The Structural Deformation Features In The Fore-mountain Belt Of The Middle Longmen Mountains

Continental dynamics is the leading edge of the research of earth science recently. Continental orogenic belt is one of the important tectonic structural units, so the new continent tectonic scientific theory must be based on the study of the orogenic belts.Longmen thrust belt is one of the main orogenic belt in China continent, belongs to the intracontinental orogenic belt of eastern margin of the Qinghai-Tibet plateau. Its internal structure is complex and has unique features. The NE-trending Longmen Mountains lie between Songpan-Ganzi fold belt and Sichuan Basin,the northern edge of which is Kunlun-Qinling orogenic belt and southern edge of which connect the Kangdian north-south tectonic zone. Longmen thrust belt becomes a key areas of a solution of qinghai-tibet plateau uplift and peripheral orogenic belt of orogenic process because of its structural position.As a hotspots of researching intracontinental orogenic belt, Geological scholars did a further research about Longmen Mountains. But it is weak in the small structural's geometry, kinematics and dynamics of Longmen Mountains.This paper discusses the characters of tectonic deformation about the three sections in the middle Longmen Orogen: Dujiangyan Section, Baishuihe-Guankou Section, and Baishuihe-Bailu Section. Using the method of small structural analytical, described the geometry, kinematics and dynamics of the fore-mountain belt of the middle Longmen Mountains. Then get the sequences of deformation and stress of this area depend on the relationships about the structural phenomena, associated with microstructure analysis,age-dating analysis, fluid inclusion thermometry and C-O isotopic element thermometry.Then get the deformation sequence of this area.It must be followed some principles to confirm the deformation sequence: Plastic deformation is earlier than brittle deformation; dense joint is earlier than sparse joint in massive rocks; The failure surface which bending and filling with veins is earlier than the one whith straightly and on veins; The failure surface filling with deformation veins is earlier than the one filling with no deformation veins; Planar X- joint is earlier than profile X- joint;The crosscutting relationship and confining relationship between deformation factors is also important. There are more than 9 times of structural deformation in the middle Longmen Mountains. The first deformation sequence was NW-SE, reflected the thrusting of the central fault during Indo-Chinese epoch, it was the initial thrust nappe function. The second deformation sequence was NE-SW,mainly formed the early formation of planar X joints and the deformation of the klippens. The third deformation sequence was S-N, it reflected the left-lateral movement of central fault. The fourth deformation sequence was NW-SE, it reflected the right-lateral strike-slip of central fault and the thrust-motion of front fault. The fifth deformation sequence was vertical stress, it formed the normal faulting of both central fault and front fault. The sixth deformation sequence was NE-SW,it was existed in both central fault and front fault. The seventh deformation sequence was S-N, it reflected the left-lateral movement of central fault and the deformation of longxi nappe during Early-Middle Yanshan epoch. The eighth deformation sequence was NW-SE, it reflected the thrust-motion of Longmen Mountains between Late Yanshan epoch to Early Himalayan epoch. The ninth deformation sequence was vertical stress, it reflected the slumped overthrust movement of Longmen Mountains.Longmen mountain nappe tectonic belt experienced several stress times. Klippens of middle Longmen Mountains were uplifted in Indo-Chinese epoch and slipped during yanshan period because of the gravity, and the formation time of the Klippens were different. The results of this paper show that the stress times of trailing edge of the klippens are obviously more than the leading edge.The deformation of Longmen mountains developed from NW to SE, explained byâ‘ The times of structural deformation in the central fault of Longmen Mountains is more than the front fault, and the formation time is earlier than the front fault. There are more than 7 times of structural deformation in the central fault and 4 times of the front fault.â‘¡The structural deformation is different of both sides of central fault, west of which is mainly ductile deformation but the east is brittle deformation.â‘¢T he stress times of trailing edge of the klippens are more than the leading edge.