Neogene Deformation Of The Litang Fault System In Eastern Tibet And Its Implications On Regional Geodynamics

Since the Cenozoic,the collision and convergence between the India and Asia plates created the most prominent morphotectonic unit on Earth-Tibetan Plateau.How the plateau has deformed and expanded is one of the fundamental scientific questions,which is critical to the continental geodynamics.In particular,the mechanisms of the large-scale strike-slip faults in the plateau interior has accommodated the stress of the India-Asia collision remains at the center of debate.There are two competing models.One model suggests that major strike-slip faults play a significant role in the evolution of the Himalayan-Tibetan orogen by facilitating the eastward lateral extrusion of mega-blocks(or microplates)of the Tibetan Plateau.The faults act as localized deformation concentrated on the boundaries of rigid blocks at high slip rates.The other model suggests a limited role of such faults in making discontinuities in the velocity field.The faults,behaving as diffuse structures at a lower slip rate,serve as passive markers in the deformation field of the brittle upper part of the lithosphere.The ~400-km-long Litang fault system across the Chuandian block serves as a crustal-scale active strike-slip fault zone,and thus provides a unique opportunity to examine how intracontinental major strike-slip faults have accommodated stress propagated laterally from the plate boundaries.Based on integrated approaches of remote sensing,structural analysis,and paleostress inversion,this thesis conducted new mapping on the central segment of the LTFS in the Chuandian block,eastern Tibet.The mapping results offer new constraints on the geometry,kinematics,and paleostress of the fault system.Apatite,zircon(UTh)/He and apatite fission track thermochronology combined with thermal modeling allow a reconstruction of complete cooling and exhumation histories of the rocks in the fault zone.The main conclusions include:(1)The geometry,kinematics,and pattern of the Litang fault system is reappraised.The Main Litang fault zone splits into 5 strands forming left-lateral,right-stepping enechelon faults under NW-directed compression.These faults have been inherited by transtensive structures under N or NNE-directed extension that is composed of left-lateral,left-stepping en-echelon faults,including Cuopu fault,Maoya fault,Litang fault,Huncha fault,Jiawa fault and Dewu fault that control Neogene-Quaternary basins.(2)Thermal modeling with multiple low-temperature thermochronology data within the fault zone reveals three episodes of accelerated cooling and exhumation at 38-35 Ma,16-13 Ma,and 7-5 Ma.The late Eocene rapid exhumation can be ascribed to the reactivation of the Garze-Litang suture and relevant compressive structures controlled by NE-directed compression associated with the northward under thrusting of the Indian lithosphere.The middle Miocene and late Miocene-Pliocene rapid exhumation are interpreted to record the onset of transpression and transtension of the LTFS,respectively.Using accumulative left-lateral displacement of 33.6-47.1 km,the long-term average horizontal slip rate of the fault system is 2.85 ± 0.75 mm/yr.(3)The incipient development of the Litang fault system coeval with other major strike-slip faults in the Chuandian block indicates a significant kinematic transition in the middle Miocene in eastern Tibet.The rigid mega-block extrusion and rotation at an earlier stage have been replaced by a new deformation style predominated by micro-block extrusion,which has been accommodated by both localized deformation on the block margins and distributed deformation in the block interior.We propose that a modern-like deformation pattern has been largely established since the middle Miocene times in eastern Tibet.