Jurassic Tectono-Sedimentary Evolution History Of The Western Yanshan Fold-Thrust Belt,North China,and Its Tectonic Implication

Located in northern part of the North China Craton,the E-W trending Yanshan fold-thrust belt represents a typical intracontinental deformation zone that was surrounded by plate convergences of multi-direction.In this case,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 Jurassic deformation mechanism,tectonic evolution history and plate tectonic setting in the Yanshan belt.In this study,we applied detailed sedimentary facies analysis,provenance analysis,thrust fault-related fold and corresponding growth strata resolving in the Mesozoic Xiahuayuan basin and Chicheng basin in the Zhangjiakou area of western Yanshan belt,trying to rebuild the paleoenvironment of and the “source-to-sink” system in these two basins,and to figure out the tectonic evolution sequence in Jurassic with integration of geochronology.Our works in sedimentary and provenance analysis suggests that both basins were dominated with complete alluvial fan-fan deltalacustrine sedimentary environment during basin-filling episode.The Xiahuayuan basin and Chicheng basin are typical flexural basin with development of thrusting in basin margins.And the marginal thrusting activities controlled fault-related folding and development of corresponding growth strata in both basin margins and basin-fillings.In basis of zircon U-Pb geochronology in growth strata,we reconstructed the “sourceto-sink” systems of western Yanshan belt in Late Triassic-Early Jurassic,Middle Jurassic and Late Jurassic-Early Cretaceous 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.This tectonic transition demonstrated that the subduction of Paleo-Pacific plate under East Asia has initially dominated the deformations in Yanshan belt.