Tectonic Research And Three-dimensional Modeling Of Sandui Region In North Section Of Longmen Mountains

The Longmenshan orogenic belt is regarded as “Geological Bermuda” bygeologists and attracts the attention of scholars from home and abroad due to itsoverthrust structure and klippen. Seniors conducted in-depth research on the tectonicof Longmen Mountains, and achieved excellent results. However, thethree-dimensional data about the Longmen Mountains has not been found. Based onthe field geological survey of Sandui region in northern section of the LongmenMountains and the use of Gocad software for three-dimensional data modeling,through displaying data model and geological interpreting of the three-dimensionalmodel of the structural characteristics of Sandui region, certain achievements has beenmade.The basic method of three-dimensional modeling by Gocad software is the use ofsemi-intelligent data model such as borehole data, seismic data, geophysical data, etc.The structure modeling “work flow” process during building the complete model isthe key point, wherein it comprises nine steps: loading data�'establishingstratigraphic column�'establishing model boundaries�'fault modeling�'horizonmodeling�'Well hierarchical data constraints�'editing contact relationships�'thedetection and modification of cross-cutting relationships betweenlayers�'establishing an intermediate layer. After completing of the workflow, thethree-dimensional model could be created by using surface data endowed withgeological significance of the fault plane and other aspects of the workflow.Through field geological investigation and data analysis, structuralcharacteristics of nappe of Sandui region have been studied in depth. Based on remotesensing data, topographic features, combined with the basic theory of nappe structureand previous studies, according to the geometric characteristics of the main fault andthe relationships between faults, Gocad3D modeling software has been used forthree-dimensional numerical simulations. However, the geological structures of studyarea are complex, stratigraphy folds are diverse, which are essentially overturnedfolds, and lack of seismic, drilling, geophysical and other deep control data. Therefore,basic modeling methods by Gocad software are not applicable in the study area. In this paper, the four main profiles, a number of sub-sections and numerous short sectionsare used to control the model. Due to the lack of depth control data, during the surfacemodel creation process, the operation steps should less than the basic ones: loadingdata�'establishing stratigraphic column�'establishing model boundaries�'faultmodeling�'horizon modeling�'editing contact relationships. After a simpleworkflow operation, it should be used to establish the geological significance of themodel endowed with three-dimensional surface modeling.After completion of the model, it needs to be demonstrated with the geologicalinterpretation. Three-dimensional models created in Gocad software can be rotated,scattered, cut sections and a single geological solid can be analyzed, etc. And on aplane geological map these functions are no way to achieve, this also reflects thesuperiority of Gocad software structure modeling.During the geological interpretation process of the three-dimensional model, wefound that: the region nappe structures mainly distribute along NE-SW direction,Nappe sliding main surface faults are tendencies of the NW thrusts to imbricateportfolio.From northwest to southeast, deformation of thrust faults were from ductile tobrittle-ductile then brittle deformation, internal nappe folds closed to the invertedpleat-relief erect, construct strength from strong to weak, levels from deep to pale.The interpretation results of three-dimensional geological model combined withinterior arranged materials are used to analyze the structural formation and evolutionprocess of the study area:①During the pre-middle Indo-China movement, LongmenMountains formed giant nappe with central fault (F2) and former hillfracture(F5)-based main sliding surface by "pre-stretching" type expansion mode.②During thrusting forward extrusion process at late Indo-China movement, when thenappe striker was blocked inactivity, the former hill fracture(F5) was covered byJurassic clastic rocks, Songlinpo Fault(F5) was formed.③From Yanshanian toHimalayan movement, inside giant nappe appeared a series of folds and secondaryimbricate thrust faults, Shizishan Fault(F4) and Qilixiang Fault(F3) were formed insuccession, these reverse faults formed secondary nappe with "after-stretching" typeexpansion mode. During late thrust extrusion process, defunct central fault movedagain, and finally located at Majiawan-Chenjiashan Facture(F2). Finally, since the latethrust effect, Lishuba Fault(F1) replaced Majiawan-Chenjiashan Fault(F2) as thecentral fault, it was still active during the5.12Wenchuan earthquake.Klippes in study area were formed by overthrust from northwest to southeast direction. Klippe body formed by the lower Cambrian strata consisting of its root zoneis located in Qilixiang-Songlinpo Nappe, its formation time, which may be earlyHimalayan, was significantly earlier than that of the upper klippe. Klippe formed bythe Devonian strata, its root belt was covered by Lishuba Nappe. The formation timeshould be the same period as piedmont molasse(Dayi conglomerate) accumulation,namely the Pliocene(N2)-Early Pleistocene(Q1).Through the study of area nappe formation sequence, the conclusions have beendrawn that the nappe tectonic of Sandui region in the north Longmen Mountains was"pre-stretching" type expansion mode at early period, due late giant nappe striker wasblocked inactivity, nappe internal fractured, the newly formed nappe covered over theold nappe, namely the "after-stretching" type expansion mode. And klippes structuresof study area are strong evidences for the "after-stretching" type expansion mode.