Fault System Of The Qukou Sag As An Intraplate Pull-apart Basin

The Qikou Sag is located in the north-central of the Huanghua Depression, which in the Bohai Bay Basin. The Qikou Sag, developed as Cenozoic oil sag, is oil–gas-bearing faulted basin, which is one of the most important oil and gas exploration regions for CNPC. Although previous studies reveal the structures of the Qikou sag, fault systems and dynamic mechanisms, the segmentation, the development and migration, activity of faults, the relationship between the basement-involved slipping fault systems and Cenozoic extensional fault systems in the Cenozoic sedimentary cover remain still unclear. It is vital for undstanding of the structural patterns of Cenozoic sedimentary cover. Based on interpretation and analysis of a large number of seismic profiles, we draw rose diagram of fault strikes for each Cenozoic structural layers and fault growth index map for major faults. Analysising the relationship of slipping fault systems and Cenozoic extensional fault systems in the sedimentary cover and dynamic mechanism, the conclusions are summarized as follows.1) Four dextral strike-slip fault zones developed in the basement of the Qikou sag. They called the Cangdong dextral strike-slip zone, the western dextral strike-slip zone, the coastal dextral strike-slip zone and the Yangerzhuang dextral strike-slip zone, respectively. The Cangdong Fault is the west boundary of the Qikou Sag, the Yangerzhuang Fault is the east boundary, the Haihe Fault and the Hangu Fault are the north boundary, and The Qidong Fault is the south boundary. The coastal transfer zone is south-north-striking,which means that the master faults are west-east-striking.2) The extensional fault system in the sedimentary cover is closely related to strike-slip zone under the Cangdong Fault. The faults in the south of the Qikou Sag, such as the Gangxi Fault, the Gangdong Fault and the Nandagang Fault, had intensive activity during the Es3 sedimentation, of which fault growth indexes are larger than those of the faults in the north of the Qikou Sag. The fault growth indexes of the Dazhangtuo Fault and the Tangjiahe Fault are larger than those of the faults in the north and south of the Qikou Sag during the Es2 sedimentation . The Changlu Fault has intensive activity during the Es1 sedimentation, and the other faults are very weak. So activity of the faults migrated from southwest to northeast during the Es sedimentation. The second-order faults in the Qikou Sag have intensive activity in Ed. The faults have weak activity in Neogene. The basement-involved strike-slip faults in the footwall of the Cangdong Fault control the activity of the second-order faults migrating from southwest to northeast, it impies that the Cangdong Fault is a dextral strike-slip fault and controls the pull-apart faulting of the Qikou Sag.3) The Qidong Fault and the Zhangdong Fault in the north of the Chengbei Fault Zone have more intensive activity than the Zhaobei Fault and the Yangerzhuang Fault in the south of the Chengbei Fault Zone during the Es2 sediementation. The intensive active zone still in the north of the Chengbei Fault Zone during the Es1 sediementation, which migrates southward in the Chengbei Fault Zone during the Ed sediementation. The faults have weak activity in Neogene.4) Cenozoic fault systems of the Qikou Sag is mainly controlled by basement-involved strike-slip and extensional fault system in the sedimentary cover. Their coupling between basement-involved strike-slip and extensional fault systems has two cases, the one is accommodated by forced folds related to strike-slip faulting, such as, underwater heave and slope; the other is tranfered by a series of left-step NE-striking en echelon normal faults such as the Nandagang Fault, the Gangdong Fault, the Gangxi Fault and the Dazhangtuo Fault in the plane, and the other group of en echelon faults include the Yangerzhuang Fault, Zhaobei Fault, Zhangdong Fault and the Qidong Fault.5) Tectonic evolution and mechanism of the Qikou sag is the result of the Pacific Plate eastward retroggressive subduction and the Indian Plate progressive subduction. The indentation of the Indian Plate to the Eurasian Plate causes an extrusion in the Eurasian Plate. Mantle flow under the Indian Plate and the North China Plate flows eastward through mantle channel flow under the Eurasian Plate which drives mantle upwelling and crustal rifting in the East China. With the continued injection of mantle material, the mantle upwelling location in the Qikou Sag had a northeastward migration, resulting in intensive fault activity of the migration from the southwest to northeast in the Qikou Sag.