Mechanics Mechanism And Its Quantitative Analysis Of The Formation Of Longmenshan Faults

The mechanism of structural mechanics is a new branch of science,especially the quantitative analysis of it is less.Based on the butterfly distribution theory of plastic zone around the soft medium space,the mechanical analysis of normal fault,reverse fault and translation fault is carried out,the yield function of butterfly plastic zone around the soft body under three-dimensional stress state is derived,and the simulation method of increasing stress by displacement loading is designed Based on the layer and structural section,it is proposed that the fault originates from the X-type failure of the hard rock mass around the weak geological body in the fault,and the plate movement of the crust is the basic power of this failure.After the formation of the X-type failure,the continuous movement of the plate will form the normal fault,reverse fault and strike slip fault in accordance with Anderson's principle,and the geomorphic characteristics of the Longmenshan fault zone in different time periods are predicted There is a good agreement between them.This paper provides an effective method for quantitative analysis of the mechanism of structural mechanics,which has important theoretical and engineering significance.According to the theory of butterfly-shaped plastic zone,the butterfly-shaped failure zone centered on the weak anisotropic body will appear in the rock mass with holes or weak anisotropic body in the high deviating stress field.The butterfly-shaped failure zone consists of four butterfly leaves,which are located on both sides of the maximum principal stress and the minimum principal stress,and are symmetrically distributed between the axes of the two butterfly leaves.The angle between the butterfly plastic zone and the butterfly plastic zone is about 80°?100°.The butterfly plastic zone has the characteristics of directionality,sensitivity and infinite expansibility.Based on this,we put forward the mechanical conditions of reverse faults,normal faults and strike-slip faults formation:reverse faults are mainly caused by horizontal compression,normal faults are caused by larger vertical stress and smaller horizontal stress,strike-slip faults are caused by vertical stress between maximum horizontal principal stress and minimum horizontal principal stress,and the existence of weak bodies in vertical direction.According to the three-dimensional numerical model,under the action of plate movement,different types of geological weak bodies and even homogeneous rock strata will produce plastic zones with a certain direction.Relatively speaking,the stress required for the butterfly-shaped plastic zones produced by voids or geological weak bodies is smaller,the plate movement time required is shorter,and the geological weakness is weaker.The dominant expansion surface of plastic zone is easier to form around the body.On the basis of elastic-plastic mechanics,considering the circular geological soft body,and based on the Mohr-Coulomb failure criterion,the author deduces the plastic failure law of surrounding rock mass of geological soft body under the action of intermediate principal stress.First,under the condition of other parameters unchanged,the shape and radius of the plastic zone around the geological soft body are affected by the lateral pressure coefficient.With the increase of the lateral pressure coefficient,the shape of the plastic zone varies roughly in the form of "butterfly-ellipse-circle-ellipse-butterfly".Secondly,the development of plastic zone has catastrophe,which may occur during the development of butterfly-shaped plastic zone.Under the action of small stress increment,plastic zone can expand infinitely in a certain direction,and the change of lateral pressure coefficient,vertical stress,cohesion and friction angle will lead to the infinite increase of radius of plastic zone.And similar results can be obtained under different fracture criteria.Based on the plate movement characteristics,geometric distribution characteristics and rock mechanics parameters of Longmenshan fault zone,the numerical calculation model of Longmenshan fault zone and its adjacent area is established,and the actual loading conditions are designed.The Mohr-Coulomb model is selected as the calculation criterion to study the mechanical mechanism and ground of fault formation in Longmenshan fault zone.The spatial and temporal distribution characteristics of stress and the evolution process of regional tectonic system are studied.After a time span of about 3.20 million years,with the extrusion of the plate,plastic zones with a certain direction appear successively in Longmen Mountain and its adjacent areas.The development direction of these plastic zones is roughly 40° from the horizontal plane,and there are conjugate faults,which correspond to the actual Longmen Mountain fault.Therefore,it can be concluded that the development of faults F1,F2 and F3 starts from the middle to both sides,and gradually develops to the surface on the upper side,and faults F1,F2 and F3 form faults in turn.With the regional plate compression movement,the horizontal stress of the rock mass in the region will inevitably increase,and the maximum and minimum principal stress ratio of the crustal rock mass exists a limit phenomenon.The ratio of the measured points in different depths is between 3.05 and 3.44.The principal stress ratio of the three faults in the region is obviously higher than that in the surrounding area,which conforms to the expansion conditions of the butterfly-shaped plastic zone.Geological weak bodies and their surrounding areas are more likely to form dominant extension planes of plastic zones,but have little influence on the development speed and shape of subsequent plastic zones.The formation of regional tectonic system in Longmen Mountains is the result of plate compression,surface erosion and sedimentation.After the formation of the fault zone,the Longmenshan Mountains and the western Sichuan Plateau to the left continued to uplift,with an average uplift rate of about 1.38 mm/yr;the western Sichuan depression to the right of the Longmenshan fault zone had only a small uplift,and there was a large gap between the two.Longmen Mountain and its Songpan-Ganzi Plateau region experienced uplift-denudation,while Sichuan Basin region experienced sedimentation.Based on the simulation results and mineral assemblage analysis,the evolution process of strata and geomorphology in Longmen Mountain and its adjacent area was inferred,including the marine coverage before Permian,the mountain uplift and marine degradation during Triassic,the rapid uplift west of Longmen Mountain after Jurassic,and the deep rock mass exposed to the surface under the action of erosion.Up to now,the Songpanganzi block,Longmenshan fault zone,western Sichuan depression,Longquanshan and central Sichuan basin appear from west to East under the action of plate compression,erosion and sedimentation.The simulation results are basically consistent with the actual topography.In conclusion,this paper enriches the theoretical connotation of butterfly plastic zone,and puts forward the viewpoints of convergence of the limit of principal stress ratio with the increase of depth,low dependence of butterfly plastic zone phenomenon on the criterion,weak sensitivity of butterfly plastic zone expansion to intermediate principal stress,and large-scale direction invariance of plastic zone development.The application field of butterfly plastic zone is expanded,and the leap from mining engineering to seismology and structural geology is realized.The research methods,contents and results of this paper provide a good idea for the process inversion of fault formation,and have a good guiding significance for the earthquake disaster near the fault,the evolution of topography and geomorphology,as well as the roadway support,rockburst and coal and gas outburst accidents in coal mine production.