Mechanism of Structure Variations at Rifted Margins in the Central Segment of South Atlantic:Insights from Numerical Modeling

Rifted margins in the central South Atlantic portray spatial variability in terms of preserved width and thickness,which relates to complex rift-related fault activities.However,there is still a lack of systematic and quantitative explanations for the causes of the variations that are observed along the paired rifts.To elucidate this issue,2D viscous-plastic thermomechanical numerical models are applied to capture the behavior of deformation,in which we investigate the effects of extensional rate,crustal strength and thickness on crust-mantle coupling,and timing of transition from rifting to breakup.Our numerical experiments demonstrate that crust-mantle decoupling accounts for crustal hyperextension,and that incorporating moderate-intensity rheology into lower crust may yield insights into the hyper-extended crust and asymmetric architecture observed in the central South Atlantic.The results also suggest that undulations in lithospheric basement cause asymmetric mantle upwelling.The lower crust of fold belts takes priority to be thermally weakened over craton and induces rift migration simultaneously.A new mechanism for the formation of failed rift is described,where the mechanical decoupling derived from thermally weakened lower crust gives access to dual rift migration.These results reinforce the interpretation on how crustal rheology shapes margins architectures and highlight the first-order effects of crust-mantle coupling.

Evolution of the Extensional Rifted Yunnan-Guizhou-Guangxi Oceanic Basin During the Late Hercynian to Middle Indosinian Stage

On the basis of exhaustive researches on the facies sequences and depositlonal evolutionary process of various depositional systems, the genetic stratigraphic framework of the extensional rifted oceanic basin, which has undergone strong structural destruction, has been reconstructed by means of dynamic genetic stratigraphic analysis. Five depositional episodes have been distinguished from various isochronous stratigraphic boundaries and stratigraphic sequences with the three-dimensional structure of each depositional episode analysed in detail. The tectonic paleogeographic environment corresponding to different stages of each depositional episode has been reconstructed for individual depositional system tracts. And the evolution history of this rifted basin has been divided into four stages' initial rifting and oceanization of continental crust, stretching and spreading of the basin, subduction and basin differentiation, and convergence and collision. A NNE-trending intracontinental soft collision suture was left after the closing of the basin.

Tectonic characteristics and structural styles of a continental rifted basin:Revelation from deep seismic reflection profiles

The Fushan Depression is a half-graben rifted sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The Paleogene Liushagang sequence is the main hydrocarbon-bearing stratigraphic unit in the sub-basin. Using three-dimensional（3-D）seismic data and logging data over the sub-basin, we analyzed structural styles and sedimentary characteristics of the Liushagang sequence. Five types of structural styles were defined： ancient horst, traditional slope, flexure slope-break, faulted slope-break and multiple-stage faults slope, and interpretations for positions, background and development formations of each structural style were discussed. Structural framework across the sub-basin reveals that the most remarkable tectonic setting is represented by the central transfer zone（CTZ） which divides the sub-basin into two independent depressions, and two kinds of sequence architectures are summarized：（i） the western multi-stage faults slope;（ii） the eastern flexure slope break belt. Combined with regional stress field of the Fushan Depression, we got plane combinations of the faults, and finally built up plan distribution maps of structural system for main sequence. Also, we discussed the controlling factors mainly focused on subsidence history and background tectonic activities such as volcanic activity and earthquakes. The analysis of structural styles and tectonic evolution provides strong theoretical support for future prospecting in the Fushan subbasin and other similar rifted basins of the Beibuwan Basin in South China Sea.

Formation Water Geochemistry and Its Controlling Factors: Case Study on Shiwu Rifted Sub-basin of Songliao Basin, NE China

