Characteristics Of Rifting Interfered By Magmatic Diapirism

Current research about the rift basin has reached some mature understandings on issues such as structure, sedimentary, magmagitism and formation mechanism. Magmatic activities are very usual during the development of a rift basin, however, the structural or tectonic relationship between them is fairly unkown, and very few studies have reached that aspect. Based on a synthetic study of the structure, sedimentary and magmatism of Jizhong Rift of Bohai Bay Basin, we have found the development of Jizhong Rift was highly effected by the local magmatic diapirism.Section restoration is used in this study to determine the active period of the main boundary faults of Jizhong rift. The overall structure of the rift is featured as S type, which is chacterized by SE dipping boundary fault in the northern part, NWW dipping boundary fault in the southern part and a N-S striking dextral transfer zone in the middle part. The evolution of the main boundary faults could be divided into three stages. During the deposition of Ek-Es4, the active boundary fault in the north is Daxing Fault and in the south is Maxi-Liulu Fault, while the transfer zone was consisted of several N-S striking normal faults like Chu-an Fault and Maozhou Fault, and some en echelon normal faults between them. During the deposition of Es3-Es2, the faults within the transfer zone gradually ceased, while the boundary faults in the north and south remained active. During the deposition of Esl-Ed, the Daxing Fault was replaced by the formation of Niudong Fault and Maxi-Liulu Fault was replaced by Maxi-Liu18Fault, and also the transfer zone was shifted toward east and became a narraw N-S striking dextral shear zone.The sendimentary of Jizhong rift was directly controlled by the evolution of the main boundary faults. During the period of Ek-Es2, the lacustrine that was bounded by Daxing and Maxi-Liulu Fault was larger, with the subsiding and depositing center in Langgu Sag, which was jointed with Baxian Sag at that time. During the period of Es1-Ed, the replacement of the Daxing Fault by Niudog Fault coursed the Langgu Sag to stop its rifting and subsequently became flood plain, and meanwhile the Baxian Sag to develop lacustrine independently, the replacement of Liulu Fault by Liu18Fault made the Raoyang Sag to retreat toward northwest. As a result, the alternation of the main boundary faults made the Jizhong rift narrower.The magmatic activities in the Jizhong rift were focused in the period of Es4and Es3, which generated massive flood basalts, sub-volcanic facies diabase, and eruptive facies tuff in the Langgu Sag and Wenan Slope. Based on the interpretation of3-D seismic data and magnetic data, the intrusive mass could be found beneath the Niutuozhen Heave, the Fengheyin Heave and Suning area of Raoyang Sag, which were emplace at the end of Es4and the end of Es3(the latter two), respectively. The intrusive mass beneath the niutuozhen Heave is the largest, which is more than10km wide in the profile.The magmatic diapirism would force the strata and the boundary fault, if any, above to uplift, forming an equivalent-scale dome, and subsequently, local high-angle unconformity. The boundary fault reshaped by magmatic diapirism would still be active, and generate wedge-shape strata. The reshaped fault will gradually become inactive and be replaced by a new boundary fault loacated on another side of the dome, controlling the formation of a new rift. On the view of a whole rift, a certain volume of magmatic diapirism will shut down the rifting process of its outer side, reshping the basin into a narrower one.