Mechanism of formation of superdeep sedimentary basins: lithospheric stretching or eclogitization?

The superdeep North Caspian, South Caspian, and Barents basins have their sedimentary fill much thicker and the Moho, correspondingly, much deeper than it is required for crustal subsidence by lithospheric stretching. In the absence of large gravity anomalies, this crustal structure indicates the presence under the Moho of a thick layer of eclogite which is denser than mantle peridotite. Crustal subsidence in the basins can be explained by high-grade metamorphism of mafic lower crust. The basins produced by lithospheric stretching normally subside for the first ～100 myr of their history, while at least half of the subsidence in the three basins occurred after that period, which is another evidence against the stretching formation mechanism. According to the seismic reflection profiling data, stretching can be responsible for only a minor part of the subsidence in the Caspian and Barents basins. As for the South Caspian basin, there has been a large recent subsidence event in a setting of compression. Therefore, eclogitization appears to be a realistic mechanism of crustal subsidence in superdeep basins.     

Formation mechanisms of ultradeep sedimentary basins: the North Barents basin. Petroleum potential implications

Consolidated crust in the North Barents basin with sediments 16–18 km thick is attenuated approximately by two times. The normal faults in the basin basement ensure only 10-15% stretching, which caused the deposition of 2–3 km sediments during the early evolution of the basin. The overlying 16 km of sediments have accumulated since the Late Devonian. Judging by the undisturbed reflectors to a depth of 8 s, crustal subsidence was not accompanied by any significant stretching throughout that time. Dramatic subsidence under such conditions required considerable contraction of lithospheric rocks. The contraction was mainly due to high-grade metamorphism in mafic rocks in the lower crust. The metamorphism was favored by increasing pressure and temperature in the lower crust with the accumulation of a thick layer of sediments. According to gravity data, the Moho in the basin is underlain by large masses of high-velocity eclogites, which are denser than mantle peridotites. The same is typical of some other ultradeep basins: North Caspian, South Caspian, North Chukchi, and Gulf of Mexico basins. From Late Devonian to Late Jurassic, several episodes of rapid crustal subsidence took place in the North Barents basin, which is typical of large petroleum basins. The subsidence was due to metamorphism in the lower crust, when it was infiltrated by mantle-source fluids in several episodes. The metamorphic contraction in the lower crust gave rise to deep-water basins with sediments with a high content of unoxidized organic matter. Along with numerous structural and nonstructural traps in the cover of the North Barents basin, this is strong evidence that the North Barents basin is a large hydrocarbon basin.     

The superdeep North Chukchi Basin: formation by eclogitization of continental lower crust,with petroleum potential implications

The large North Chukchi Basin in the northeastern Eurasian shelf is filled with up to 22 km of sediments, which is far thicker than filling a basin upon oceanic crust would require. The basin sedimentation began 380 Myr ago, and about 16 km of sediments have been deposited for the past 125 Myr, long after the oceanic crust would have completed its subsidence. This fact is in favor of the continental instead of oceanic crust origin. Rapid basin subsidence appears to be driven by a mechanism other than crustal stretching as the latter has no evidence over the greatest part of the basin area. The suggested basin formation model implies a transformation of gabbro into denser eclogite in the lower crust and related contraction of mafic rocks. To sustain consolidated crust beneath 22 km thick sediments, the layer of dense eclogites under the granitic layer must be at least ～25 km thick. The presence of basement flexures formed at several stages of the basin evolution indicates a considerable loss of lithospheric rigidity under the effect of fluid infiltration from small mantle plumes. The fluids catalyzed the eclogitization and thus increased the subsidence rate. Rapid subsidence apparently occurred in Barremian-Albian time when the basin had accumulated up to 11.5 km of sediments. Besides the Early Cretaceous event, there were, possibly several older events of rapid subsidence. This basin subsidence history, along with the evidence of steep lithospheric flexure, is a known feature of large petroleum basins. Therefore, the North Chukchi Basin may be expected to be an oil and gas producer.     

On formation mechanisms of deep sedimentary basins: Is there enough evidence for eclogitization?

This is a critical comment on the model of basin formation by eclogitization of mafic crust suggested by E. Artyushkov. The eclogitization model bears uncertainties in average parameters (thickness, density, pressure) of lithospheric mantle, crust, and sediments, which may bias the estimates of subsidence magnitude. Main pitfalls, however, lie in high-pressure petrology: The lithostatic pressure is insufficient for eclogite to form in the lower crust beneath deep basins. It is shown that linear extrapolation of laboratory data on the gabbro-to-eclogite transition onto the field of relatively low pressures and temperatures in the lithosphere is incorrect. The hypothesized role of hot mantle fluids in the gabbro-eclogite transformation appears doubtful in terms of both petrology and kinetics of metamorphic reactions. Eclogite volumes in none of well known eclogitic sites agree with those required for eclogitization-driven subsidence. Artyushkov’s criticism of the extension basin formation model is not quite just. There are recent models of a two-layer lithosphere that imply a possibility of brittle and ductile deformation at different crust rheologies. The models we refer to predict most of extension to occur in mantle lithosphere rather than in the crust, this extension being able to produce deep continental basins.     