A common way to trace fluid flow and hydrocarbon accumulation is by studying the geochemistry of formation water. This paper focuses on the spacial distribution of the geochemical features of the formation water in the Shiwu Rifled Basin and its indication of the water-rock interaction processes. The hydrodynamic field controls the spacial distribution of formation water. Due to the penetration of meteoric water, the salinity is below 4,500mg/L at the basin margin and the severely faulted central ridge and increases basin ward to 7,000-10,000mg/L. The vertical change of formation water can be divided into 3 zones, which correspond respectively to the free replacement zone （〈1,250m）, the obstructed replacement zone （1,250m-1,650m） and the lagged zone （〉 1,650m） in hydrodynamics. In the free replacement zone, the formation water is NaHCO3-type with its salinity increased to 10,000mg/L. The formation water in the obstructed replacement zone is Na2SO4-type with its salinity decreased to 5,000mg/L-7,000mg/L because of the dehydration of mud rocks. The formation water in the lagged zone is CaC12-type, but its salinity decreases sharply at a depth of 1,650m and then increases vertically downward to 10,000mg/L. This phenomenon can be best explained by the osmosis effect rather than the dehydration of mud rocks. The relationships between Cl^--HCO3^- and Na^＋＋K^＋-Ca^2＋ show that the initial water-rock interaction is the dissolution of NaCl and calcium-beating carbonate, causing an increase of Na^＋-K^＋-Ca^2＋-Cl^- and salinity. The succeeding water-rock interaction is albitization, which leads to a decrease of Na^＋ and an increase of Ca2＋ simultaneously, and generates CaCl2-type fluid. The above analysis shows that the geochemical evolution of formation water is governed by the water-rock interactions, while its spacial distribution is controlled by the hydrological conditions. The water-rock interaction processes are supported by other geological observations, suggesting that formation water geochemistry is a viable method to trace the fluid-rock interaction processes and has broad applications in practice.

Formation mechanisms of good-quality clastic reservoirs in deep formations in rifted basins:A case study of Raoyang sag in Bohai Bay Basin,East China

In order to reveal the development mechanism of high-quality clastic rock reservoir, the basic characteristics of Sha-3 Member of the Shahejie Formation in the Raoyang sag, Bohai Bay Basin, are analyzed based on cores observation, thin-sections and SEM images, and petrophysical properties measurements as well. It is found that high-mature composition and texture, early oil charging, and dissolution are the main factors controlling the formation and preservation of pores in deep reservoirs. Compaction is the major factor destructing pores, whereas formation overpressure is conducive to the preservation of original pores, high compositional and medium textural maturity can enhance the resistance capacity to compaction and protect primary pores. Early oil charging could lead to temporary cessation of diagenesis and thus inhibit the cementation. When organic acids entered reservoir formations, considerable amounts of secondary pores were formed, leading to the local improvement of petrophysical properties. When predicting good quality belt in exploration of deep basin, it is recommended that the superimposing effects of the multiple factors(overpressure, early oil charging, compositional and textural maturity, diagenesis) be taken into consideration.

Development and exploration practice of the concept of hydrocarbon accumulation in rifted-basin troughs: A case study of Paleogene Kongdian Formation in Cangdong sag, Bohai Bay Basin

Based on the merged 3 D seismic data, well logging, formation testing, analysis and testing data, the structural evolution, sedimentary reservoirs, thermal evolution of source rocks were investigated of Paleogene Kongdian Formation in the trough area of Cangdong sag, Bohai Bay Basin. A conventional-unconventional hydrocarbon accumulation pattern in the trough area of rifted basin was revealed. The reservoir forming elements in the trough area of Cangdong sag have a zonation feature in terms of reservoirs and source rocks. There are two types of reservoir forming models, primary trough and reformed trough. The formation and evolution of trough controlled the orderly distribution of conventional oil to unconventional oil in the trough. Particularly, structural reservoirs occur in the upper part of the trough, stratigraphic-lithologic reservoirs are likely to form in the delta front deposits at the outer ring of trough, the middle ring transitional belt is the favorable site for tight oil reservoirs, while the fine grain deposits zone in the inner ring is shale oil and gas exploration area. The study has pointed out the new domains and directions for searching reserves in the secondary exploration of mature oilfields.

Preservation of a Paleoproterozoic rifted margin in the Himalaya:Insight from the Ulleri-Phaplu-Melung orthogneiss

Orthogneiss within the Paleoproterozoic strata of Lesser Himalayan sequence across the Himalaya has been variably linked to development in a continental arc setting, Indian basement, or a continental rift.New whole rock and trace element geochemical data and U/Pb zircon geochronology indicate that the granitoid protoliths to these rocks were derived from upper crustal sources in the Paleoproterozoic and have within-plate, A-type affinities. This is consistent with their generation in a rifted margin and is compatible with paleogeographic reconstructions that indicate an open boundary for present-day northern India in the Paleoproterozoic.

Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone:Implications for the Opening of the Neo-Tethys Ocean

In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).

Magmatic-tectonic response of the South China Craton to the Paleo-Pacific subduction during the Triassic:a new viewpoint based on Well NK-1

The Nansha Block(NB)is one of the blocks separated from the southern margin of the South China Craton(SCC)by the western Pacific subduction,which contains rich information of geodynamic and tectonic transformation.To reveal the essence of western Paleo-Pacific subduction during the Triassic period,Well NK-1 in this block was selected for petrographic study,and published research data from other cooperative teams were compared.A double-cycle pattern of basic to neutral magmatic volcanism was established,and 36 lithological rhythmic layers and representative cryptoexplosive breccia facies and welded tuff bands were identified.Combined with a reanalysis of published geochronological data,geochemical elements,and isotope geochemistry,we found that the rock assemblages could be divided into an intermediate-acid dacite(DA)series(SiO_(2)>65%)and basaltic(BA)series(Co<40μg/g),which was formed during the early Late Triassic((218.6±3.2)–(217.9±3.5)Ma).BA exhibits obvious calc-alkaline island-arc magmatic properties:(^(87)Sr/^(86)Sr)_i ratio ranging 0.70377–0.71118(average:0.70645),^(147)Sm/^(144)Nd ratio ranging 0.119–0.193(average:0.168),and chondrite-normalized rare earth element(REE)curves being flat,while DA exhibits remarkable characteristics of subducted island-arc andesitic magma:(^(87)Sr/^(86)Sr)_i ratio(0.70939–0.71129;average:0.71035),εNd(t)value(-6.2–-4.8;average:-5.6)andε_(Hf)(t)value(-2.9–-1.7,average:-2.2)show obvious crust-mantle mixing characteristics.BA and DA reveal typical characteristics of island-arc magma systems and typeⅡenriched mantle(EM-Ⅱ)magma.BA magma was likely resulted from the process whereby the continental crust frontal accretionary wedge was driven by the Paleo-Pacific slab subduction into the deep and began to melt,resulting in a large amount of melt(fluid)joined the asthenosphere on the side of the continental margin.In contrast,DA magma was likely resulted from the process whereby the plate front was forced to bend with increasing subduction distance,which triggered the upwelling of the asthenosphere near the continent and subsequently led to the partial melting of the lithospheric mantle and lower crust due to continuous underplating.The lithospheric thinning environment in the study area at the end of Triassic created suitable conditions for the separation between the NB and SCC,which provided an opportunity for the formation of the early intracontinental rift during the later expansion of the South China Sea(SCS).

Structural features in the mid-southern section of the Kyushu–Palau Ridge based on satellite altimetry gravity anomaly

The Kyushu–Palau Ridge(KPR),an anti-S-shaped submarine highland at the center of the Philippine Sea Plate(PSP),is considered the residual arc of the Izu–Bonin–Mariana Island Arc,which retains key information about the cessation of the Western Philippine Basin(WPB)expansion and the Parece Vela Basin(PVB)breakup.Herein,using the new generation of satellite altimetry gravity data,high-precision seafloor topography data,and newly acquired ship-borne gravity data,the topographic and gravity characteristics of the KPR mid-southern section and adjacent region are depicted.The distribution characteristics of the faults were delineated using the normalized vertical derivative–total horizontal derivative method(NVDR-THDR)and the minimum curvature potential field separation method.The Moho depth and crustal thickness were inverted using the rapid inversion method for a double-interface model with depth constraints.Based on these results,the crust structure features in the KPR mid-southern section,and the“triangular”structure geological significance where the KPR and Central Basin Rift(CBR)of the WPB intersect are interpreted.The KPR crustal thickness is approximately 6–16 km,with a distinct discontinuity that is slightly thicker than the normal oceanic crust.The KPR mid-southern section crust structure was divided into four segments(S1–S4)from north to south,formed by the CBR eastward extension joint action and clockwise rotation of the PVB expansion axis and the Mindanao fault zone blocking effect.

