Early Cretaceous Deposits Thrust Related In Xuanhua Basin,Western Part Of Yanshan Fold Thrust Belt,North China

The nature,active time,deformation process and movement driving mechanism of the main period of Yanshanian movement are still controversial.The detailed analysis of the deformation styles and sedimentary filling characteristics of the fold thrust belt and sedimentary basin in the North China Craton is the key to solve the above disputes,accurately grasp the deformation time limit and deformation mechanism of the main Yanshanian movement(b curtain),and further reveal the tectonic process,coupling mechanism and dynamic background of the basin mountain system.In this paper,Zhangjiakou Xuanhua basin is selected as the research object,which is well preserved Mesozoic strata and confined by basin margin structure.Through systematic basin analysis of the early Cretaceous Tuchengzi Formation in the basin,including stratigraphy,sedimentology,structural geology,isotope chronology and so on,a complete chronostratigraphic framework is established,and its sedimentary system is restored To clarify the relationship between the sedimentary process of target strata and basin margin structure.On the basis of large-scale field mapping,the upper and middle strata of Tuchengzi Formation are divided into fluvial facies braided river delta facies alluvial fan facies by sedimentary facies analysis;According to the difference of fluid dynamics during the formation of lithofacies,10 types of sedimentary facies associations are identified,namely,matrix supported conglomerate(GMM),massive conglomerate(GCM),graded bedding conglomerate(GCG),schistose conglomerate(GCS),grain supported conglomerate(GCO),cross bedding conglomerate(GCC),massive sandstone(SM),tabular cross bedding sandstone(st)and massive mudstone(m).The analysis of sedimentary facies reveals that the sedimentary environment mutates vertically and near the growing strata,from River Delta lake facies to alluvial fan facies.The provenance characteristics also changed significantly with the change of sedimentary facies,including the change of paleocurrent from north to south in the early stage,and the change of gravel composition from granite and gneiss debris to dolomite gravel.The growth strata related to thrusting and fault spreading of Mapu thrust fault were identified in the horizon near the sedimentary environment transformation(delta deposit at the lower part),and the sedimentary age of the stratum 142.3 ±4.6Ma,which limits the activity time of Mapu thrust fault with NE-SW strike to early Cretaceous.The formation of the alluvial fan in the upper part of the Tuchengzi Formation and its relationship with the underlying fluvial delta lacustrine deposits,the spatial location(southern basin),provenance(southern basin)and stratigraphic location(above the growth strata)of the alluvial fan suggest that its formation is controlled by the Mapu thrust fault,but its formation lags behind the thrust.According to the surface trace and growth stratigraphic types of growth structures in the study area,it is inferred that Xuanhua basin was in NW-SE compressional stress environment in the early Cretaceous,and NW progressive plate subduction of Izanagi plate may be the driving force of the thrust.