Exhumation Processes And Evolution Of The Gaoligong Mountains At The Southeastern Margin Of The Qinghai-Tibet Plateau Since The Cenozoic

A number of large strike-slip orogenic belts trending perpendicular to the margin of the Qinghai-Tibet Plateau are distributed in the southeastern margin of the Plateau.Their tectonic deformation and exhumation histories are of great significance to understand the orogenic processes within the continent and the growth as well as the expansion mechanism of the plateau.The Gaoligong Mountains(GLGM),the westernmost zone near the boundary of the Indian subduction plate,is a large-scale structural belt formed due to the southward extrusion of the Plateau materials on the southeastern margin and also large-scale strike-slip between blocks located in the southern segment of the Bangonghu-Nujiang suture.The orogenic processes and geomorphological evolutions of these continental-scale orogenic belts are important for revealing the deformation mechanism that entered the interior of the Asian continent after the collision of the Indo-Asian plates since the Cenozoic.First,the starting time and dynamic process of the expansion of the Tibet Plateau to the southeastern margin have always been the focus of debate between domestic and foreign geoscientists;secondly,the other margin of the Tibet Plateau is usually surrounded by steep mountains,while the topographic of southeast Tibet gradually decreases outward from the interior of the plateau,forming a gentle slope where water vapor can climb freely.This gentle slope is divided by a large strike-slip shear zones.The research on the exhumation and uplift process of the Gaoligong Mountains is beneficial to reveal the evolution process of the gentle slope terrain in this area that stretches for thousands of kilometers;in addition,the geographical environment of this area is harsh,the level of research is low,and the related research results are controversial.It is necessary to study and compare the results of its research with the uplift process of other edges.Based on this,this paper mainly uses the low-temperature thermochronology method and numerical modeling method to conduct a detailed study and discussion on the exhumation and uplift process and tectonic evolution process of the Gaoligong Mountains and the entire southeastern margin of the Tibet,respectively.The following main insights have been obtained.In this paper,low-temperature thermochronological methods including apatite(UTh)/He(AHe),zircon(U-Th)/He(ZHe),and apatite fission track(AFT)were applied to date the samples from the vertical profile of the Gaoligong Mountains Range to reveal the exhumation processes that occurred in the late Cenozoic.Our results show that the Gaoligong Mountains(GLGM)has experienced two stages of rapid exhumation events since the late Cenozoic: in the middle Mioceneand the late Pliocene.It is generally believed that at the beginning of the Eocene,the northeastern corner of the Indian plate and the Eurasian plate collided obliquely outside the eastern Himalayan syntaxis(EHS),and the GLGM responded positively and started a right-lateral strikeslip movement synchronously with the EHS in the Oligocene.The rapid exhumation event from the early to middle Miocene may be the result of the large-scale strike-slip movements and the lateral compressions in the area,while this rapid exhumation event in the late Pliocene was mainly related to climate and river erosions,fault activities and the overall uplift of the earth’s crust.The GLGM trends N-S in the north but changes to a S-E trend in the south.The comparisons of the thermal age data in the different areas of the Gaoligong Mountains Range disclose that the exhumation rate in the northern section of the Mountains is significantly higher than that in the southern section.These tilting exhumation processes resulted in the topographic features of the GLGM,higher in the north and lower in the south.When comprehensive thermochronological data are used to study the cross transect of the GLGM,it is found that the systems with different closure temperatures have similar waveforms except a small offset in the horizontal direction,and the peaks of the different thermochronological data appear at different positions,which may be related to the isothermal line migrations and the lateral accretionary orogeny.Through the comparative analyses of the comprehensive data,we found that the GLGM experienced slow cooling(0.2mm/yr)in the late Eocene and early Miocene(～35 to ～20Ma)and began to rapidly denude in the Miocene(～20 to ～10Ma)with the exhumation rates varying between 0.7-0.8mm/yr.,The exhumation rate decreased to ～0.35mm/yr in the late Miocene(～10Ma).In general,the rates varied greatly with space.This study reveals the