Yanshan Epoch Tectonic Superposition In Changping Xuejiashiliang Region

Changping Xuejiashiliang Region, located in the western segment of YanshanOrogenic Belt, underwent intensively tectonic deformation and magma activity inYanshan epoch. Based upon the tectonic deformation and magma activity in the region,tectonic superposition in Yanshan epoch was discussed in this article.As a result of complicated tectonic deformation, different direction and age foldsand faults developed in Xuejiashiliang Region. Fold strikes are mainly northeast andnorthwest. Fault strikes mostly include two direction: northeast faults, e.g.Thirteen Mound-Xiazhuang reverse thrust;and northwest faults, e.g.Shaling-Xiazhuang fault, south of Xiazhuang fault. Thereinto, northeast faultsformed more early;northwest faults formed last and cut the early-formed ones.Magmatic activity, developed extremely in the study area, include 4 assemblages,viz. middle Jurasic Thirteen Mound vocanic basin, late Jurasic Changyuan complexbodies, early early-Cretaceous Xuejiashiliang complex body and late early-CretaceousNankou bimodal dike swarm. Thereinto, Xuejiashiliang complex body is asynkinematic intrusion.,which formed in the same tectonic setting.Thirteen Mound-Xiazhuang reverse thrust cut Thirteen Mound volcano basin, butwas cut by Xuejiashiliang complex body, which shows that the reverse thrust tookplace later than the volcano basin but earlier than the complex body. Time limit of theThirteen Mound-Xiazhuang reverse thrust, ascertained by syntectonic metamorphicintrusion, is 151-159Ma. The emplacement time of Nankou bimodal dike swarm is120Ma. Nankou bimodal dike swarm can be considered as direct evidence oflithosphere extension caused by Mesozoic North China low-intrusion.Two compression deformation and two extension deformation took place atJurassic-Cretaceous period. Tectonic system of Jura-early Cretaceous happenedreverse. Compression and extension direction is different from the middle Jurassic tothe early Cretaceous period. We think that it is compression situation in the early stage of the EarlyCretaceous, which is not extension situation. It is extension situation in the later stageof the Early Cretaceous.