Late Mesozoic-Cenozoic Deformation And Uplift Processes Of The Daliang Shan Tectonic Belt Along The Southeastern Margin Of The Tibetan Plateau,and Its Dynamic Mechanism

The Daliang Shan（“Shan”means mountain in Chinese）Tectonic Belt along the southeastern margin of the Tibetan Plateau is located in the transfer zone between the Western Pacific tectonic domain and the Tethys-Himalaya tectonic domain.The Daliang Shan Tectonic Belt is an ideal site for revealing the westward extension of South China Plate and the eastward growth of the Tibetan Plateau.However,systematic and detailed studies of structural deformation and uplift-exhumation processes of the Daliang Shan Tectonic Belt are still lacking.In this study,based on the detailed geological survey and structural analysis along regional frame profiles,we record the isotopic dating of fault rock and multiple low-temperature themalchronological studies,in order to find out the structural geometry and kinematic characteristics of the Daliang Shan Tectonic Belt,its tectonic deformation periods and associated deformation timing,and constrain its uplift-exhumation history.Integrated with previous studies,this study allow us to reconstruct the tectonic evolution of Daliang Shan Tectonic Belt during the late Mesozoic to Cenozoic,and reveal detail of the westward extension of South China Plate during the late Mesozoic and the eastward growth of the Tibetan Plateau during the Cenozoic.The main conclusions in this study are as follows:（1）Daliang Shan Tectonic Belt is bounded by the Daduhe River and is close to Daxiangling Tectonic Belt to the north,the Ebian-leibo Fault to the east,the Lianfeng Fault to the south and close to the Daloushan Tectonic Belt in the west of Xuefeng Tectonic Belt,and the Xiaojinhe Fault in the west.The Upper Triassic-Early Cenozoic in the study area formed the Mesozoic Xichang Basin,which was unified with the Sichuan Basin to form the proto-Sichuan Basin during the Mesozoic,and experienced Cenozoic strong structural transformation and uplift-exhumation processes.The Daliangshan Tectonic Belt is a Cenozoic intraplate orogenic belt deforming the Mesozoic Xichang Basin,and represents an intraplate gradual basin-mountain boundary in the southwest margin of the present Sichuan Basin.（2）Three orientations of structures in the Daliang Shan Tectonic Belt have been identified and are approximately SN-,NW-and NE-trending.In the study area,the deformation strain is generally weak,and the strata generally dip with low-angle or nearly horizontal,and are characterized by open-closed folding.The strong strain zones are mainly concentrated near the fault zone,and the strata steep dip or even reversed to form box or inverted folds.Based on the structural analysis and paleostress inversion of the conjugate X-type joints,magnetic fabrics,fault scratches and superimposed folds,it shows that the Daliang Shan Tectonic Belt has experienced at least five-phase of deformation events in succession:NW-SE compressional fold→NE-SW compressional fold and thrust→nearly EW compressional thrust→NW-SE compressional transition compression→and NNW-SSE compressional strike-slip deformation.（3）On the main fault system（Anninghe Fault,Zemuhe Fault,Shimian-Zhaojue Fault）isotopic dating of brittle faults was carried out using fault gouge authigenic illite K-Ar dating and calcite in-situ LA-MC-ICP-MS dating.Fault gouge was separated into four size fractions of 1-0.5（or 2-0.5）,0.5-0.2,0.2-0.1 and<0.1μm,which were then analyzed by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction analysis and K-Ar isotopic dating.The clay minerals in each size fraction mainly include illite and chlorite,with the illite characterized by 2M₁,1M_d and a small amount of 1M polytypes.The K-Ar age and the proportions of 2M₁ illite relative to 1M/1M_d illite progressively decrease with decreasing grain size fraction.SEM and TEM imagery of the finest size fraction（<0.1μm）confirm that most illite crystallites are characterized by well-preserved prismatic platy and fiber habits,suggesting in situ neocrystallization of illite.K-Ar ages for different size fractions are mainly impacted by the physical mixture of high temperature 2M₁ illite and low temperature 1M_d/1M illite and are hence characterized as mixture ages.We suggest that the 2M₁ illite content which dominates the coarser size fractions represents either a detrital compontent inherited from the host rock