| The Longmenshan orogenic belt is regarded as “Geological Bermuda” by geologists and attracts the attention of scholars from home and abroad due to its overthrust structure and klippen. Seniors conducted in-depth research on the tectonic of Longmen Mountains, and achieved excellent results. However, the three-dimensional data about the Longmen Mountains has not been found. Based on the field geological survey of Majiaoba region in northern section of the Longmen Mountains and the use of GOCAD software for three-dimensional data modeling, through displaying data model and geological interpreting of the three-dimensional model of the structural characteristics of Majiaoba region, certain achievements has been made.The basic method of three-dimensional modeling by GOCAD software is the use of semi-intelligent data model such as borehole data, seismic data, geophysical data, etc. The structure modeling “work flow” process during building the complete model is the key point, wherein it comprises nine steps: loading data'establishing stratigraphic column'establishing model boundaries'fault modeling'horizon modeling'Well hierarchical data constraints'editing contact relationships'the detection and modification of cross-cutting relationships between layers'establishing an intermediate layer. After completing of the workflow, the three-dimensional model could be created by using surface data endowed with geological significance of the fault plane and other aspects of the workflow.Through field geological investigation and data analysis, structural characteristics of nappe of Majiaoba region have been studied in depth. Based on remote sensing data, topographic features, combined with the basic theory of nappe structure and previous studies, according to the geometric characteristics of the main fault and the relationships between faults, GOCAD 3D modeling software has been used for three-dimensional numerical simulations. However, the geological structures of study area are complex, stratigraphy folds are diverse, which are essentially overturned folds, 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 five main profiles, a number of sub-sections and numerous short sections are used to control the model. Due to the lack of depth control data, during the model creation process, the operation steps should more complex than the basic ones: collecting data'arranging data'checking data'loading data'establishing model boundaries'fault modeling'horizon modeling'editing contact relationships'establishing model. After a simple workflow operation, it should be used to establish the geological significance of the model endowed with three-dimensional surface modeling.After completion of the model, it needs to be demonstrated with the geological interpretation. Three-dimensional models created in GOCAD software can be rotated, scattered, cut sections and a single geological solid can be analyzed, etc. And on a plane geological map these functions are no way to achieve, this also reflects the superiority of GOCAD software structure modeling.Based on the geological analysis results obtained from the three-dimensional model, in combination with the data collected indoors and the studies into the characteristics of the structural assemblage within the study area, it could be inferred that the faults within the investigated area are all imbricate thrust ones with the inclination of northwest, trend of northeast to southwest and the napping direction going along northwest to southeast. No evidence of normal faults has been found. Moreover, it is not difficult to see from the section of the model that the napping distance becomes shorter from northwest to southeast. Therefore, it could be concluded that the front of the nappe that takes these faults as its major detachment surfaces was blocked when it thrust towards southeast. As a result, it couldn’t thrust forward smoothly. It also indicates that the faulting in the southeast stopped earlier than in the northwest. Only in this way, can it block the moving forward of the nappe from the northwest. According to the stratigraphic development situations within each nappe, the stop time of the faulting, etc., it could see that the pushing distance of the nappe becomes smaller from northwest to southeast. Thus, we can come to the conclusion that the tectonic stress within the investigated area is decreased, the tectonic deformation is relieved and the napping distance is shortened gradually along the northwest to the southeast.By virtue of the established the three-dimensional model within the study area, the author conducts a geological analysis of the fault structure, imbricated nappe, fold and spatial distribution among them and clarifies the structural configuration of geologic body. GOCAD software can be used to accomplish the three-dimensional modeling of above-mentioned complex geologic body and the calculation of its actual area, projected area, volume, etc., having broad application prospect in terms of geological analyses. |
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