Growth Structures And Growth Strata Of The Qianjiadian Basin In The Western Yanshan Fold And Thrust Belt,North China

The northwestward subduction of the Izanagi Plate beneath the eastern Eurasian Plate during the late Mesozoic caused a series of compressional deformations in the Yanshan fold and thrust belt,but debates continue regarding the tectonic deformation timing,tectonic properties and relationship between tectonic uplift and sediment accumulation in the intermontane basins.We studied the sediments,sources,basin margins and intrabasin structures of the Upper Jurassic-Lower Cretaceous Tuchengzi Formation in the Qianjiadian Basin?QJB?,North Beijing.The results suggest that the Tuchengzi Formation of the QJB is divided into three members.Member 1 is dominated by conglomerate and mudstone,which are mainly fan-delta deposits;Member 2 is dominated by mudstone and siltstone,which are mainly lacustrine deposits;Member 3 is dominated by conglomerate,which are mainly alluvial fan deposits.A fault-propagation fold-type?FPFT?growth structure and growth strata developed on the western edge of the QJB,and that the top part of Member 2 and Member 3 of the Tuchengzi Formation are growth strata controlled by a limb rotation mechanism.The offlap-type progressive unconformity and the onlap-type progressive unconformity developed in the growth strata.Two small thrust faults that developed in the QJB are characterized by the Mesoproterozoic Xiamaling Formation thrust over the Tuchengzi Formation,which may have controlled the development of the fault-bend fold-type growth strata.An analysis of the"source-sink"process shows that the gravel composition is consistent with the stratigraphic lithology of the hanging wall of the Shangyi-Pingquan fault?SPF?,and the paleocurrent directions are generally from NE to SW,and Member 3,from North to South,has obvious sedimentary face changes from fan root-fan middle-fan margin.Therefore,the hanging wall succession of the SPF was the main source area of the Late Jurassic-early Early Cretaceous strata in the QJB rather than the hanging wall of the Qianjiadian thrust fault?QJTF?.A zircon ²⁰⁶Pb/²³⁸U age of 140.8±2.4 Ma from the volcanic rocks at the bottom of the FPFT growth strata represents the initiation time of the FPFT growth structure.The discovery of the FPFT growth structure and growth strata in the QJB proves that the QJB was an intermontane flexural basin controlled by fold and thrust structures during the early Early Cretaceous.Near the E-W-trending SPF,the NE-SW-trending QJTF and two small intrabasinal thrust faults may constitute a unified right-lateral strike-slip system in response to the northwestward flat subduction of the Izanagi Plate beneath the East Asian continent during the Jurassic-Early Cretaceous.