Petroleum geological characteristics and exploration targets of the oil-rich sags in the Central and West African Rift System

Based on seismic,drilling,and source rock analysis data,the petroleum geological characteristics and future exploration direction of the oil-rich sags in the Central and West African Rift System(CWARS)are discussed.The study shows that the Central African Rift System mainly develops high-quality lacustrine source rocks in the Lower Cretaceous,and the West African Rift System mainly develops high-quality terrigenous organic matter-rich marine source rocks in the Upper Cretaceous,and the two types of source rocks provide a material basis for the enrichment of oil and gas in the CWARS.Multiple sets of reservoir rocks including fractured basement and three sets of regional cap rocks in the Lower Cretaceous,the Upper Cretaceous,and the Paleogene are developed in the CWARS.Since the Late Mesozoic,due to the geodynamic factors including the dextral strike-slip movement of the Central African Shear Zone,the basins in different directions of the CWARS differ in terms of rifting stages,intervals of regional cap rocks,trap types and accumulation models.The NE-SW trending basins have mainly preserved one stage of rifting in the Early Cretaceous,with regional cap rocks developed in the Lower Cretaceous strata,forming traps of reverse anticlines,flower-shaped structures and basement buried hill,and two types of hydrocarbon accumulation models of"source and reservoir in the same formation,and accumulation inside source rocks"and"up-source and down-reservoir,and accumulation below source rocks".The NW–SE basins are characterized by multiple rifting stages superimposition,with the development of regional cap rocks in the Upper Cretaceous and Paleogene,forming traps of draping anticlines,faulted anticlines,antithetic fault blocks and the accumulation model of"down-source and up-reservoir,and accumulation above source rocks".The combination of reservoir and cap rocks inside source rocks of basins with multiple superimposed rifting stages,as well as the lithologic reservoirs and the shale oil inside source rocks of strong inversion basins are important fields for future exploration in basins of the CWARS.

Protectivity of Freeze Dried Inactivated Rift Valley Fever Vaccine

Background: Vaccinations for animals are crucial for food production, animal welfare, public health, and animal health. They are an affordable way to stop animal sickness, increase food production efficiency, and lessen or stop the spread of zoonotic diseases to humans. Animal vaccines that are both safe and efficacious are vital to modern culture. The vaccine should induce a strong, protective and prolonged immune response against the antigenic factor. In order to achieve these goals, novel vaccination techniques and an efficient adjuvant are required to render the vaccine immunogenically protective and trigger a strong immune response. Aim: Our study aims to promote and enhance the immunogenicity against RVF virus disease through lyophilized inactivated RVF vaccine through induction of early cellular, high and prolonged humeral immunity in vaccinated animals using cabopol as stabilizer and Saponin or normal saline as a diluent at time of vaccination. Moreover, manufacturing of these vaccines is easy to be done. Results: The gained results revealed that RVF freeze-dried vaccine with Carbopol that reconstituted using Saponin elicited better immune response than that reconstituted using normal saline (NaCl). The cell mediated immune response as represented by lymphocyte blastogenesis and phagocytic activity were markedly increased with high levels when we used Saponin as a diluent than that in group vaccinated with vaccine diluted with NaCl, on the other side the humeral immune response in group vaccinated using the Saponin as diluent is more detected and stayed within the protective level till the end of 11th month post vaccination (1.5 TCID50) while the immune response induced after using normal saline as a diluent stayed within the protective level till the end of 10th month post vaccination (1.8 TCID50). Conclusion: The use of Saponin as a diluent for reconstitution of the freeze dried RVF vaccine is preferable than the use of normal saline enhancing both sheep cellular and humeral immune response.

Geochemical and Petrological Insights into the Neoproterozoic Moumba Metabasites: Implications for Crustal Processes in the West Congo Belt (Republic of Congo)

The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which extends from southwest Gabon to the northeast of Angola. This study aims to constrain the geochemical signature Nemba complex of West Congo belt from the petrograhic and geochemical study on the whole rock. The petrographic data from this study show the Moumba metabasites are made up of amphibolites, metagabbros, epidotites and greenschists interstratified in the Eburnean metasediments and affected by mesozonal to epizonal metamorphism characterized by the retromorphosis of intermediate amphibolite facies minerals into greenschist facies. Whole-rock geochemical data indicate that these metabasites are continental flood basalts (CFB) of basic nature and transitional affinity emplaced in intraplate context. These continental flood basalts are generated from magma originating from a significantly enriched shallow mantle plume and this magma then contaminated by the continental crust during their ascent. The reconstruction of tectonic signature suggests that West Congo belt would result from closure of an ocean basin with subduction phenomena. This collision would be marked by the establishment of ophiolite complex. We show that this model is incompatible with the CFB nature of metabasites and the orogenic evolution of Neoproterozoic. It does not seem that we can evoke a genetic link with a subduction of oceanic crust, because the paleogeography of Neoproterozoic (Rodinia) is marked by intracontinental rifts linked to opening of Rodinia. We therefore suggest the non-existence of ophiolitic complex in western Congo belt and reject the collisional model published by certain authors. We confirm the currently available intracontinental orogen model.

