Cenozoic Tectonic Evolution Of The North Altyn Fault

Revealing how the Tibetan plateau-bounding faults have evolved in time can effectively constrain the uplift and expansion mode of the plateau,which is not only of scientific significance in deciphering the dynamics of intracontinental orogeny,East Asian monsoon evolution,and the migration of Asian hominin,but also of practical importance in the evaluation of intracontinental seismic disasters and the exploration of ore deposits in bedrock terrains and oil-gas fields in basins.The North Altyn fault is located on the northern edge of the Tibetan Plateau,separating the rigid Tarim basin to the north from the relatively soft Tibetan Plateau to the south.However,the Cenozoic tectonic evolution of the North Altyn fault and its role in the rise and expansion of the northern Tibetan Plateau is not clear.This work investigated the geometry and kinematics of the North Altyn fault,based on remote sensing analyses and structural mapping in the fault zone,together with the subsurface deformation through interpretations of seismic reflection profiles in the southeastern Tarim Basin.We then determined the deformation timing of the North Altyn fault,which is constrained by the exhumation history of Altyn Shan based on low-temperature thermochronology and the sedimentary response in the southeastern Tarim Basin.Finally,we reconstructed the Cenozoic tectonic evolution of the North Altyn fault,and explored its implications for the middle Miocene reorganization of the Altyn Tagh fault system and the growth of the northern Tibetan Plateau.The following innovative findings are drawn.1.The western segment of the North Altyn fault is a NEE-trending,basementinvolved high angle(>45°)fault,with dip direction varying along strike,which together with two newly identified Qigelek fault and Xinjianglisai fault,constitutes a narrow(<20 km)transpressional shear zone,producing Cenozoic shortening of ～9-12 km by compression component.The North Altyn fault goes eastward along the Jianglisai fault and enters into the Altyn Shan,supporting large-scale(>120 km)left-slip movement on the fault.To the north of the fault,the southeastern Tarim Basin features very weak Cenozoic deformation,with a maximum shortening amount of only ～0.85 km.The Cherchen fault is not the deformation front of the North Altyn fault,as its Cenozoic activity only occurred west of the North Altyn fault;the two faults may behave as complementary structures to absorb the strike-slip deformation north of the Altyn Tagh fault.2.Bedrock fission-track analyses and thermal modelling results indicate that,the North Altyn fault zone and Altyn Shan were extensively buried by sedimentation during the early Cenozoic to the middle Miocene,which was locally interrupted by ～40 to 35 Ma exhumation proximal to the fault;Since ～17-15 Ma,the North Altyn fault zone and Altyn Shan have experienced widespread but heterogeneous exhumation: the blocks in the fault zone tilted from west to east along the fault strike,and the Altyn Shan to the south tilted towards the southeastern Tarim Basin.This result is supported by the sedimentary response of southeastern Tarim Basin: the Paleogene and first member of Neogene(～66-16 Ma)with stable stratum thickness were deposited in a broad lake,which may cover the Altyn Shan and be connected with the northwestern Qaidam Basin;the second member of Neogene and upper strata(≤16 Ma)has accumulated as a set of upward-coarsening molasse sequences,signifying that the Altyn Shan to the south started to rise at ～16 Ma.3.The North Altyn fault experienced two stages of evolution in the Cenozoic.During Eocene(～40-35 Ma)to Miocene(～17-15 Ma),the North Altyn fault was a leftslip fault with deformation concentrated on the fault plane;the Jinyanshan thrusts as the eastern terminal structure absorbed the strike-slip offset,leading to the uplift and exhumation of the Jinyanshan.Since the middle Miocene(～ 17-15 Ma),the North Altyn fault has shifted from left slip to transpression,resulting in strong exhumation of Altyn Shan and compressive deformation of southeastern Tarim Basin accompanied by leftslip transpressional deposition.This finding provides independent evidence supporting the middle Miocene reorganization of the Altyn Tagh fault system: in the early Cenozoic,the North Altyn fault was the main strike-slip boundary on the northern edge of the Tibetan Plateau;at ～17-15 Ma,the middle part of the Altyn Tagh fault formed and propagated rapidly to its northeastern end,rendering the North Altyn fault a transpressive fault with a more reverse component.Thus,whether it is the North Altyn fault in the early Cenozoic or the Altyn Tagh fault formed in the middle Miocene,their left-slip movement indicates that there is a long-term strike-slip boundary between the northern margin of the Tibetan Plateau and the Tarim Basin in the Cenozoic.4.This study demonstrates that the Tarim Basin did not undergo large-scale intracontinental subduction along the North Altyn fault beneath the Tibetan Plateau,as indicated by:(1)there is no foreland architecture developed in the southeastern Tarim Basin,as Cenozoic sequences do not thicken towards the Altyn Shan as a wedge shape;instead they thin towards the mountain due to the basal shear of the North Altyn fault;(2)no fold-thrust belt is developed in the southeastern Tarim Basin,where the Cenozoic compressive structure is occurred mainly by converting Jurassic normal faults,featuring with basement-involved fault propagation folds;(3)the Cenozoic total shortening of southeastern Tarim Basin and North Altyn fault zone is no more than 20 km,which does not meet the requirement of ～100 km shortening at front of Altyn Shan under large-scale subduction.