Tectonic Chronology Constraint On The Main Faults Bounding The East Sichuan-Yunnan Block And Implications For Tibetan Plateau Kinetics

Sichuan-Yunnan block in East Tibetan Plateau is a transition zone of topography from the higher Tibet Plateau to the lower Yangtze block. This area had experienced the collision between the India Plate and the Eurasia Plate, along with crustal shortening and thickening, and finally result in the eastern margin of the Tibet Plateau. Based on the timing of structural deformation, rock cooling and exhumation, and tectonic uplift, the critical chronological records conduce to establish spatial-temporal framework of the tectonic evolution in Sichuan-Yunnan block in order to cast enlightenment on dynamic mechanism of Cenozoic Tibet Plateau formation. However, the current limited studies restricted the awareness of this very complex expansion and growth process of the east Tibetan Plateau.Thesis on the basis of the field investigation in the east Sichuan-Yunnan block, the Cenozoic geological and structural geomorphology along two typical tectonic belts, the Xianshuihe fault and the Litang fault, and its adjacent areas for the study. Combined observations across faults and interpretation of active structual geomorphology by remote sensing images, we clarify the geological units around the study area, the spatial distribution characteristics of fault zones, and then analyze the properties of the geometry and kinematics on the cross-sections within the fault zones, which reveal the basic characteristics of structural deformation and active periods. We collect bedrock samples along key atitulde profiles to date and analyze zircon U-Pb, apatite (U-Th)/He, apattite and zircon fission track ages. Three-dimensional thermal-kinematics numerical modeling precisely constrains the spatial-temporal evolution along the fault zone and surroundings to resolve fault activity history and tectonic uplift. The corresponding swaths extracted from digital elevation models aid to reveal influence factors between the tectonic exhumation and surface erosion process. Comprehensive discussion including previous studies help us to establish spatial-temporal framework of the tectonic evolution in Sichuan-Yunnan block since Cenozoic to reveal the kinetics of the eastern margin of Tibet plateau.Analysis of structural deformation phases related to the fault activity and zircon U-Pb dating show, Zheduoshan granite along the southeast segment of the Xianshuihe fault consists of three rock units with different structural deformation and ages ranging from Jurassic to Miocene, and it means the Xianshuihe fault has experienced long-term and complex activity history. A small piece of mylonitized coarse porphyritic monzonite granite flanks the western edge of the Zheduoshan granite with strong oriented foliation, implying it is syntectonic granite. The pegmatitie dyke with weaker oriented foliation invaded mylonitized granite slightly later, yields zircons U-Pb age of-155Ma, which limited the emplacement to Jurassic era.The onset of the Cenozoic Xianshuihe fault is Oligocene ～32-27Ma. The syntectonic ductile deformation markers and kinematics indicate, it results from the initial activity of the compressive dextral shearing. The Miocene Zheduoshan granite is a set of undeformed massive structure uniform fine grained biotite monzonite granite, and intruded into the Jurassic and Oligocene deformed units, which proves not to be syntectonic granite, as commonly suggested. Zircon U-Pb dating of igneous rocks indicate the main emplacement is between ～19-15 Ma, occurred in the south and then to the north by the second episode or pulsatile emplacement.Our apatite and zircon fission track results combining previous 39Ar/40Ar data, constrain the Zheduoshan granite experienced three stages of cooling exhumation stage of evolution since the emplacement of early Miocene. The first stage during the period of ～15-9 Ma after emplacement, experienced a slow cooling exhumation, exhumation rate is between 0.01-0.1 km/Ma. The second stage represents initial activity of the Xianshuihe sinistral strike-slip fault since ～9 Ma, leading to the strong exhumation period between ～9-4 Ma that related to the regulation of the Xianshuihe compressive faulting. Intense sinistral strike-slip movement causes the overall uplift of rock block along these