Geological characteristics of the southern segment of the Late Sinian–Early Cambrian Deyang-Anyue rift trough in Sichuan Basin, SW China

Based on the latest drilling, seismic and field outcrop data, the geological characteristics(e.g. strata, development and sedimentary evolution) of the southern segment of the Late Sinian–Early Cambrian Deyang–Anyue rift trough in the Sichuan Basin are analyzed. First, the strata in the southern segment are complete. The first to second members of Dengying Formation(Deng 1 + Deng 2) are found with relatively stable thickness(400–550 m), and the third to fourth members(Deng 3+ Deng 4) show great thickness difference between the marginal trough and the inner trough, which is up to 250 m. The Cambrian Maidiping Formation and Qiongzhusi Formation in southern Sichuan Basin are relatively thin, with the thickness changing greatly and frequently. Second, the Deyang–Anyue rift trough extended southward during the Deng 4 period, affecting southern Sichuan Basin. Compared to the middle and northern segments of the rift trough, the southern segment is generally wide, gentle and shallow, with multiple steps, and alternating uplifts and sags, which are distributed in finger shape. Third, the Deng 1 + Deng 2 in southern Sichuan Basin records the dominance of carbonate platform and unobvious sedimentary differentiation, and the Deng 4 exhibits obvious sedimentary differentiation, namely, basin–slope–secondary slope–slope–secondary slope–platform margin–restricted platform, from the inner trough to the marginal trough. Fourth, the rift trough in southern Sichuan Basin has evolved in four stages: stabilization of Deng 1–Deng 2, initialization of Deng 3–Deng 4, filling of Maidiping–Qiongzhusi, and extinction of Canglangpu Formation.

Trace-element geochemistry and S–O isotopes in the fluorite-barite mineralization of Merguechoum,Moroccan eastern Meseta:insights into ore genesis to the Pangea rifting

The Merguechoum fluorite-barite mineralization,located in the Eastern Meseta of Morocco,is hosted in the Late Hercynian granite.The ore consists of fine crystals of fluorite 1,massive barite 1,euhedral crystals of fluorite 2,and barite 2 with calcite and minor quartz and sulfides.The Merguechoum ore deposits have never been investigated.This study was the first contribution that studied the genesis of fluorite and barite.The ore occurs as dissemination within granite intrusion and also fills the NE-SWtrending meter-sized fractures and faults.The values of the total Rare Earth Elements and Yttrium(REY)and the ratios of LREY/HREY,Y/Ho,Tb/Ca,and Tb/La indicate that the Merguechoum fluorite precipitated from hydrothermal fluids,likely basinal brines,which interacted with the Hercynian granite.The REY data indicate that the ore-forming fluids of the early stage have intensely interacted with the Hercynian granite compared to those of the late ore stage.The gradual decrease in the europium(Eu/Eu^(*)),yttrium(Y/Y^(*)),and cerium(Ce/Ce^(*))anomalies and a low concentration ofΣREY observed in the second ore stage compared to the first ore stage suggest an increase in p H and fO_(2)and by inference a decrease in temperature during the evolution of the hydrothermal system.This evolution could be explained by fluid mixing between the ascending basinal hydrothermal fluids and the diluted sulfate-rich meteoric water barite separates from selected samples reveal that the dissolved sulfates(SO_(4)^(2-))were derived from Permian–Triassic sulfates and/or coeval poreseawater sulfates.The proposed fluid mixing triggered the precipitation of an early-stage F-Ba assemblage followed by the second-stage F-Ba mineralization.Geologic fieldwork,REY inventories,and isotope data point to the ore genesis during the Permian–Triassic extensional tectonic activity concerning the Pangea rifting.This extensional tectonic environment is likely the driving force that mobilized a large amount of the ore-forming basinal brines along the available faults and fractures to the loci of ore deposition.