thermal evolution history of the GLGM and discusses the evolution of the southeastern margin of the Qinghai-Tibet Plateau.Since the late Eocene,the southeastern Tibet has been exhumated and uplifted as a whole.At this time,the continuous strike-slip of large ductile shear zones occurred in the western part of the region.Since the middle Miocene,the thrust structures and faults activities have drastically enhanced in the eastern part.The southeastern Tibet has begun to rapidly denude and uplift since the early Pliocene,and the extensive tectonic movements occurred.The regional exhumations and uplift processes were mainly controlled by the two aspects: one was the downward flow of high-potential energy materials in the plateau to the low-potential energy region(southeast direction);the other was the lateral accretionary orogeny of the Indian plate and the South China plate.The two effects played different roles in different geological historical periods.At the beginning of the collision in the late Eocene,lateral accretion was the dominant effect;and the plateau material downflow was dominant in the middle Miocene.The southeastern margin of the plateau has been jointly affected by vertical uplift and lateral accretion,leading to the spatial-temporal differences of the tectonic movement and exhumation and uplift.In this study,numerical modeling methods(Pecube and Glide)were used to reveal the spatiotemporal evolution of the exhumation rate of the GLGM since the Miocene.At the same time,the role of lateral accretionary orogens in the process of Mountains debonding and uplift was also discussed.A total of 37 low-temperature thermochronological age data were collected in the region,and the data were mainly distributed along the GLGM.From the Glide inversion results,it can be seen that the exhumation of the GLGM has two main characteristics.The southern part of the Mountains range started to exhumation first,and the exhumation rate in the northern part of the Mountains range was significantly faster than that in the northern part since the Pliocene.Since the Early Miocene,the southern part of the GLGM was the first to begin to exhumation than the north part,and the maximum exhumation rate is about1.8 km/Myr.This phenomenon of exhumation and uplift in the south prior to the north is contrary to the lower crustal flow theory,indicating that the lower crustal flow model is not suitable for explaining the dynamic mechanism of exhumation and uplift of the mountains in this area.In the Glide numerical modeling of the GLGM,it is found that the Longmen Mountains and Songpan-Ganzi-Yidun Blocks located in the eastern part of the southeastern Qinghai-Tibet Plateau at the beginning of the Oligocene first experienced exhumation and uplift,while the Longmen Mountains and Songpan-Ganzi-Yidun Blocks,located in the eastern part of the southeastern Qinghai-Tibet Plateau,first occurred in the Oligocene.The Three river area in the west of the margin has only entered the stage of rapid exhumation and uplift since the Early Miocene.The exhumation and uplift in the Three river area gradually expanded from south to north,and the main reason for this is related to the oblique subduction of the Indian plate in this area.Since the Eocene,the Indian plate has begun to subduct and push in the NNENE direction at the southeastern margin of the Tibet Plateau,resulting in the gradual welding of several island arcs and plates in this area.In the Oligocene,this area completely formed a complete block was squeezed and collided with the Yangtze plate,and at the same time,it was squeezed from the interior of the Qinghai-Tibet Plateau to this area,resulting in the Longmenshan area and the Songpan-Ganzi-Yidun block being the first to start exfoliating and uplifting,and the surrounding areas.Fractures also appear and become active from this point on.Since the early Miocene,the area south of 26°N in the Three River areas has been the first to be affected by the subduction and extrusion of the Indian plate.In the late Miocene,the influence of this subduction and push was transmitted to the entire southeastern margin of the Qinghai-Tibet Plateau.Deformation and uplift began to occur overall.It can be seen that the main factors affecting the uplift process of the southeastern margin of the Tibet Plateau are the lateral extrusion of the Mountains caused by the subduction of the Indian plate and the blocking of the Yangtze plate,and the gravity flow of materials from the Tibet Plateau.Therefore,the late Eocene began to be dominated by lateral extrusion,and the Miocene turned to be dominated by gravity flow.Since the Pliocene,the exhumation and uplift of Mountains ranges have been simultaneously affected by,gravity flow,fault activity and surface erosion etc.