which has been summarized into three major stages.(1)During the Paleocene to Eocene(?65-36.5 Ma),the uplift of western Kunlun was mainly located in the Songpan-Ganzi-Tianshuihai and South Kunlun terranes,with insignificant influence on the southwestern Tarim basin.(2)During the Oligocene(?33-22.6 Ma),the Songpan-Ganz-Tianshuihai and South Kunlun terranes continued to uplift and supplied detritus to the southwestern Tarim Basin,which indicated the onset of the southwestern Tarim foreland basin.The depocenter(foredeep)probably migrated northwards during this period.(3)Since the Miocene(?22.6-0 Ma),the North Kunlun terrane has been uplifted and supplied detritus to the southwestern Tarim foreland basin together with the South Kunlun terrane.The depocenter(foredeep)migrated northwards by at least ?56 km.6.An integration of northward migration of the orogenic wedge in previous studies,shift from low-to high-energy deposition,provenance shift from the Songpan-Ganzi and South Kunlun terranes to the South Kunlun and North Kunlun terranes,and northward migration of the depocenter by at least ?56 km in this study,leads this study to propose the upper-crustal boundary between western Kunlun and the Tarim basin to be a movable feature.These outcomes lend support to the proposal that substantial lower Tarim plate(?56 km if calculated from the magnitude of the northward depocenter migration)has subducted southwards beneath western Kunlun.7.These results,in combination with southward migration of enhanced exhumation in western Kunlun,lead to a speculation that the growth of the northwestern Tibetan Plateau can be divided into two stages.At Stage I during the Eocene to Oligocene,the terranes,including the Songpan-Ganzi,South Kunlun and North Kunlun terranes that constutite the interior of western Kunlun,were uplifted gradually from south to north,until the North Kunlun terrane was uplifted and supplied detritus for the southwestern Tarim Basin.At Stage 2 since the Miocene,the deformation has migrated both northwards into the southwestern Tarim Basin and southwards into the South Kunlun and Songpan-Ganzi terranes bounded by the Tiklik fault.This process results in fold-and-thrust belts of the SW Tarim Basin and a southward younging trend of thermochronological age in the interior of western Kunlun.This two-stage evolution possibly reflects a change of the Cenozoic growth mechanism of the northwestern Tibetan Plateau,and we speculate that the first stage is controlled by the northward indentation of the Indian plate,while the second stage is governed by the southward underthrusting of the lower crust and lithospheric mantle of the Tarim plate under western Kunlun.