Tectonic paleogeography of Late Sinian and its significances for petroleum exploration in the middle-upper Yangtze region, South China

The paleotectonic pattern,lithofacies paleogeographic features,sedimentary evolution and its controlling effects on hydrocarbon accumulation assemblages during the depositional period of the Sinian Dengying Formation in middle-upper Yangtze region were investigated based on outcrops,drilling,log and seismic data.The study shows that,(1)Affected by the breakup of the Rodinia supercontinent,the middle and upper Yangtze areas were in extensional tectonic environment during the depositional period of Dengying Formation.The carbonate platform was structurally differentiated.Intra-platform depressions controlled by syndepositional faults developed,forming a tectonic-paleogeographic pattern of"three platforms with two depressions".(2)During the depositional period of the first and second members of the Dengying Formation,rimmed platforms and intra-platform fault depressions developed in upper Yangtze area and isolated platform developed in middle Yangtze area,and there was the Xuanhan-Kaijiang ancient land block in eastern Sichuan.The depositional period of the third member of the Dengying Formation is the transformation period of tectonic-paleogeographic pattern,when a set of shallow water shelf sediment rich in mud was deposited due to transgression on the background of the eroded terrain formed in EpisodeⅠof Tongwan Movement.The sediment of the fourth member of the Dengying Formation inherited the paleogeographic pattern of the first and second members of the Dengying Formation in general,but the Deyang-Anyue intra-platform fault depression further expanded,and the middle Yangtze platform evolved into two separated platforms.(3)Tectonic-sedimentary differentiation and evolution of carbonate platform in the Sinian gave rise to two types of accumulation assemblages with wide distribution and great exploration potential,which are platform margin and intra-platform.

Reconstruction of the Triassic Tectonic Lithofacies Paleogeography in Qiangtang Region, Northern Qinghai-Tibet Plateau, China

The Triassic petrostratigraphic system and chronologic stratigraphic sketch have been updated and perfected in the Qiangtang area, Qinghai-Tibet Plateau based on the integrated 1：250000 regional geological survey and the latest research progeny. The first finished 1：3000000 Triassic tectonic lithofacies paleogeographic maps in the Qiangtang area shows that the Triassic tectonic unit in the Qiangtang area can been divided into three parts from north to south： northern Qiangtang block; Longmucuo-Shuanghu suture zone; and southern Qiangtang block. The early-middle Triassic tectonic paleogeography in the Qiangtang area is divides into three sub- units： northern Qiangtang passive continental marginal basin （NQPB）, Longmucuo- Shuanghu residual basin （LSRB） and southern Qiangtang residual basin （SQRB）. The NQPB can be subdivided into four paleogeography units： The Tanggula-Zangxiahe shallow and bathyal sea; The Wangquanhe- Yingshuiquan carbonate platform; The Rejuechaka-Jiangaidarina littoral- shallow sea; and Qiangtang central uplift. The above units of The NQPB possess EW trend, geomorphology high in the south and low in the north, the seawater depth northward. The basinal paleo-current direction is unidirectional, and basinal tectonic subsidence center is in accord with the depo-center, located in the Tanggula-Zangxiahe belt, north of the basin. The sedimentation and tectonic evolution of the NQPB are characterized with passive continental marginal basin. The Qiangtang central orogenic denuded area （ancient land） may be as a sedimentary materials source of the NQPB. SQRB can be divided into two units： Duoma carbonate platform and southern Qiangtang neritic-deep sea. The late Triassic tectonic paleogeography in the Qiangtang area is the framework of the ＂archipelagic-sea＂ as a whole, and it may be divided into three sub-units： northern Qiangtang back- arc foreland basin（NQFB）, Longmucuo-Shuanghu residual basin（LSRB） and southern Qiangtang marginal-sea basin（SQMB）. Thereinto, NQFB can be divided into five paleogeography units： the Zangxiahe-Mingjinghu bathyal basin characterized with the flysch; the Tanggula shallow-sea shelf with the fine-clastics; the Juhuashang platform with carbonates; the Tumenggela-Shuanghu coastal- delta with coal-bearing clastics and the Nadigangri- Geladandong arc with volcanics and tuffs. In transverse section, the NQFB fills is wedge-shaped, and the sediments characterized with thicker in north and thinner in south, and with double materials derived from the Ruolagangri orogenic belt in north and the Shuanghu central orogenic belt in south. The late Triassic depocenter of NQFB is located in the middle of the basin, the Yakecuo-Bandaohu-Quemocuo belt, but the subsidence center in the north, the Zangxiahe- Mingjinghu belt, and basinal tectonic subsidence center not concordant with the depo-center. Late Triassic, the SQMB may be divided into three sub-units： Xiaochaka shallow-sea; Riganpeicuo platform~ and South Qiangtang southern bathyal basin. In transverse section, the basement of the SQMB is characterized with low in the northern and southern, but high in the middle; forming wedge shaped sediments with thicker in the north and thinner in the south; the sedimentary materials derived from the Qiangtang central uplift and Nadigangri arcs in north. The late Triassic subsidence centre of the SQMB is located in the northern （Xiaochaka area）, but the depocenter in the southern （Qixiancuo Suobucha area）. The sedimentation and tectonic evolution of the SQMB are characterized with marginal sea.

Plate tectonics in the late Paleozoic

As the chronicle of plate motions through time, paleogeography is fundamental to our understanding of plate tectonics and its role in shaping the geology of the present-day. To properly appreciate the history of tectonics--and its influence on the deep Earth and climate-it is imperative to seek an accurate and global model of paleogeography. However, owing to the incessant loss of oceanic lithosphere through subduction, the paleogeographic reconstruction of ＇full-plates＇ （including oceanic lithosphere） becomes increasingly challenging with age. Prior to 150 Ma ~60% of the lithosphere is missing and re- constructions are developed without explicit regard for oceanic lithosphere or plate tectonic principles; in effect, reflecting the earlier mobilistic paradigm of continental drift. Although these ＇continental＇ re- constructions have been immensely useful, the next-generation of mantle models requires global plate kinematic descriptions with full-plate reconstructions. Moreover, in disregarding （or only loosely applying） plate tectonic rules, continental reconstructions fail to take advantage of a wealth of additional information in the form of practical constraints. Following a series of new developments, both in geo- dynamic theory and analytical tools, it is now feasible to construct full-plate models that lend themselves to testing by the wider Earth-science community. Such a model is presented here for the late Paleozoic （410-250 Ma） together with a review of the underlying data. Although we expect this model to be particularly useful for numerical mantle modeling, we hope that it will also serve as a general framework for understanding late Paleozoic tectonics, one on which future improvements can be built and further tested.