Sedimentary Basin Evolution And Its Tectonic Implication In Lingyuan,eastern Yanshan

Located in northern part of the North China Craton,the Yanshan fold-thrust belt represents a typical intracontinental deformation zone.The Yanshan belt has experienced multi-phases deformational events during Mesozoic period and shaped a complicated basin-mountain system.It is now widely accepted that the Yanshan belt was controlled with widespread extensional tectonics after Early Cretaceous(ca.135 Ma),but the tectonic property and structural evolution of this belt in Jurassic was still under great debate among geologists.Intermontane basins in the Yanshan belt honestly recorded the tectonic activities.Thus,exploring research of syn-tectonic sedimentation and the corresponding structures in a basin-mountain system is a useful tool to reveal the Triassic-Jurassic deformation mechanism,tectonic evolution history and plate tectonic setting in the Yanshan belt.In this study,we applied detailed geochronology,sedimentary facies analysis,provenance analysis,thrust fault-related fold and corresponding growth strata resolving in the Mesozoic Dengzhangzi basin and Guojiadian basin in the Lingyuan area of eastern Yanshan belt,trying to propose a stratigraphic division scheme,rebuild the paleoenvironment and the“source-to-sink”system in these two basins,and to figure out the tectonic evolution sequence in Triassic-Jurassic with integration of geochronology.According to the geochronological results,we determined the ages and sequence of various stratigraphic units in the study area.From bottom to top,the Laohugou Formation formed during the Middle Triassic;the Shuiquangou Formation,which unconformably overlies the Laohugou Formation,formed during the Late Triassic;and the Dengzhangzi Formation conformably overlies the Shuiquangou Formation.The maximum depositional age of the upper Dengzhangzi Formation is Late Triassic.The Guojiadian Formation is Middle-Late Jurassic in age.Our works in sedimentary and provenance analysis suggests that Dengzhangzi basin was dominated with complete alluvial fan-fan delta-lacustrine sedimentary environment during Middle Triassic-Late Triassic/Early Jurassic,and Guojiadian basin was dominated with braider river-alluvial fan sedimentary environment during Middle Jurassic.The Dengzhangzi basin and Guojiadian basin are typical flexural basin with development of thrusting in basin margins.Based on the paleo-sedimentary environment restoration results and basin margin structure analysis,we reconstructed the"source-to-sink" systems of eastern Yanshan belt in Middle Triassic-Late Triassic/Early Jurassic and Middle Jurassic periods.The evolution of "source-to-sink"systems indicates a major tectonic transition in Yanshan belt at Middle Jurassic(ca.170 Ma),from a N-S contractional setting in Late Triassic-Early Jurassic to a NW-SE contractional setting in Middle Jurassic-Early Cretaceous.After Middle Jurassic,this tectonic transition demonstrated that the subduction of Paleo-Pacific plate under East Asia has initially dominated the deformations in Yanshan belt.