Major Unconformities,Tectonostratigraphic Frameword,and Evolution of the Superimposed Tarim Basin,Northwest China

The Tarim basin experienced a complex tectonic evolutionary history from Sinian to Ceno zoic. Eight largescale and more than 20 subordinate unconformities defining tectonosequences of dif ferent protobasins formed in various tectonic settings have been identified within the Phanerozoic in the Tarim basin, their distribution determining the general characteristics of sequence stratigraphic framework of the basin. Tectonostratigraphic unit I （magasequence） consists mainly of the Sinian Sys tem, which formed in a rift or aulacogen setting and can been subdivided into two subordinate strati graphic units （supersequences）. Unconformity （Tgg） between Sinian and Cambrian with surface karsti fication is regarded as a postrift unconformity. Tectonostratigraphic unit II comprises the Cambrian and the Ordovician and can be divided into six subordinate tectonostratigraphic units, recording the tectonogeographic evolution of the prototype basins from Cambrian to Early Ordovician passive car bonate continental margin or cratonic depression and the Late Ordovician submarine to neritic ret roarc foreland and cratonic depressions. The tectonic uplift related to the formation of the unconformity Tg5-2 resulted in the remarkable change in basin tectonic setting from a passive divergent to an active convergent, with the development of the Tazhong （塔中） uplift, the Tangguzibasi （塘古孜巴斯）, and the northern depression at the end of the Middle to the early Late Ordovician. The widespread angular unconformity Tg5 formed by a relatively strong compressive deformation, which caused an abrupt tec tonogeographic change of the basin from abyssal to a neritic setting in response to the collision andassociated tectonic deformation of the North Kunlun （昆仑） orogenesis during the Late Ordovician to the Early Silurian. Tectono stratigraphic unit III is composed of the Silurian and the Lower to Middle Devonian and character ized by the development of fluvial or deltaic and clastic littoral and neritic deposits. Largescale terrigenous clastic depositional wedges progra dated from the north to south in the southeast ern slope of the basin indicate the continuously shallowing and uplifting along the northern basin margin. Tectonostratigraphicunit IV includes the Upper Devonian, Carboniferous, and Permian and can be classified into two sub ordinate tcctonostratigraphic sequences. The angular unconformity （Tg3） at the base of the unit is the most widespread unconformity and the strong compression and uplift of the basin during this period has been suggested to be related to the collision of the Tianshan （天山） orogenesis and resulted in fun damental change in tectonic geomorphology with higher to the northeast and lower to the southwest. Tectonostratigraphic unit IV records another tectonic cycle from weak extension to compression in ba sin setting and is composed mainly of nearshore elastic deposits of embayment basin f＇flls. From the Triassic, the Tarim basin evolved into a period characteristic of development of intracontinental de pressions and marginal foreland basins and experienced several cycles from rapid subsidence to strong uplift and deformation, resulting in superimposition and reformation of differently orientated proto basins filled with a series of regional depositional cycles bounded by major unconformities and consist ing of extremely thick alluvial and lacustrine deposits. The Kuqa foreland depression in the northwest ern basin margin developed since the Triassic and deposited a elastic wedge of the Mesozoic to Cenozoic more than I00 000 m in thickness, which progradated and thin towards the southern Tabei （塔北） forebulge. The largescale sedimentary cycles from alluvial, fluvial to lacustrine, and finally fluvial deposits are attributed to the results of foreland tectonisim from active to relatively quiet stages. The foreland tectonisim was active during the Triassic, relatively quiet during the Jurassic, and active again from the Late Jurassic to the Cretaceous. To the Eogene, the depression subsided again and the com pression intermittently increased, resulting in a series of faulted and folded structural belts.

Late Mesozoic tectonostratigraphic division and correlation of the Bohai Bay basin: Implications for the Yanshanian Orogeny

We present the results of Mesozoic sequences of the Bohai Bay basin in North China, based mainly on geochronology and interpretations of seismic profiles and logging data. Five tectono-stratigraphic sequences are defined: Lower-Middle Triassic, Lower-Middle Jurassic, Upper Jurassic, Lower Cretaceous and Upper Cretaceous. Based on an analysis of detrital zircons, the clastic rocks recorded two intervals of Jurassic magmatic activity, during 180–175 and 160–152 Ma, which can be correlated to the Nandaling and Tiaojishan Formations in the Yanshan area, respectively. However, since Jurassic volcanic rocks have not yet been found in the Bohai Bay basin, we speculate that these two stages of activity were mainly concentrated around the periphery of the North China Craton(NCC) rather than within. Based on an analysis of zircons from volcanic rocks and pyroclastics, the early Cretaceous magmatism in the Bohai Bay basin can be divided into two stages, 125–120 and 110–100 Ma,which can be correlated to magmatism in the eastern part of the NCC. The zircon ages indicate an absence of volcanic activity during the late Tuchengzi and Zhangjiakou periods which may correlate to the uplift of the Bohai Bay basin in the late Late Jurassic. Comparison of the development of Mesozoic basins and sedimentary strata in the central-eastern part of Yanshan tectonic belt and the the Bohai Bay basin indicates that the two areas are generally comparable, but with substantial differences.The central-eastern part of the Yanshan structural belt lacks Early-Middle Triassic strata, and the Bohai Bay basin lacks late Jurassic-early Cretaceous strata. Based on research results from late Mesozoic sedimentary structures in the central and eastern parts of the Yanshan tectonic belt, we infer that episode A of the Yanshanian Orogeny was weak in the Bohai Bay basin and its periphery, while episode B of the Yanshanian Orogeny had a strong influence on the Bohai Bay basin and its periphery. The available data reveal differences in the expression of these episodes: the Bohai Bay basin is characterized by vertical uplift, and the northern margin of NCC is characterized by horizontal compression uplift.

Early-middle Mesoproterozoic tectonic evolutionary history of the southwestern Yangtze Block,China:lithostratigraphic,geochronologic and elemental geochemical constraints

During the Early-Middle Proterozoic era,three major lithostratigraphic unit associations,namely Hekou-Dahongshan,Dongchuan,and Kunyang-Huili Groups,were established for the metamorphosed volcanicsedimentary rocks exposed in the southwestern Yangtze Block (SWYB).The integration of petrology, geochemistry and geochronology constrains tectonic framework and evolution of the SWYB,in which four sets of SHRIMP U-Pb zircon ages were obtained from the volcanic rocks interbedded within the Middle Proterozoic successions:1800-1600 Ma,1600-1300 Ma,1300-1100 Ma,and 1100-1000 Ma. Major and trace elemental analysis indicate that four key tectonic evolutionary stages,each coinciding with the above radiometric age set,of the SWYB during the Early-Middle Mesoproterozoic.The SWYB was characterized by an east-westerly trending rift in the Hekou,Dongshan,and Dongchuan areas,and separate basin-forming events during 1800-1600 Ma arid 1600-1300 Ma,respectively.In the SWYB,an intracontinental rift basin and a rift basin occurred in the Caiziyuan-Matang and Laowushan areas, respectively in 1300-1100 Ma ago.During 1100-1000 Ma,the SWYB was characterized by the closure of the Caiziyuan-Matang rift-ocean basin,collision between the Huili Blocks and Kunyang Blocks,and presence of volcanic ares in the Tianbaoshan and Fulingpen areas.Accordingly,the SWYB represents a new basin that records the relatively complete assembly process of the Rodina during the Early-Middle Mesoproterozoic era.