Cenozoic Uplift And Exhumation Of The Yalong Thrust Belt On The Eastern Margin Of The Tibetan Plateau And Its Implications For The Outward Extension Of The Plateau

The Yalong thrust belt,the southern continuation of the Longmen Shan thrust belt,formed during the Tibetan Plateau eastward expansion,and defined an important transition zone of tectonic geomorphology and geophysical feature in eastern Tibet.The Yalong thrust belt locate at the western margin of the Yangtze block and marked a NE-trending wide high relief zone.However,the topographic gradient across the belt descend gradually to low elevations,its deformation was underestimated and few investigations have been involved in recent studies consequently.Numerous studies have revealed rapid denudations since the Late Miocene in eastern Tibet,and it was linked to the onset of the surface uplift.However,recent studies from the hanging wall of thrust belts reported early Cenozoic denudation events,revealed a multi-stage deformation history in eastern Tibet.As a part of the thrust belts,the Cenozoic tectonic history and deformation pattern of the Yalong thrust belt is still unclear,and its role in the plateau eastward expansion is worth further discussion.In this dissertation,four vertical elevation profiles are designed across different branch faults of the Yalong thrust,and low–temperature thermochronology(AHe,AFT,and ZHe)and thermal models are used to investigate the Cenozoic exhumation pattern across the belt.In addition,a large number of previous low–temperature thermochronological data,river terrace ages,and basin-wide ¹⁰Be erosion rates data in and around the Tibetan Plateau are collected and modeled to analyze the Cenozoic erosion spatial-temporal pattern rates of the Tibetan Plateau at different time scales.Based on the above results,we discussed the Cenozoic uplift and exhumation of the Yalong thrust belt and its implications for the outward extension of the Tibetan Plateau.Our new dating results and thermal history suggest that the Yalong thrust belt recorded the early Cenozoic uplift and denudation events,revealing a multi-stage and differential deformation history.The belt experienced accelerated denudation since the Late Eocene and went faster in the late Miocene.In the hinterland of the thrust belt,the hanging wall and footwall of the Yunongxi fault have undergone significant differential exhumation since 40–30 Ma,revealing the quasi-synchronous deformation event with the Longmen Shan thrust belt,then the eastern margin of the plateau has been confined.In the front of the thrust belt,the Jinping Shan areas experienced two rapid uplift and denudation events in the late Oligocene(?24 Ma)and Middle Miocene(17–14 Ma).The Cenozoic early event is 15–10 Myr later than that in the hinterland,which may reveal the differential deformation among blocks or the eastward expansion of the plateau.The temporal and spatial pattern of Cenozoic denudation rates modeled by the previous low–temperature thermochronological data suggest that eastern and northeastern Tibet experienced accelerated uplift and exhumation since the late Eocene,while the Tian Shan area was relatively late(until the Miocene).A more widespread and faster exhumation period occurred across Tibet in the Late Miocene.Spatially,the faster denudation area is consistent with the tectonic activities(faulting)zones.Moreover,fluvial erosion rates calculate by the terrace and basin-wide ¹⁰Be erosion rate also reveal striking differential denudation,revealing the long-term control effect of tectonic activities on the surface denudation process.Based on the above results and the plateau expansion model proposed by Rohrmann et al.(2012),we proposed that the eastern margin of the Tibet extended to the Jinping Shan–Longmen Shan and West Qinling areas at ca.30 Ma,and the northeastern margin extended to the southern edge of Qaidam basin and defined by the East Kunlun fault.This early Cenozoic accelerated outward expansion and deformation of the plateau is a long-range response to the collision between India and Eurasia.The overall large scale of the late Miocene uplift and deformation may have been driven by the delamination of the lithosphere within central Tibet or locally due to the inflation of the lower crust into southeastern and eastern Tibet.