Analysis On Structural Characteristics In The Micangshan Area, NW Sichuan Basin

The Micangshan structure, located at the northwestern side of Sichuan basin and the northern margin of Yangzi Craton, is surrounded by Qinlin orogen to the north, Dabashan structure belt to the east and Longmenshan thrust belt to the west. Its formation and evolution has a close relationship with the surrounding orogens and thrust belts, making itself a key area for studying the geological evolutional history of northern Sichuan basin and adjacent orogens. Based on interpretation and analysis on high resolution seismic profiles, this dissertation studied the characteristics of structures in Micangshan area, and discussed the geological evolutional history as well. The main conclusions are as follows:1. Micangshan is mainly a large-scale basement-involved structure with approximately E-W strike. It can be divided into two units with different deformation characteristics by the gypsum layer at the base of Lower Triassic Jianglingjiang Formation: deformation of the upper unit is weak and simple, while that of the lower unit is fairly strong and complicated, representing as a series of duplex structures with north-dipped thrust faults. What's more, erosion, strata thinning and thickening can be identified under the gypsum layer, indicating the existence of structural deformation in Early Triassic.2. The basement-involved thrust fault system in Micangshan linked to the thin-skinned thrust fault system in Longmenshan through lateral fault ramp, which is composed of several east-dipping, basement-involved reverse faults and forms the western boundary of the Micangshan thrust fault system.3. A series of nearly NE-trending folds, which are slightly deformed in the shallow layers, are developed to the southern side of Micangshan. The western part of these folds are mainly NE-trending, indicating the controls of Longmenshan thrust fault system. However, toward the east, they gradually change to E-W trending, indicating stronger and stronger effect of the Micangshan thrust fault system.4. Since the Mesozoic era, the Micangshan area experienced four times structural deformations in the Early Triassic, the Latest Jurassic, the Early Cretaceous and the Late Cenozoic time respectively. The first and second deformations were fairly weak. The present structural framework in Micangshan area basically formed during the third deformation period, when the Micangshan strong uplifted, and intensified in the Cenozoic deformation (the fourth one).