Cenozoic Southeastern Progressive Growth Of Sichuan-yunnan Block,Southeastern Tibet Plateau

The collision between the Indian Plate and Eurasian Plate during the Cenozoic created the Tibet Plateau,known as the third pole of the world.The generation of the Tibet Plateau has a profound impact on today’s geomorphology and water system pattern.Its uplift also dominates the formation and evolution of the eastern Asian monsoon,and has a global impact on climate and environmental changes.Therefore,the study of the Tibet Plateau growth and its mechanism is vital for the human to understand the crust process,climate as well as human living environment change.In this case,southeastern Tibet play a significant role in the research on the plateau growth dynamical model.The geomorphic studies show the stepped structure presented in southeastern Tibet.The topographic boundaries of these steps are corresponded to the Muli and Jinhe-Qinghe thrust,indicating the fault-controlled geomorphic generation.Therefore,this study focuses on the Muli and Jinhe-Qinghe thrust in the Sichuan-Yunnan block,which is identified as the tectonic-geomorphic boundary of the SE Tibet.The detailed field structure observations and low-temperature thermochronology dating were conducted to reveal the kinematics,as well as the potential differential exhumation history across the fault.Furthermore,the sedimentary sequence and provenance studies were conducted on the Paleogene sediments in the Yongsheng basin,to explore the evolution of drainage patterns and its relation with the tectonic process in SE Tibet during Eocene-Oligocene.Finally,the Cenozoic tectonic-geomorphic evolution history and corresponding dynamic mechanism of the SE Tibet Plateau are discussed based on this study and other published data.The NE-SW trending Muli fault is considered as the first tectonic-geomorphic boundary of SE Tibet.Its field observation shows the Muli fault is characteristic by the Permian limestone or basalt thrust on top of the Triassic sediments.The klippe structure at the front of the fault indicates its horizontal thrusting distance is more than 13 km.The age-elevation profile collected in the hanging wall of the fault and subsequent QTQt inverse thermal modeling documented two episodes rapid cooling events.The differential cooling history is documented across the Muli thrust,indicating fault-controlled differential uplift and subsequent exhumation in the hanging wall.Hence,the early thrust movement of the Muli thrust is during ～30-20 Ma,and re-activated between ～12-10 Ma.The more rapid exhumation rate during 12-10 Ma is triggered by the positive feedback between the re-activated of the thrusting and regional fluvial erosion.The field structure analysis of the Jinhe-Qinghe fault shows it trends parallel to the Muli thrust,and transferred the Sinian dolomite or Paleozoic strata on top of the Mesozoic sedimentary.The kinematics of the fault is dominated thtust motion with a minor leftlateral component in some areas.Several cross-sections indicate the vertical displacement triggered by the Jinhe-Qinghe thrust is estimated to be 0.6-3.6 km.The low-temperature thermochronology samples were collected from the Baishagou granite,in the hanging wall of the Jinhe-Qinghe thrust.Its age-elevation relationship and corresponding inverse thermal modeling result documented ～20-16 Ma fast cooling event.A differential cooling history existed when compared with the published thermal history documented in the footwall of the Jinhe-Qinghe thrust.This fast exhumation is not contemporaneous with the climate change or regional river incision event.Therefore,we proposed it was caused by the fast exhumation of the Baishagou granite as a result of the uplift in the hanging wall.That means the Jinhe-Qinghe thrust is active during the 20-16 Ma.To reveal the impact of the southeastward expansion of the Tibet Plateau on the drainage pattern evolution,the sedimentary sequence and provenance analysis was conducted for the Yongsheng basin,located between the Muli and Jinhe-Qinghe thrust.The new sedimentary sequence study divided Ninglang formation into four units.The field lithology and sedimentology indicate the sedimentary facies of unit1 to unit4 are piedmont to fluvial facies,shore-shallow lacustrine facies,fluvial facies,and lacustrine facies respectively.The depositional age of unit4 is constraint by the mean UPb age of the youngest five detrital zircons and the U-Pb age of the granite intruded into the unit4 sediments.Combined with published work,the depositional age for unit1 to unit3 is between the Early to Middle Eocene,and ～44-33 Ma for unit4.The provenance analysis is conducted to reveal the interrelationship between different Eocene basins in SE Tibet.The multidimensional scaling result shows a major southward river draining the Gonjo,Jiahcuan and Midu basins,of which the Yongsheng and Chuxiong basins are tributaries during the Early to Middle Eocene.A new southward river draining northern Gonjo,Mula and Yongsheng basins was generated in the Late Eocene result from the tectonic event.Finally,this was disrupted by the uplift of the Tibetan Plateau,which caused the Yongsheng basin lost its source in the Eocene–Oligocene transition.Based on synthesizing new studies above and published paleo-elevation,paleoclimate,geophysics results,a multi-stages tectonic-geomorphic evolution model of the SE Tibet Plateau is proposed.During the Eocene to Early Miocene,the tectonic process dominated the southeastward growth and surface uplift of the Tibet Plateau.It is characterized by the in-sequence southward thrusting bounded by the left lateral Ailao Shan-Red River shear zone.This stage corresponds to the regional paleo-elevation data and drainage reorganization events,indicating that most of the SE Tibet almost gained its present elevation before the Middle Miocene.Since the middle to late Miocene,it was a landscape modification process triggered by the positive feedback between the tectonics and climate.It is characterized by rapid river erosion along large river canyons and unusually younger thermochronology ages near fault zones.This process reshaped the previous elevated plateau surface,and finally established the present-day “relict landscape” in the SE Tibet Plateau.