Numerical Simulation Of Crustal Deformation Dynamics In Southeastern Tibet

The southeast margin of Tibet is located in the area bounded by Tibet plateau,South China block,the Sunda block and India plate.The boundaries of the area are two large strike-slip fault systems that are Xianshuihe –Anninghe-Zemuhe-Xiaojiang fault in the east and Sagaing fault in the west,mixed by continent collision,plateau expansion,Burma subduction,Sumatra subduction and Philippine subduction.This region has undergone complex deformation history due to the evolution of the Tethys Ocean since Mesozoic.The complex inherited units interweave with various dynamic processes,so that the crustal deformation mechanism in this area has not been clearly understood.For these aims,we first summarize the strain rate distribution characteristics of this region by using the current long-term 3D crustal deformation data(GPS horizontal and precise horizontal vertical velocity fields).In order to evaluate the rotation of GPS velocity,the deformation fields of different reference frames are used to estimate the rotation centers.The contribution of each rotation boundary to the current crustal velocity is analyzed based on the estimated rotation center and numerical simulation tested.Then numerical models were established to analyze the deformation mechanism of the classical crustal deformation in the area(the uplift of Gongga Mountain in western Sichuan,the subsidence of central Yunnan relative to western Sichuan and southern Yunnan,and the east-west extension of southern Yunnan).Finally,on the basis of summarizing the previous research work and the work we got,a joint model of the present crustal deformation on the southeastern margin of the Tibet is proposed.Specifically,this work mainly contains the following research contents:1.The long-term horizontal and vertical deformation fields that have been published are used to summarize the main features of the long-term crustal deformation in the Tibet.The reliability and accuracy of the current long-term crustal deformation features are determined by comparing with the results of previous studies,and the possible formation mechanism is discussed.2.Based on the velocity fields of different frames of reference,the rotation center around the eastern Himalayan Syntaxis on the southeast edge of the Tibet was estimated,and the contribution of the rotation effect to the observed velocity field was evaluated.3.The high resolution seismic wave velocity structure published in the study area was used to calculate the gravitational potential energy and estimate the strain rate generated by the gravitational potential energy,and the influence of gravity on crustal deformation in this area was discussed.4.Taking uplift of Gongga Mountain as an example,the uplift mechanism of the high terrain on the turning point of fault strike of Xianshuihe –Anninghe-Zemuhe-Xiaojiang system is analyzed by using the oblique extrusion numerical model.And the influence of river incision and middle and lower crustal flow on the orogenic process is discussed.5.We combined with deep imaging and geochemical results to determine the position and physical parameters of lithosphere high-velocity body generated by the Emeishan mantle plume.After that we used 3D numerical simulation of thermodynamics to discuss the evolution of topography,fault pattern,surface gravity change and deep mantle flow produced by lithosphere high-velocity body.6.By using the deep imaging result and 2D thermodynamic numerical technique,the stress state of rocks generated by the spontaneous subduction of Plates in Myanmar and the deep mantle cyclotron flow were simulated,the control mechanism of the current horizontal crustal extension process in southern Yunnan was analyzed,and the relationship between the SKS wave observation results and the deep mantle cyclotron flow and lithospheric deformation was discussed.7.Based on the distribution characteristics of focal mechanism solutions and numerical simulation results,the present state and dynamic performance of the Subduction of the Myanmar plate are analyzed,and its influence on crustal deformation in the southwest Yunnan region is discussed.8.In order to evaluate the effect of far-field subduction on the Red River fault in the study area,the relationship between the two-way subduction process of the Sunda block and crustal deformation in the present southern Yunnan region is discussed using numerical simulation results.The possible mechanism of Miocene polarity reversal in Red river fault is also speculated.Through the above research,the conclusions and understandings obtained in this paper can be summarized as follows:1.The large dextral shear zone and the rotating deformation of the Sagaing fault and the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault system in the southeastern TibetThrough the analysis of the current horizontal strain field and the relevant numerical simulation work in this paper,it is found that the strain field in the southeastern Tibet is mainly controlled by the Sagaing fault in south and the Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault system in north.Within the range of600 km long and 700 km wide,the two fault systems are interwoven with the complex pre-existing structures in the region,thus forming the unique strain characteristics.The main evidences are as follows:(1)According with the direction of principal extension strain rate,the strain rate rotates with clockwise around the northwest end of Red River fault,while the principal extrusion strain rate and shear strain rate are mainly concentrated in the vicinity of two major strike-slip fault systems.This feature means that the kinematics model in this area should be dominated by boundary movement of the main fault,while each active block is subjected to the torque generated by the movement of the strike-slip fault and then rotates and deforms.This understanding is highly consistent with the normal faults and the extensional focal mechanism solutions near Nanting River fault,Chenghai fault and Litang fault.(2)From the point of view of fault slip rate,although there are ancient suture zones such as Honghe fault,Lijiang-Xiaojinhe fault,Jinshajiang fault and Lancang River fault in the study area,their current slip rates are all below 5mm/a.It is not only difficult to compare with Anninghe-Zemuhe fault,but also difficult to compare with Xianshuihe fault,Xiaojiang fault and Sagaing fault.Therefore,driven by the current tectonic forces,the active faults in this region should be controlled by both the fault systems.In terms of the geological slip rate,the million-year-old scale slip rate in these areas is also no more than 5mm/a.(3)From the perspective of long-term vertical deformation field and topographic distribution,the rapid uplift area is mainly concentrated in the strike-slip fault transition area.For example,Gonggashan area is located at the intersection of Xianshuihe fault and Anninghe fault,Luoji Mountain where Anninghe fault and Zemuhe fault intersect,and Jiaozi Mountain where Zemuhe fault intersects with Xiaojiang Fault.The entire western Sichuan region is under the extrusion effect from the Tibet and has a high uplift rate.However,the uplift rate decreases gradually from the Xianshuihe fault in the north of the western Sichuan block to the south,while the Chenghai fault in the southwest Yunnan province is mainly subsidence.The central Yunnan block is located at the direct intersection of the two fault systems,and is in the subsidence area relative to the southern Yunnan and the western Sichuan.This phenomenon is also confirmed by the Neogene-quaternary basin deposition which is widely distributed throughout the region.(4)From the perspective of block rotation,the GPS velocity field relative to the South China block can be used to obtain the rotation center consistent with the geological rotation results,which is located near(96.10°E,27.4°N).According to the angular velocity distribution obtained from the rotation center,the angular velocity around the eastern Himalayan syntaxis in the study area is 2 degrees/million years,and the inner ring is faster than the outer ring.This understanding is in good agreement with the results obtained by using paleomagnetic and geological methods.(5)Rotation numerical simulation test results show that the boundary conditions from the inner ring contribute up to 50% to the entire regional crustal deformation,of which the tangential rotation contribution accounts for 40% while the radial boundary action accounts for 10%.In fact,the rotation component accounts for 70%.Between two large strike-slip fault systems,the differential motion is absorbed mainly by means of block rotation.(6)Through the numerical test of uplift mechanism in Gongga Mountain area,it is found that extrusion uplift and three-dimensional non-uniform denudation are the prime forces for the rapid uplift of Gongga Mountain since nearly 9Ma.However,luoji Mountain and Jiaozi Mountain in the south cannot accumulate terrain rapidly due to the stretching effect in the southwest direction.The present high residual topography may be caused by the reconstruction of non-uniform denudation in this area.This means that the extrusion effect of the Tibet is still the main control effect in the area north of Longmenshan-Jinpingshan,and the rapid uplift of Gongga is the best evidence.(7)From the numerical simulation of the high-velocity body in Emeishan basalt,it can be seen that the high-velocity l body generated after the transformation of Emeishan basalt has important indicative significance to the current crustal deformation dynamics.The high velocity body in the shell,which is nearly round,has directly confirmed that the central Yunnan block was not subjected to large scale internal deformation such as extrusion and stretching in Cenozoic.If large-scale deformation occurs,the annular body in the lithosphere is difficult to retain.However,if only rotational and extruded deformation occurs,it will well explain the fast-sliding boundary fault and the near-circular high velocity body in the deep lithosphere,as well as the relatively slow uplift.2.Gravity driven terrain flowIn addition to the subduction in the far field,some scholars believe that the deformation of GPS observed today can be explained by the gravity gradient generated by the terrain.Based on the above considerations.In this paper,the differential stress generated by the gravitational potential energy gradient in the study area is calculated,and the strain rate generated by the gravitational potential energy is estimated in combination with the regional viscous structure.The results show that gravity drive has great influence on the Lijiang-Xiaojinhe fault,and the direction of differential stress is consistent with the observed direction of tensile strain rate.It can be seen from the bending degree of the topography contour line of 1500 m and the Moho interface contour line of 40 kilometers that the deep material of The Tibet Plateau does not flow out of the Tibet Plateau in a large scale.Only the topography is like water and expands from the plateau to the surrounding under the action of gravity.It is uncertain in the model of this paper whether the gravity driving process involves the middle and lower crustal flow.3.The spontaneous subduction process in Myanmar and the lithospheric tensionFrom the perspective of subduction,the direct cause of whether a high mountain(Subduction zone in the Andes of South America)or a back-arc basin(Sea of Japan)is whether the override plate is squeezed or stretched,and the most critical control factor is whether the trench moves forward or retreats.As for the Burma arc,it bends nearly 150 Km from east to west,which fully indicates that the subduction process of The