or may be an authigenic product formed during the earlier ductile deformation.The1M_d/1M illite component which is dominant in the finest size fraction represents the timing of brittle faulting.Based on illite age analysis method,we determined authigenic 1M_d/1M illite K-Ar ages of Late Eocene-Early Oligocene（～50-28Ma）for the Anninghe Fault,Zemuhe Fault and Shimian-Zhaojue Fault,indicating fold-and-thrust deformation during the early Cenozoic in the study area.The illite age analysis of the fault gouge collected from the Moxi Fault in the south segment of Xinshuihe Fault revealed～20Ma aged 2M₁ illite and～9Ma aged 1M_d illite,which represented the Early Miocene ductile faulting and Late Miocene brittle sinistral strike-slip faulting,respectively.LA-MC-ICPMS dating of syn-tectonic calcite veins from the Shimian-Zhaojue Fault yielded U-Pb age of～8-4Ma,indicative of the sinistral strike-slip deformation in the study area since the Latest Miocene.（4）Multiple low-temperature thermochronology analysis combined with thermal history modeling of the Ma’an Shan,Xiaoxiangling surface elevation profiles and drill-hole Xide-1 depth profile suggested that the Daliang Shan tectonic belt experienced multiple-stage heterogeneous cooling and exhumation processes during the Cenozoic.The profiles of Xiaoxiangling and drill-hole Xide-1 in the study area reveal that the sedimentary burial temperature increased from Late Triassic to Early Cenozoic in the study area.After reaching the maximum burial depth during～50-40Ma,the study area experienced a slow cooling and exhumation process during～40-25Ma.Three rapid cooling and uplift events occurred successively during late Oligocene-early Miocene（～25-20Ma）,Middle Miocene（～16-12Ma）and Late Miocene（<～8Ma）,with exhumation rates of～0.57-1.3,～0.12 and～0.25km/Ma,respectively.The Ma’an Shan area in the north of the study area experienced a slow uplift-exhumation process during～20-10Ma,followed by two rapid uplift-exhumation events during the late Miocene（～10-7Ma）and late Pliocene-early Pleistocene（～2Ma）,with exhumation rates of～1.23 km/Ma and～0.57-1.07km/Ma,respectively.Given that the present geothermal gradient of～30°C/km and surface temperature of～10°C,we suggest that the Daliang Shan Tectonic Belt underwent exhumation magnitude of～5.7km during the Cenozoic.（5）Integrating tectonic deformation and low-temperature thermochronology studies from the western South China Fold Belt,as well as the central Tibetan Plateau and its periphery,we have reconstructed four-stage tectonic evolution model of the Daliang Shan Tectonic Belt from the Late Mesozoic to Cenozoic.（a）.During the Late Cretaceous to Early Cenozoic,the south margin of Daliang Shan Tectonic Belt underwent fold deformation under NW-SE-oriented compression,which reduced the extent of the Proto-Sichuan Basin.These were triggered by the northwestward extension of South China Plate responding to the westward rapid subduction of the western Pacific Plate during the late Cretaceous.（b）.During Eocene-Early Oligocene（～50-28Ma）,the far-field compressional force of the hard collision between the India and Eurasian Plates resulted in fold-and-thrust deformation and associated slow uplift-and-exhumation process of the Daliang Shan Tectonic Belt under the NE-SW-oriented compression,which had contributed to the destruction of Proto-Sichuan Basin.（c）.The Daliang Shan Tectonic Belt underwent further intense thrust faulting and uplifting during the Late Oligocene to Early Miocene due to E-W-oriented compression,which can be attributed to the lateral horizontal compressional pressure resulted from the establishment of the present high elevation and flat topography in the central Tibetan Plateau.（d）.The Daliang Shan Tectonic Belt experienced two-stage extrusion since the early Miocene,which was triggered by decoupling deformation between the lateral extrusion of the upper crust of the Chuandian block and its middle-lower crust channel flow.During～20-10Ma,the study area experienced transformation compression deformation under the NW-SE compression within the influence of sinistral strike slip of Xianshuihe Fault.Since～8Ma,with the final connection of the Xianshuihe-Anninghe-Xiaojiang Fault system,the present tectonic pattern of Chuandian block demonstrated clockwise rotation around the eastern Himalayan syntaxis.Sinistral strike-slip deformation also occurred in the time period,caused by NNW-SSE-oriented compression.