Jurassic Intracontinental Deformation And Sedimentary Record

Along with convergence of multiple plates towards the East Asia,the North China block underwent regional intracontinental deformation and orogensis during the Middle-Late Jurassic,which formed a series of intracontinental orogenic belts surrounding the rigid undeformed Ordos basin.Among these belts,the W-E-trending Yinshan-Yanshan tectonic belt and the NNE-trending Taihang Mountains are considered to be the typical representatives.The northeastern Ordos basin is located to the intersection area between these two belts,so that it is the ideal place for studying the closure of the Mongol-Okhotsk Ocean,the subduction of the Paleo-Pacific plate,and their corresponding intracontinental orogensis.This paper takes the Daqingshan tectonic belt and the northern Taihang Mountains as the research objects.Based on the recognition of growth strata,detailed structural and fault kinematic analysis,and geochronological study,a three-stage tectonic evolution is proposed for the Daqingshan tectonic belt.Furthermore,the uplift history of the Taihang Mountains has also been revealed by theprovenance analysis of sedimentary basin and the reconstruction of prototype basin.Finally,this paper discussed the superposed process between the Mongol-Okhotsk tectonic domain and the Paleo-Pacific tectonic domain.The main research achievements are introduced as follows:(1)In the Daqingshan area,the Middle Jurassic Changhangou Formation is generally attached with overturned anticlines along the deformation front that involves the Early-Middle Jurassic coal-bearing sequences,and shows typical growth gemeotries,indicating synchroneity between the sedimentation and the folding deformation.The initiation of intracontinental orogensis in the Daqingshan area has been constrained to the late Middle Jurassic(ca.170 Ma),according to regional correlation and zircon U-Pb dating for a volcanic ash layer(163.7±1.0 Ma)preserved in the upper Changhangou section.Furthermore,an increasing uplift rate has been delineated by the transition of growth geometries from onlap and overlap in the lower section of Changhangou Formation to offlap in the upper section.(2)A three-stage tectonic evolution and related paleostess fields has been established in the Daqingshan area.The first stage was characterized by the Early-Middle Jurassic N-S extension(200-170 Ma),which controlled the initial formation of the Shiguai basin.The second stage was associated with N-S compression(170-160 Ma),which controlled the initial development of the Daqingshan fold-and-thrust system marked by W-E-trending compressional structures.The last stage was performed as NW-SE compression(160-145 Ma),resulting in the development of several structural reentrants along the previous W-E-trending compressional structure,the occurrence of many intrabasinal thrust faults cutting the Changhangou growth strata,and the pronounced eastward migration of the depocenter of the Late Jurassic Daqingshan Formation.(3)The initial uplift of the Taihang Mountains has been considered to be synchronous with the deposition of the Middle Jurassic Yungang Formation,consistent with the onset of intracontinental orogensis in the Daqingshan area.However,reconstruction of the Caogoubu prototype basin indicates deposition of the Jiulongshan Formation was controlled by N-S compression.This feature implies that the NW-SE compression had not yet dominated regional deformation until the Late Jurassic when the violent eruption of Tiaojishan volcanics and the widely development of NE-trending foreland basins infilled by the Tuchengzi synorogenic conglomerates,that is to say,the uplift rate of the Taihang Mountains was accelerated markedly during the Late Jurassic.(4)Along with the progression of NE-trending Paleo-Pacific tectonic system from east to west,the W-E-trending Mongol-Okhotsk tectonic system was gradually overprinted and obscured westwards.Finally,the NW-SE compression began to dominate regional deformation during the Late Jurassic,including the Daqingshan area that is located in the eastern segment of the Yinshan belt.