faults, and forming ～20 km width strip of active high-altitude mountain topography. The cooling and exhumation have migrated from the south of Zheduo section to north of the Longbu and Taizhan sections. The total amount of exhumation is ～9 km. There is significant differential uplifting between the exhumated ranges and the hinterland plateau surface with minor exhumation during Cenozoic, by comparasion the different depth of the paleo-partial annealing zone, vertical offset across the fault reaches ～7 km, indicates a large part of the actually horizontal slip rate transformed to vertical exhumation rate on the main structure of exhumation. Latest period of early Pliocene since ～4 Ma, exhumation rate significantly reduced in all three sections, at the meantime, a large number of fine pegmatite veins with U-Pb age of 4 Ma intruded the migmatite zone, accompanied by severe cooling ～330° C/Ma, magmatic hydrothermal partial melting as a reulst of decompression melting along the fault. It is contemporary with the sedimentary response along the Anninghe-Xiaojiang fault era, which means Xianshuihe fault extended to Anninghe-Xiaojiang fault at the time. Cumulative total offset along western border of Zheduoshan granite is about ～52km, defining the minimum horizontal slip rate of ～4.3-5.7 mm/yr.Litang fault in the central of the Sichuan-Yunnan block, we observed geometry, structural deformation, and kinematic of the fault, and interprete the complex fault zone and its relationship with pull-apart basins by remote sensing image of Landsat ETM+. It consists of the North Cuopu, the north Maoya, Litang, South Jawa and Dewu fault, indicating a larger extending than previously. The analysis of geometry and kinematic shows that the spatial distribution of the fault segments stretching en echelon or slightly oblique to absorb part of the left-lateral motion by the pressure releasing bends. The left-lateral motion is contemporaneous with the normal component along with all segments. Our apatite (U-Th)/He, apatite and zircon fission track data, precisely constrain exhumation history of the footwall bedrock due to the onset of the Litang fault system since 5-7Ma. Total exhumation increase from north Maoya of ～3.8 km northwest to north Cuopu of ～5.3km, which means tectonic uplift controlled by fault zone expanded northwestward, the corresponding minimum vertical offset also increased from ～1000 m to ～1700 m. Fault zone cut pre-existing Garzi-Litang suture zone and Yidun arc belt, the offset reached 6-14 km, average slip rate of ～0.9-2.8 mm/yr, which means Litang fault system is a transtensive zone and play a role in accommodating the eastward extrusion of the Sichuan-Yunnan block.Combining comprehensive studies, tectonic and geomorphic evolution of the Sichuan-Yunnan block are roughly divided as 37-20 Ma,17-9Ma and 7-4Ma three main stages. 37-20 Ma, major boundary faults of the Sichuan-Yunnan block started initial activity since Cenozoic. The Xianshuihe fault activity, forming a large northern boundary fault. Meanwhile, along the southern border Jiali and Ailaoshan-Red River fault constitute the large southern strike-slip faults. This two boundary faults of the Sichuan-Yunnan block occurred limited extrusion. Records of the low-temperature thermochronology indicate, Sichuan and Yunnan block surface stayed as tectonic quiet period.-17-9 Ma, strong thrust along Jinhe-Jinghe Fault and Xianshuihe fault compressional left-slip activities, shortening and thickening of Plateau accompanied by the main rivers of the region under severe cutting erosion and partial response of deposition.7-4 Ma, the collision of the Indian plate subduction squeeze horn continued northward migration, a massive reorganization process of the Sichuan-Yunnan block faults occurs, the fault zone to convert the transtensive faults. The formation of today’s tectonic framework of Sichuan and Yunnan block mainly occurred during the later two stages. Multi-stage activities along Xianshuihe fault since the Oligocene and Miocene transtensive Litang fault is not supported by lower crustal flow model. The Sichuan-Yunnan block significant rotated during the period of the early to mid-Miocene, and thus it can not be a good model with limited extrusion since mid Miocene. Therefore, we favor "hidden plate-tectonic" model, such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation.