The Geomorphological Characteristics And Kinematic Mechanism Of Tagarma Rock Avalanche

The rock avalanche has a large volume,high velocity,long distance,accompanied by huge energy release while moving.It often shows abnormal mobility,which will cause serious damage once instability occurrs.Until now,it's still one of the most mysterious phenomeno n in the geological field because of its complex forma tion mechanism,sedimentology,kinematics and dynamic mechanism.Therefore,it's of great significance to study the sedimentology,kinematics and dynamics mechanism of different types of rock avalanches.The Tagarma rock avalanche with the volume of 1.1×10~8km~3 and the movement length of5.43km developed in Gonggela fault of Pamir Plateau.The accumulation zone have a series of geomorphological landforms,including toreva block,lateral levees and arc-shaped transverse ridges.Its substrate developed a layer of fractured garvels and pebbles due to the strong interaction between the body and the substrate.Therefore,it will provide valuable geological evidence for the study of the Tagarma rock avalanche.The arc-shaped transverse ridges are are rare,this geomorphological landform is the only cases having this special landform in the world except Elm rock avalanche.And although a layer of fractured garvels and pebbles has been found in other cases,it has not been interpretation thoroughly.In this paper,we take the Tagarma rock avalanche as the research object.Based on the detailed field survey,remote sensing and indoor analysis,this paper conducts detailed research on various sedimentological characteristics on the the surficial and internal sedimentology in the long-distance movement path of the Tagarma rock avalanche,aiming to reproduce the transportation and explain the long-runout mechanisms of the Tagarma rock avalanche.It provided a reference for the study of kinematics and dynamics rock avalanches.Fristly,based on remote sensing and field investigation,a series of landforms,including toreva block,lateral levee,arc-shaped transverse ridges and strike fault,were introduced in detail.Secondly,And based on field investigation,laboratory test,indoor statistics and remote sensing,this paper studied the fragmentation and arrangement of magablock along the movement path and the sedimentary structure along the depth direction.Then,combined with previous research results,the paper also analyzed the influence of debris fluidization characteristics and internal deformation on the movement process,and explained the formation mechanism of landforms.Finally,the movement process was inverted based o n the above analysis.the velocity of Tagarma rock avalanche was calculated by Scheidegger and superelevation methods,and the long run-out movement mechanism was described combining with the statistical data of 42 rock avalanches cases.The conclusions are as follows.According to the movement characteristics and the accumulation characteristics of the megablock in different positions during the migration process,the landslide was divided into the source zone,the transition zone and the accumulation zone on sufacial accumulation.The source zone developed three sets of preferential structures,of which two sets are X-shaped,controlling the left and right boundaries of the landslide respectively,and one set is the folation of the felsic gneiss gneiss.The transition zone developed toreva block with three sets of preferential structures and large-scale lateral levees.The toreva block has good integrity and low disturbance,which have three structures stayed the same with resource area.The toreva block formed by momentum transfer due to the difference of the vertical drop of the center of mass between the front and the rear sliding body.After the energy transfer,the tension fracture formed on the surface is the initial shape of the toreva block.The formation of lateral levees are due to the collaboration effect of the accumulation of the deceleration stop zone on both sides,the pushed accumulation in front of the side of the lateral deceleration zone and the fine particles produced by shear and friction.The accumulation the deceleration stop zone on both sides not only provided the propagate channel for debris flow,but alse produce a large number of fine particles by friction shear.The accumulation of coarse and fine particles strengthens the stability of lateral levees.The pushed accumulation extends the length of lateral levees gradually.The accumulation zone is composed of four arc-shaped transverse ridges,which are due to the collaboration effect of rapid deceleration,lateral spread and differential shear.The fragmentation along the movement path shows a trend of“increase-decrease-increase”,which is helpful to speculate the process of accumulation.The reason that causes the fracture trend different from other rock avalanches is related to the inertia force of coarse particles or partial shear action.The dominant orientation of the meablocks on the surface of the arc-shaped transverse ridges is parallel to the tangent direction of the ridge line,which reflects the movement process of the coarse and fine particles in the body of rock avalanche.The roarse particles wrapped by the fine particles rotate under the moment of forces,and then move in the direction with the movement of the"fluid medium".According to the change of debris fragmentation in different depth of the accumulation body,the body is divided into carapace facies,body facies,and basal facies.The fragmentation is stronger and stronger with the increase of depth,which shows the inverse grading in the deposit.The inverse grading reflect the internal deformation of the sliding body.The megablocks in carapace facies separate along the dominant structure under the gravity of megablocks and fracture by extrusion and collision,which fragmentation is dynamic fracture under gravity.The break of megablocks in body facies include compression fracture,shear fracture and stress concentration fracture under the complex stress,which is dynamical fracture under stress.The basal facies directly contact with the substrate,and the internal stress of the layer is large and complex.The main crushing is high stress concentrated fracture and friction shear fracture,which belong to dynamical fracture under huge stress.The movement process of Tagarma rock avalanche along the movement path was divided into three stages,including the collision stage,the rapid stretching stage and the compression accumulation stage.The propagatation as follows.With the sliding body triggered by earthquake,the front and rear sliding bodies due to the potential energy loss of the mass center collided.Energy transfer occurs.Subsequently,the sliding body stretched,the rear sliding body stoped gradually formed a toreva block,and the front accelerating sliding body move forward on the unconfined area with a certain angle.Then,under the effect of small lateral spread,the edge of sliding body slow down and accumulation laterally.The accumulation on both sides in the transiton zone forms a channel to restrain the sliding body movement,the front accumulation was pushed to the both sides,extending the lateral levees,and the accumulation in the front of the stable debris flow drawn into the stable debris flow and continues to fracture.Finally,the stable debris flow compressed on the flat and rough substrate,the stable debris flow deposited preferentially on both sides under the increasing lateral dispersion.The deposits resist the deceleration sliding body,resulting in the continuous deposition and the formation of arc-shaped transverse ridges.The maximum velocity obtained by Scheidegger method is 100.02m/s based on the maximum distance and 37.09m/s based on the distance of the center of mass,and the velocity obtained by superelevation method are 40.67m/s and 10.68m/s.The velocity obtained by superelevation method verified the accuracy of the velocity based on the distance of the center of mass.Due to the large resistance of the substrate of Tagarma rock avalanche,it's difficult to accelerate to 100.02m/s in the short time,which is inconsistent with the actual situation of Tagarma rock avalanche.Moreover,the research object in this paper belongs to the rock avalanche.The reason for Tagarma rock avalanche's long-distance movement on the high resistance substrate is the collaborative effect of large volume,bod y fluidization,reduction of resistance by fine particles,flat substrate and self-formed channel in the transition area.