Analysis On Structure In South Margin Of Jiuxi Depression

The experiences of Jiuxi Depression exploration indicate that the foreland thrust belt of Jiuxi Depression has great potential and broad oil and gas prospect.In this theory, we make the revolution of Tethys tectonics and the coupling relation of basin and mountain as the main line of our study. Based on surface geology and physical geography analysis data, the structural characteristics, internal textures and evolution of the foreland thrust belt of Jiuxi Depression are studied under the direction of the theory of fault-related-folds and thrust structures. Then, five conclusion were gotten, as follows:The distribution of the faults existed in the foreland thrust belt are characterized by multi-epoch and directional. The faults in the area can be divided into five groups according to their directions, which are shaped in the early of Late Yan Shan movement, the end of Late Yan Shan movement and Himalayan movement.The character of the structural styles is variety. Based on synthetic analysis data, four types and eleven subtypes of basic structural styles are identified in the foreland thrust belt of Jiuxi Depression as follows: extensional tectonics ("Y" synsedimentary normal fault, domino type half-graben), compressional tectonics (imbricate thrust structure, triangle belt, fault-related fold, duplex, stacked anticline, pop up, composed structural wedge), wrench tectonics (echelon folds) and inversion tectonics (positive inversion s tructure). The distribution of the structural styles existed in the foreland thrust belt are characterized by layer upon layer vertically and by zones laterally.The texture of the foreland thrust belt of Jiuxi Depression is a forward propagating mode. The slide surface of the thrust belt varied from old system to young system towards foreland. The structure of the belt can be divided into three zones laterally, which are the frontier fold belt in the northern Lao Junmiao anticline, the Lao Junmiao-Qing Caowan thrust-fold belt and the fore-mountain thrust-fold belt. The Lao Junmiao-Qing Caowan thrust-fold belt can be subdivided into two segments along the west to the east.The structural evolution of the foreland thrust belt of Jiuxi Depression can be divided into six stages as follows: (1) Triassic-Early-Middle Jurassic: the stage of pre-Cretaceous system basement accept sediment, (2) Middle-Late Jurassic: the stage of the old faults in northern margin of QiLian mountain reactive , folded and thrust, (3) the early of Early Cretaceous-the end of Early Cretaceous: the stage of tensional companied with slide , (4) Late Cretaceous-Eocene epoch: thrust and uplift stage, (5) Oligocene epoch- the end of Pliocene epoch: stage of vertical vibration, the end of Pliocene epoch-now: overthrust and fold stage.The controlling of tectonics for distribution of source rocks, types of traps, andtimes of hydrocarbon generation, migration and accumulation are obvious.