New understanding and exploration direction of hydrocarbon accumulation in Termit Basin, Niger

Based on the seismic and drilling data, casting thin sections, geochemical analysis of oil and rock samples, and hydrocarbon generation history simulation, the hydrocarbon accumulation characteristics and exploration direction of Termit superimposed marine–continental rift basin are discussed. The Termit basin is superimposed with two-phase rifts(Early Cretaceous and Paleogene). The subsidence curves from two wells on the Trakes slope in the east of the basin show high subsidence rate in the Late Cretaceous, which is believed to be high deposition rate influenced by transgression. However, a weak rift may also be developed. The depositional sequences in the Termit basin were controlled by the Late Cretaceous marine transgression cycle and the Paleogene lacustrine transgression cycle, giving rise to two types of superimposed marine–continental “source-sink” deposits. The marine and continental mixed source rocks developed universally in the whole basinduring the marine transgression period, and are overlaid by the Paleogene Sokor 1 reservoir rocks and Sokor 2 caprocks developed during the lacustrine transgression period, forming the unique superimposed marine–continental basin in WCARS. The early low geothermal gradient in the Termit basin resulted in the late hydrocarbon generated by the source rock of Upper Cretaceous Yogou in Paleogene. Mature source rock of Upper Cretaceous Donga developed in the Trakes slope, so that the double-source-supply hydrocarbon and accumulation models are proposed for the Trakes slope in which formed the oil fields. Due to virtue of the newly proposed hydrocarbon accumulation model and the exploration activities in recent years in the Termit superimposed marine–continental rift basin, an additional effective exploration area of about 2500 km2has been confirmed in the east of the basin. It is believed that potential domains such as Sokor 1, Donga and Upper Cretaceous lithologic traps in the southeast of the basin are key expected targets for exploration and frontier evaluation in future.

Structural features and exploration targets of platform margins in Sinian Dengying Formation in Deyang-Anyue Rift, Sichuan Basin, SW China

Based on the seismic, logging, drilling and other data, the distribution, structural types and mound-shoal hydrocarbon accumulation characteristics of platform margins of the Sinian Dengying Formation in the Deyang-Anyue Rift and its periphery were analyzed. Four types of platform margins are developed in the Dengying Formation, i.e., single-stage fault-controlled platform margin, multi-stage fault-controlled platform margin, gentle slope platform margin, and overlapping platform margin. In the Gaoshiti West-Weiyuan East area, single-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in nearly NEE direction and are shielded by faults and mudstones, forming fault-controlled–lithologic traps. In the Lezhi-Penglai area, independent and multi-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in NE direction and are controlled by synsedimentary faults;the mound-shoal complexes are aggraded and built on the hanging walls of the faults, and they are shielded by tight intertidal belts and the Lower Cambrian source rocks in multiple directions, forming fault-controlled–lithologic and other composite traps. In the Weiyuan-Ziyang area, gentle slope platform margins are developed in the Deng 2 Member, which trend in NW direction;the mound-shoal complexes are mostly thin interbeds as continuous bands and shielded by tight intertidal belts in the updip direction, forming lithologic traps. In the Gaoshiti-Moxi-Yanting area, overlapping platform margins are developed in the Deng 2 and Deng 4 members;the mound-shoal complexes are aggraded and overlaid to create platform margin buildup with a huge thickness and sealed by tight intertidal belts and the Lower Cambrian mudstones in the updip direction, forming large-scale lithologic traps on the north slope of the Central Sichuan Paleouplift. To summarize, the mound-shoal complexes on the platform margins in the Dengying Formation in the Penglai-Zhongjiang area, Moxi North-Yanting area and Weiyuan-Ziyang area are large in scale, with estimated resources of 1.58×1012 m3, and they will be the key targets for the future exploration of the Dengying Formation in the Sichuan Basin.

