Yanshanian Superposed Structural Characteristics Of West Margin Of Foreland Fold Belt Of South Dabashan

In the front of south Dabashan arcuate thrust belt, two kinds of superposed areas are formed in the west margin of foreland fold belt, the one is the cross superposed area in the northern part, and the other is a fan-shaped superposed area, which is dominated by Triassic and Jurassic in the southern part. It is meaningful to study the structural geometry and kinematic characters and formation of different types of superposed areas to further understand the intracontinental tectonics and relationship between the Qinling orogenic belt and the north margin of Yangtze block. Based on previous research results, some research methods are applied in this study, such as structural analysis by field observation and statistic of fold geometric factors, test of anisotropy of magnetic susceptibility(AMS) and so on. The discussion comprehensively analysis the superposed relationship between the west margin of south Dabashan foreland fold belt and the Micangshan nearly EW anticline, as well as the giant flexural syncline located to the south of the Mt. Micangshan during the Yanshanian intracontinental tectonic evolution after the closure of Mianlue suture in Indosinian. The most significant fruits of this study can be concluded as follows.1) The formation mechanism of the fan-shaped superposed area is preliminarily discussed. At the first step, the nearly EW giant flexural syncline to the south of Mt. Micangshan and the NNE syncline to the west of Mt. Huayingshan were superposed in the south of the study area and a broad superposed syncline area was formed, whose NE part was narrow while SW part was raletively wide. Later, the broad syncline underwent another superposition by the west margin of south Dabashan foreland fold belt, and the fan-shaped superposed area which mainly outcrop Triassic and Jurassic was finally formed at the south of study area.2) Based on previous results and boundaries of abundant materials obtained in this study, it can be inferred that the NE margin of Sichuan basin has undergone the following intracontinental tectonic evolution sequence in Yanshanian. Late J₂, a nearly EW fold was formed in the north margin of Yangtze block, due to the continuing subduction of Yangtze block towards north. Synchronously or later, the South Sichuan fold belt extended from SE to NW, and superposed with the nearly EW flexure syncline and formed the primary framework of the south part of study area. Because of the barrier of Hannan and Huangling rigid blocks, at late J3, the Dabashan thrust belt overthrusted from NE towards SW, which induced the foreland fold belt and superposed onto the former syncline. Furthermore, the tectonic regime made up of the three different tectonic stress last to at least late of K₁, however, the nearly SN compressional stress became much weaker in the late stage.3) Due to the study and analysis of the application of magnetic fabric in such a condition where the sedimentary rocks deformed in a multi-stage and multi-direction regime, plenty of useful information are obtained, and it's also suggest that this method should be based on the basic geologic background and be used carefully in intricate area.