Burma plate continues and produces the phenomenon of trench retreation,which must produce the stretching effect in the southern Yunnan area of the overlying plate.Through our numerical work,it is not difficult to find that if the subduction stops,the present observed focal mechanism solution direction and crustal strain state cannot be generated.Based on the published deep plate morphology of The Myanmar micro-plate,it can be speculated that the entire southern Yunnan region,i.e.,the end of the Sagaing fault and the area to the south of latitude 26.5 degrees north,the retreating Myanmar arc will exert an east-west stretching effect on the lithosphere in this region.In the region north of latitude 26.5 degrees,the eastern Himalayan syntaxis pushes northward,producing a "trench" advancing state,which will squeeze the rocks to the north.4.Whirl flow and mantle anisotropy in the deep mantleWhen the spontaneous subduction plate moves towards the deep mantle due to its own weight,it will cause disturbance to the mantle.Previous studies have shown that the subduction plate edge produces cyclotron flow,and the size and shape of cyclotron flow are directly related to the plate distribution.The retreat of the trench is conductive to the generation of mantle anisotropy parallel to the trench,while the advance of the trench produces mantle anisotropy perpendicular to the trench.But for The Burmese subduction,it can't be considered as the forward and backward part of the trench in the traditional sense due to its lack of pushing action required by active subduction.Based on the numerical simulation results in this paper,although the trench retreats,it can still generate a swirling flow perpendicular to the direction of the trench at the front of the plate.This cyclotron flow is in good agreement with the observed results of mantle anisotropy-SKS waves in this region.In the region north of latitude 26.5 degrees,the mantle anisotropy changes to the north-south direction,which is similar to the effect of active subduction in this region to produce trench advancing,and produces the mantle anisotropy perpendicular to the collision suture zone.5.Driving action from far field--The Sunda block two-way subduction process and Red River faultApart from the expansion of Tibet,the northward thrust of Indian plate and the subduction of Myanmar plate,is this area affected by the subduction of Sumatra and Philippines? This paper also discusses it.The published deep velocity structure revealed that the Sumatran plates existed only near the trench at a depth of 1000 km and that most of the plates had rapidly penetrated the 660 km phase transition surface and sunk deeper into the mantle.The retreatment of the trench has led to the formation of the Andaman Sea and the South China Sea.But based on current GPS velocity results,Sumatra subduction is squeezing the Sunda block.In this paper,a numerical model is established based on the P-wave velocity profile from Myanmar to the Philippines.The simulation results show that the sunda block is stretched by the subduction in Burma and squeezed by the subduction in The Philippines,and the two sides face each other and move westward.However,after the Philippine subduction was reduced,the Sunda block also had tension effect,while the Sunda block could not generate westward movement.This indicates that the motion pattern of the Sunda block is closely related to the two-side subduction process on both sides.Based on this,it can be speculated that the weakening of the current movement of Red River fault is related to the compressional boundary action of the Sunda block.Contrast the subduction and start time of subduction zones in the Philippines,can be found in the west of subduction is more ancient Myanmar,since in 27-28 ma basically is given priority to with spontaneous subduction,and the east of the Philippines subduction between 40 to 20 Ma is conducted priority to with the proto-south China sea dive first dive nearly 700 km,20 Ma later in the south China sea subducted nearly 700 km,a total of nearly 1400 miles of displacement.Conincidently,during 35-17 Ma,the Red River Fault had a large-scale left-lateral strike-slip,with the slip magnitude ranging from 500-1000 km,which was basically consistent with the length of the slabs under the Philippine subduction zone at present.Therefore,the large scale displacement of Red River Fault 35-17 Ma was mainly absorbed by the rapid subduction of the former South China Sea,and the main control function came from the subduction process on both sides of the Sunda block.The trench retracement caused by the Andaman-Sumatra subduction mainly started at 11 Ma,which was less than the time when the Red River fault had large-scale movement.Therefore,the main force source controlling the large-scale sliding and polarity change of Miocene Red River fault may come from the reversal of Philippine subduction.6.Common crust-mantle decouplingAccording to the results of this study,the crustal layer in this study area is controlled by the large dextral shear zone between the Saging fault and the Xianshuihe-Xiaojiang fault system.The topography is controlled by the flow of the terrain produced by gravity.The lithosphere is controlled by the extrusion and tension of the override plate and the subduction process in the far field.The deep mantle layer is controlled by the swirling currents generated by the deep plates.From the shallow to the deep,the acting force is not the same.No matter whether the direction of the surface GPS observation is consistent with that of the deep SKS and PMS observation,the different forces it bears will inevitably lead to the different strain states of the crust and mantle.Therefore,based on the analysis in this paper,it is believed that the crust-mantle decoupling is a common phenomenon in southeast Tibet to the huge difference in the forces from the shallow to the deep.