| A GPS observation results on the southeastern margin of Tibetan Plateau revealed that there comes out a wild range of substance extrusion clockwise rotation phenomenon around Himalaya east structural knot along with the eastern boundary of Xianshui River-Xiaojiang-Dien Bien Phu fault zone.The above conclusion breaks the previously knowledge that the Tibetan Plateau extrudes laterally and its clockwise rotation range around the east structural knot ends to the Red River fault zone,which indicates west area of Xiaojiang Fault Zone and west of Dien Bien Phu fault zone have correlated dynamic causes.So,it makes great sense to research Dien Bien Phu fault zone for the exploration of whose relationship with Xianshui River-Xiaojiang Rive-Xiaojiang Fault Zone,and further to explore the dynamics significance of Xianshui Rive-Xiaojiang Fault Zone and Dien Bien Phu fault zone who both act as the lateral extrusion outer border,so as to establish a theoretical foundation to further explore whether the Dien Bien Phu fault zone is an extension of the North to South seismic belt to the outside of our country.After the research on the tectonic evolution of Dien Bien Phu fault zone by collecting and analyzing the research results of domestic and foreign experts and scholars and understanding the basic characteristics of the Dien Bien Phu fault zone,the research on geometric characteristics of the Dien Bien Phu fault zone based on ArcGis10.0 by the use of Remote Sensing images and DEM elevation data,and the research on the impact of the terrain differences of the both sides caused by Dien Bien Phu fault zone tectonic evolution by calculating the stress field(17?23° N,101?104° E)of Dien Bien Phu fault zone and its adjacent area through epicenter mechanism solutions of fine grid searching method to the geometry,kinematics,mechanics and dynamics characters and the formation of tectonic stress field,besides,combining with the previous research results on tectonic characteristics from the shallow to the deep of the earth surface and the deep medium differences in Dien Bien Phu fault zone,this paper explores the seismogenic dynamic environment of Dien Bien Phu fault zone,and concludes the following understandings:1.In the early Hercynian Movement of Late Paleozoic Era,Dien Bien Phu fault began to shape the subduction zone,a ridge expansion produced in the process of in the East Branch of the Paleo Tethys ocean;in Early Permian Period,the subduction zone/ridge expansion closed for the first time;but.in the early and mid-term of the Indosinian movement in Early Mesozoic,the subduction zone/ridge expansion expanded and pulled apart once more forming a residual trough;after Noric Period,the pulled and expanded residual trough collided,which eventually became the suture zone between Simao intermediate plate and Indosinian plate,the former was a geosynclinal folded system and the latter took Kon Tum archicontinent core as the core.After that,Dien Bien Phu fault zone started the dextral strike slip in Jurassic.From late Jurassic to early Cretaceous,the Dien Bien Phu fault zone deposited.From early Cretaceous to late Cretaceous,Dien Bien Phu fault motion was activated again.During the late Cretaceous period,Dien Bien Phu fault zone was sinistral strike slip.At least in the later period of the late Cretaceous or in the early Eocene Epoch,the movement of Dien Bien Phu fault zone mainly was dextral strike slip,whose traction worked together with the early sinistral striked slip in Red River fault zone formed the present tectonic framework cross the east to the west of Dien Bien Phu.In late of 5Ma,the sinistral strike slip had begun and become the main characteristic of movement,lasted till the Pliocene to the Quaternary Period,with part of the normal fault sinistral strike slip,although the sinistral strike slip dislocation came up piecewise difference.At present,the fault zone of Dien Bien Phu is still mainly characterized by sinistral strike slip actively in the shallow.2.Dien Bien Phu fault zone located in the boundary of two special tectonic units,whose north side of the East is Vietnam Northwestern fold system,and southern is the Kontum massif on which the Indosinian archicontinent core made up the main body;the west of the Eat directly meets Simao block which is gradually narrowing from north to south till the southernmost of Dien Bien Phu fault zone in Thailand.Dien Bien Phu fault zone and Xiaojiang fault zone have a similar terrestrial background which means whose east directly meets the stable archicontinent core,and the west meets the unstable geosyncline fold system,thus forming the Strong-weak boundary.And because the southernmost of Dien Bien Phu fault zone is on the Indosinian archicontinent core and Simao block,under the joint action of the Shan Temma to the west,it forms a spatial and temporal distribution characteristics of earthquakes that the deformation of the northern is stronger than that in the southern;the earthquakes are shallow earthquakes;seismic activity in northern is more frequent than in the southern;and the magnitude is relatively larger.Therefore,the stable block in the east of Xiaojiang and the southeast and southwest of Dien Bien Phu,the southwest side of the Southern China block,Indochina block and Shan Temma limit Tibetan Plateau lateral extrusion clockwise rotation and force the south central of Dien Bien Phu fault zone towards W-NW.3.The main body of Dien Bien Phu fault zone is single fault line with multiple fault lines in parts of sections.There are also steps and several pull-apart basins.The fault zone scatters in Dien Bien Phu basin,and to the north of Dien Bien Phu basin it is anSN trending but to in Dien Bien Phu basin begins to gradually shift from NE trending to NNE trending with a large number of river dislocation all along the fault zone.In overall,it looks a concave bow-shape NW trending.The maximum sinistral displacement of Dien Bien Phu fault zone is 12.5km.The average slip rate is2.5mm/year since the Pliocene,and there is a shear extension to the north of the westDien Bien Phu fault zone,where the crustal stress field rotates clockwise from north Xiaojiang Fault Zone to mid-Dien Bien Phu fault zone.The shear transmission of Xiaojiang Fault Zone cross Red River fault zone leads the Dien Bien Phu fault zone strike slip since the Pliocene,and plays an important role of eastern boundary in the process of crustal deformation in North Indosinian.4.The principal compressive stress axis of structural stress field in Dien Bien Phu fault zone and its adjacent area rotates clockwise from the north,middle to the south,which together with the stress field in Xiaojiang fault zone from north,middle to south shows a clockwise rotation.Parts of sections which mainly are the south of Xiaojiang Fault zone and the north section of Dien Bien Phu fault zone,the principal compressive stress axis do not rotate clockwise,because these area have the both shear transmissions from NS trending in Xiaojiang Fault Zone and WE trending in the west of north Dien Bien Phu fault zone,which makes the stress field in this region same with the lateral extrusion clockwise trend in southeastern margin of the Tibetan Plateau.5.The main power source of the warning signs of an earthquake along the Xiaojiang-Dien Bien Phu Fault Zone and its adjacent area is the substance lateral extrusion movement form the East Himalaya structural knot in southeastern of Tibetan Plateau.It is difficult to accurately determine whether Dien Bien Phu fault zone and Xiaojiang Fault Zone is connected on the existing research works,but the existing data reflect that the outer border active intensity from the north to the south of Xianshui River-Xiaojiang-Dien Bien Phu Fault Zone gradually weakens since late Quaternary Holocene even to the Recent Epoch,which illustrates that the main power to control the whole large-scale lateral clockwise rotation movement is from the lateral exclusion of Tibet Plateau.According to the location of Dien Bien Phu fault zone in Indochina and the its geometry,kinematics,mechanics and dynamics characteristics,this paper considers that and that Dien Bien Phu fault zone,together with Xianshui River-Xiaojiang Fault Zone,composes of the lateral extrusion Passive East boundary in the southeastern margin of Tibetan Plateau terrane groups.Subject to the limitation of research data of Dien Bien Phu fault zone including on its location,earthquake,geology and geophysics,this research could be further studied in the future. |
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