Yanshanian Tectonic Evolution Of The Chengde Basin And The Adjacent Area In The Eastern Segment Of The Yanshan Fold-and-thrust Belt

Located at the northern margin of the North China block, the Yanshan fold-and-thrust belt hasoften been referred to as the cradle of Chinese geology and is famous from its strong intraplatedeformation in Mesozoic and Cenozoic. From the beginning of the last century, many researcheshave been done and much progress was achieved on the Mesozoic-Cenozoic geology and tectonicsin this area. The time and processes of major deformation events in Yanshanian, the correlation ofthe Mesozoic strata and the deformation of the major sedimentary basins are still controversial,and more researches are essential to clarify these problems.Nappe tectonic and fold deformation of late Jurassic were very typical in Chengde area,north Hebei Province, and attracted attention of domesitc and overseas experts. To researchdeeply tectonic deformation there and basin evolution in Mesozoic can help to know Yanshanianmovement and its meanning well. In this thesis, the sedimentary sequence, clast compositions,paleocurrent data of three Mesozoic sedimentary basins near Chengde were studied. On the baseof the new zircon SHRIMP and LA-ICP-MS dating of the the volcanic rocks of the Tiaojishan inthe Chengde basin and previous isotopic dating results in Yanshan area, the Middle Jurassic -EarlyCretaceous strata were redivided. Moreover, the nature, evolution and overlap of original basinswere studied and the major tectonic events of the Yanshan belt were discussed.The main progresses of the thesis are as following:1. On the basis of former isotopic dating results of the Tiaojishan (Lanqi) period volcanicrocks in the Yanshan area and our new zircon LA-ICP-MS U-Pb dating results of volcanic rocksfrom the top and the bottom of the Tiaojishan Formation in Xinglong mountain area, Chengdebasin, the initial and the terminative time of Tiaojishan period volcanic rocks in this region wasconstrained to 153±1Ma(2σ) and 156±3Ma(2σ), respectively. Comparison of isotopic datingresults of the volcanic rocks of Tiaojishan (Lanqi) Formation in Yanshan region indicate itsbottom age and top age is 158±1Ma and 153±1Ma, respectively. Therefore, the volcanic rocks ofTiaojishan (Lanqi) Formation formed in late Jurassic. The boundary age of Tuchengzi (Houcheng)Formation and the volcanic rocks of Tiaojishan (Lanqi) Formation was restricted to 153±1Ma.Theupper(top) timing of Tuchengzi (Houcheng) Formation was restricted within 134±2Ma～136±2Ma, and its deposit time was from late Jurassic to early Cretaceous. Furthermore, the age ofJiulongshan Formation below the volcanic rocks of Tiaojishan Formation is likely late Jurassic inage. The bottom age of Zhangjiakou Formation was restricted to 134±2Ma in the Yanshan region.2. In early late-Jurassic, volcanic rock of the Tiaojishan formation in the Chengde basinerupted continually. Subsequently, the depocenters of the basin shifted gradually to the north.Sedimentary characteristics of coarse fragment, palaeocurrent and provenance of the late lateJurassic Tuchengzi formation, combined with the movement characteristics of the fault in themargin of the Chengde basin indicate that the northern part of the basin was controlled by the Fengning-Longhua thrust fault.In the middle Tuchengzhi period, the southern part of the basinwas controlled by the Chengde county thrust and the Jiyuqing thrust fault. From late Jurassic toearly early-Cretaceous (Tuchengzhi period) the Chengde basin was rapidly filled by coarsefragment. The course reflected a strong intraplate deformation episode. The episode deformationoccurred during sedimentary of the Tuchenzhi formation, which resulted in dissymmetric synclineof Chengde basin. In the early Cretaceous volcanic rocks of the Zhangjiakou formation with anage of 135±1Ma in the Jiguanshan area of Chengde basin reflects the upper limit of thisdeformation. The deformation is another major phase of deformation in the Yanshan belt.3. Structural analysis of the Shuangmiao thrust sheet and the Sandaohezi thrust sheet indicatethat Shuangmiao thrust was a NNW-ward fault resulted from near N-S compression aftersedimentation of the Tuchengzi Formation in the Late Jurassic. The Sandaohezi thrust, which hasa more complex tectonic history than the Chengde basin, was formed by tectonic superposition ofmulti-stage thrust sheet prior to the Tiaojishan stage.4. On the basis of sedimentary and provenance analysis of the Triassic-latest JurassicXiabancheng basin in northern Hebei province, the evolution of the basin was studied .Appearance and sedimentary of the basin indicate that the sediments in Xiabancheng basin is offluvial facies during early stage of basin. In early Jurassic, the Xiabancheng basin was controlledby the south-ward thrust of the Pingquan-Gubeikou fault. Braided river facies sedimentary rocksfrom two lower segments of the Xingshikou Formation record the uplift and exhumation of theInner Mongolian Paleo -uplift process during this period. Subsequently, the depocenters of thebasin shifted to the north, From the late middle-Jurassic to the early late-Jurassic, the sedimentaryof Longmen Formation and Jiulongshan Formation was strongly deformed. This is the moststrong deformation period and also the major phase of the Yanshan intraplate deformation. Thisstage of deformation happened prior to 158±1Ma.