Extensional structures of the Nan'an Basin in the rifting tip of the South China Sea: Implication for tectonic evolution of the southwestern continental margin

Nan'an Basin is a giant hydrocarbon basin,but its tectonic division scheme and associated fault systems has not been well understood.Based on newly acquired seismic data from the southwestern margin of the South China Sea,this study analyzed the structural units,tectonic feature and geodynamics of the sedimentary basin.The new data suggests that the Nan0 an Basin is a rift basin oriented in the NE-SW direction,rather than a pull-apart basin induced by strike-slip faults along the western margin.The basin is a continuation of the rifts in the southwest South China Sea since the late Cretaceous.It continued rifting until the middle Miocene,even though oceanic crust occurred in the Southwest Subbasin.However,it had no transfer surface at the end of spreading,where it was characterized by a late middle Miocene unconformity(reflector T3).The Nan'an Basin can be divided into eight structural units by a series of NE-striking faults.This study provides evidences to confirm the relative importance and interplay between regional strike-slips and orthogonal displacement during basin development and deformation.The NE-SW-striking dominant rift basin indicates that the geodynamic drivers of tectonic evolution in the western margin of the South China Sea did not have a large strike-slip mechanism.Therefore,we conclude that a large strike-slip fault system did not exist in the western margin of the South China Sea.

Sedimentary Processes and Depositional Characteristics of Coarse-grained Subaqueous Fans along Steep Slopes in a Lacustrine Rift Basin:A Case Study from the Dongying Depression,Bohai Bay Basin,China

Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin,China.The insufficient understanding of their sedimentary processes,depositional patterns,and controlling factors restricts efficient exploration and development.Coarse-grained subaqueous fans in the Yong′an area,Dongying Depression,are investigated in this study.These fans include nearshore subaqueous fans,and sublacustrine fans,and their sedimentary processes,depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology.Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence.Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction.While the topography was gentle,sublacustrine fans did not develop.During the late weak tectonic activity stage,late sublacustrine fans developed with multiple stages superimposed.Frequent fault activity and related earthquakes steepened the basin margin,and the boundary fault slopes were 25.9°–34°.During the early–middle deposition stage,hyperpycnal flows triggered by outburst floods developed.During the late deposition stage,with weak tectonic activity,seasonal floods triggered hyperpycnal flows,and hybrid event beds developed distally.

Geochemistry and petrogenesis of the Paleoproterozoic orthogneisses and granitoids of the Banded Gneissic Complex,central Rajasthan,NW India:Implications for crustal reworking processes

Granitoids and orthogneisses(1.7 Ga)from the northern part of the Banded Gneissic Complex(BGC-II domain),Aravalli Craton(NW India)are geochemically analyzed to understand the geodynamic condition and crustal evolution processes.The samples are metaluminous to peraluminous(molar A/CNK:0.74±2.12),and characterized by Eu-anomaly ranging from 0.17 to 1.06,Fe_(2)O_(3)^(T)/(Fe_(2)O_(3)^(T)+MgO)from 0.8 to 0.9 and high magmatic zircon saturation temperature(>830°C)which are the features suggestive of A-type granite affinity.Tectonic discrimination diagrams classify the samples as post-collisional extensional A2-type granites.Geochemical characteristics along with trace element ratios[(Y/Nb)_(N)=0.15 to 4.33(avg.0.76),(Th/Nb)_(N)=4.63 to 255.47(avg.63.13),(Th/Ta)_(N)=1.37 to 9.84(avg.8.86),(Ce/Pb)_(N)=0.05 to 3.05(avg.1.43)]indicate that the rocks were derived from a plagioclase-rich and garnet-free crustal source under lowpressure conditions.Further,it is also proposed that tonalite-trondhjemite-granodiorite(TTG)rocks which occur dominantly in southern Rajasthan(BGC-I)are precursors and their partial melting led to the generation of the studied A-type granite samples.The studied samples also bear close geochemical similarity with A-type granites of similar age(1.7 Ga)near the Sakhun-Ladera region of northern BGC-II.The studied A-type granites are believed to be coeval to similar aged A-type granites of the Khetri and Alwar sub-basins of the North Delhi Fold Belt(NDFB).They are comparable in age and magmatic history to recorded A-type magmatism in North America and parts of the Chinese craton.The large geographical extents of synchronous A-type granites are proposed to be related to the Columbia Supercontinent assembly(ca.1.7 Ga;postcollisional granites).Thus,based on the studied extensional granites,we surmise that BGC-II was part of the Columbia Supercontinent.