Mesozoic Tectonic Evolution And Late Reforma-Tion Of The Northern North-south Tectonic Belt,China

The Northern North-South Tectonic Belt,locating in the conjunction region among the Ordos Block,the Alxa Block,the Qinling-Qilian Orogenic Belt and the Xing-Meng Orogenic Belt,is not only the boundaries separating the East and West China's continental tectonics,but also the key area for studying the interaction and transformation between these two mainlands.Over the past decades,different opinions have been proposed to describe the original tectonic framework and evolution process of the tectonic belt during the Mesozoic period,due to the strong late reformation effect since the Cenozoic.This study chose the Northern North-South Tectonic Belt,especially its middle-south segment,as the study area.The field observation,data from oilfields and coalfields,together with comprehensive means of geological analysis,sedimentology,tectonics,isotope chronology,thermochronology,geochemistry and provenance analysis method were applied to discuss the tectonic periods of the study area since the Mesozoic and to analyze the stratigraphic distribution,sedimentary condition and provenance to reconstruct the original basin and evaluate the petroleum potential of the middle-south segment.Based on a large amount of thermochronology data accompanied with previous study,the tectonic events of the Northern North-South Tectonic Belt could be divided into various of episodes with different geological settings:Late Triassic-Early Jurassic period,Late Jurassic-Early Cretaceous,Late Cretaceous-Paleocene,Eocene and Late Miocene.Based on these,this paper is mainly aimed at reconstructing the original basin of the middle-south segment of Northern North-South Tectonic Belt during the main period in Mesozoic,e.g.Late Triassic,Early-Middle Jurassic and Early Cretaceous,by analyzing the tectonic events,residual strata distribution and transformation characteristics together with field investigation,drilling,seismic data and relevant tests.The author believes that the Upper Triassic Yanchang Formation was widely deposited in the middle-south segment of Northern North-South Tectonic Belt and connected with the Ordos Basin,shortly after the deposition,the sediments were strongly reformed and suffered erosion due to the Indosinian movement,leading basically no residual Yanchang Formation sediments in this area.When it comes to Early-Middle Jurassic,the paleo uplifts formed during Indosinian period were gradually suffered deplanation,the study area was widely spread with the Yan'an Formation sediments and connected with the Ordos Basin again.The tectonic event occurred in Late Jurassic to the early stage of Early Cretaceous resulted in strong uplift of the study area and the separation between the Liupanshan Basin and the Ordos Basin.By the time at the late stage of Early Cretaceous,the tectonic activity gradually weakened and the sedimentary area expanded,leading the study area finally connected with the Ordos Basin.The uplift event in the middle-south segment of Northern North-South Tectonic Belt during Late Triassic-Early Jurassic could be controlled by the Qinling-Qilian Orogenic Belt;while the north segment was closely related to the interaction between the Ordos Basin and the Alxa Block.The uplift event during Late Jurassic-Early Cretaceous was referred to the formation of a secondary extrusion convergence belt and thrust nappe tectonic belt in Northern North-South Tectonic Belt caused by multi direction convergence effect of adjacent plate surrounding the North China Block.The tectonic event during Late Cretaceous-Paleocene period was characterized by regional uplift that was responded to the convergence between China massif and Pacific Block and Siberia Block and the deep continental geodynamic effect.The uplifting event in the Eocene occurred in tensional environment and was the response of the peripheral rifting and breakup event of the Ordos Basin but not of the uplift of the Tibetan plateau.While in the Late Miocene,the tectonic uplift event was widely distributed in the margin of the Tibetan plateau,indicating the extrusion expansion of Tibetan plateau didn't spread to the middle-south segment of Northern North-South Tectonic Belt until Late Miocene.It is noteworthy that based on the contrastive study of tree arcuate tectonic belts in the study area,it is suggested that the tectonic event was strong in south and weak in north,which is consistent with the northeastward expansion of the Tibetan plateau.There are three series of potential source rocks deposited in the middle-south segment of Northern North-South Tectonic Belt:Yan,an Formation in Jurassic and Liwaxia and Madongshan-Naijiahe Formations in Cretaceous.The source rocks in the Yan,an Formation are characterized by good-great level of source rocks with high content of TOC and mature organic matter which are type ?2 and ?.Most of the source rocks in Madongshan-Naijiahe Formations can be considered as good source rocks in low maturity stage,only the strata in lower subsection is just mature.The source rocks in the Liwaxia Formation are poor and non source rocks in the upper subsection,while good source rocks in the lower subsection,the level of thermal evolution is mature-highly mature.The type of kerogen in the two sets of source rocks in Cretaceous are type ? and ?.Based on the abundance,type and maturity of the organic matter,it is indicated that the Jurassic-Cretaceous hydrocarbon source rocks,especially those overlain by thick Cenozoic sediments,have good prospects for oil and gas resources.