| “Yanshan movement” is an important process of the evolution of the North China Craton,and its active time limit,tectonic attributes,deformation styles,deformation intensity and influence range have always been the focus of discussion.The sedimentary basin records the evolution of the structure in and around the basin,which is of great significance to the regional tectonic evolution.In this paper,the Middle Jurassic Haifanggou formation in the east of Ningcheng basin in western Liaoning Province is taken as the main research object,which in the eastern part of Yanshan fold thrust belt.Through stratum section surveying and sedimentary facies interpretation,we established the lithostratigraphic framework and sedimentary facies belt distribution model of the basin,and we established the possible source-sink system according to gravel composition statistics.Combined with the discovery of syntectonic sedimentation,we revealed the nature of Ningcheng basin during the sedimentary period of Haifanggou formation,restored the basin evolution process,and constructed regional tectonic evolution sequence.The unconformity at the bottom of the Haifanggou formation corresponds to the initial phase of the Yanshan movement,and the unconformity between the top of the Haifanggou formation and the tiaojishan formation corresponds to the end of phase A of the Yanshan movement.The study of the Haifanggou formation in the basin is helpful to understand the structural properties and deformation characteristics of phase A of the Yanshan movement in western Liaoning Province.The research results of basin lithology and sedimentation show that the Haifanggou formation is a set of terrigenous clastic deposits,which contains a lot of volcanic materials.The grain size changes from coarse to fine to coarse from the bottom to the top,corresponding to the bottom alluvial fan-lacustrine clastic deposits and the top delta sandstone-mudstone deposits.The original basin center is located in the northwest of the study area.From the edge to the center,the fine clastic composition increases continuously,while the coarse clastic composition decreases to disappear.The conglomerate gravels of Haifanggou formation are mainly composed of granite and gneiss,mixed with some volcanic gravels,indicating the provenance area dominated by Archean basement on the southeast side.Controlled by the Lingyuan-Beipiao thrust fault on the southeast side,it is a piggyback basin developed on the hanging wall of the thrust fault.According to the geometry and kinematics analysis of synsedimentary structure,the Haifanggou formation in the study area was controlled by NW-SE compression.At about168?166Ma and ?165Ma,a relatively strong compression event occurred in the Haifanggou formation respectively,which resulted in the formation of rotational growth strata in the wing near the hada fire and retrogradation overlap unconformities controlled by many small faults.At about165?163Ma,the overall stratigraphic angle is gentle,and the top of the formation is nearly parallel unconformity contact with tiaojishan formation,so there may be no strong tectonic activity.Combined with previous studies,we believe that during the initial phase of the Yanshan movement and phase A,western Liaoning was controlled by NW-SE compression.During the initial phase,the NE trending thrust faults were mainly produced and further developed in western Liaoning,which laid the foundation for the volcanic events of the later tiaojishan formation;the final stage of phase A did not have strong tectonic activity,which may be that other factors used the early structure to cause the large-scale volcanic eruption.By comparing the subduction of the Mongolia-Okhotsk tectonic belt with that of the paleo-Pacific plate,this paper tend to think that the latter controlled the tectonic evolution here. |
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