Seismic velocity structure of a large mafic intrusion in the crust of central Denmark from project ESTRID

The origin of regional sedimentary basins is being investigated by the ESTRID project (Explosion Seismic Transects around a Rift In Denmark). This project investigates the mechanisms of the formation of wide, regional basins and their interrelation to previous rifting processes in the Danish–Norwegian Basin in the North Sea region. In May 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central Denmark. The data confirms the presence of a body with high seismic velocity (> 6.5 km/s) extending from a depth of  10–12 km depth into the lower crust. There is a remarkable Moho relief between 27 and 34 km depth along this new along-strike profile as based on ray-tracing modelling of PmP reflections. The lack of PmP reflections at a zone of very high velocity in the lowest crust (7.3–7.5 km/s) suggests a possible location of a feeder channel to the batholith. The presence of volcanic rocks of Carboniferous–Permian age above the intrusion (mafic batholith) suggests a similar age of the intrusion. An older obliquely crossing profile and two new fan profiles deployed perpendicular to the main ESTRID profile, show that the batholith is about 30–40 km wide. The existence of this large mafic batholith supports the hypothesis that the origin of the Danish–Norwegian Basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late Carboniferous and early Permian. The data and interpretations from project ESTRID will form the basis for subsidence modelling. Tentatively, we interpret the formation of the Danish–Norwegian Basin as a thermal subsidence basin, which developed after widespread rifting of the region.     

What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma？

The Trans-North China Orogen （TNCO） was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton （NCC）. Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greeustone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic （TTG） magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, -2250 Ma 2110-21760 Ma and -2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-are basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the dosing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.     

Thermal Backstripping from Vitrinite Reflectance Profiles in Comparison with an Extensional Model for Passive Continental Margin Basins on Northern Continental Shelf of South China Sea

INTRODUCTIONVitrinitereflectance(R.)canprovideagreatdealinfor-mationonthermalhistoryofabasin.Itisafunctionofmaximumpaleotemperatureandeffectiveheatingtime,inotherwords,afunctionofdepositionalrateandgeothermalgradientwithirreversiblecharacteristics.Butitalsohasacumulativeeffectthroughoutgeothermalevents.Paleotemperaturesaremainlycontro1ledbythebasalheatflowandheatgenerationfromradioactivesourceswithinthecrust.Themodelforde-terminingpaleoheatflowoflinearchangewithtimefr0mvitri-nitereflecta…     

板块运动对中国近海新生代盆地沉降及充填的控制作用

太平洋板块、印度板块和欧亚板块的演化对中国近海沉积盆地的沉降及充填具有控制作用。根据地幔对流及地壳拉伸特征可将中国近海沉积盆地沉降类型划分为被动、主动和组合热沉降型3种。不同沉降类型分别具有不同的盆地结构,其中被动热沉降型以断陷为主,主动热沉降型以坳陷为主,组合热沉降型则是两种盆地结构的叠加或侧加。中国近海北部板内沉积盆地沉降类型以被动热沉降为主,远离海洋,受海侵影响较小,以陆相沉积体系为主;中部板缘沉积盆地沉降类型为被动侧加主动热沉降,水体整体较浅,坡折及三角洲发育规模小;南部板缘沉积盆地沉降类型也为被动侧加主动热沉降,水体整体较深,坡折及三角洲发育规模大。     

Relation between alternations of uplift and subsidence revealed by Late Cenozoic fluvial sequences and physical properties of the continental crust

Reversals in vertical crustal motion, alternations between uplift and subsidence over time scales of hundreds of thousands of years or more, have been identified in Late Cenozoic fluvial sequences in many regions worldwide. They form a class of fluvial archive that is distinct from the extreme stability observed in Archaean cratons and the monotonic uplift or subsidence that is widely observed in other regions. Such alternations between uplift and subsidence are characteristic of regions of Early or Middle Proterozoic crust, where the initial crustal consolidation included the development of a thick ‘root’ of mafic material at the base of the crust; the present study focuses on localities with this crustal type in the USA and eastern Europe. It has previously been suggested on the basis of uplift modelling that this style of crustal behaviour occurs only in regions where the mobile lower‐crustal layer is relatively thin. This study supports this conclusion on the basis of independent geothermal calculations, which indicate that such alternations between uplift and subsidence occur where the mobile lower‐crustal layer is ≤～7 km thick. An understanding of this phenomenon, in relation to the understanding of vertical crustal motions induced by surface processes (and thus by climate change) in general, therefore requires analysis of the properties and dynamics of the mobile lower‐crustal layer; detailed analysis of fluvial sequences thus contributes unique information in this area.     

The rift origin of the Vilyui basin (East Siberia), from reconstructions of sedimentation and mechanical mathematical modeling

Results of modeling of the formation of the Vilyui sedimentary basin are presented. We combine backstripping reconstructions of sedimentation and thermal regime during the subsidence with a numerical simulation based on the deformable solid mechanics. Lithological data and stratigraphic sections were used to “strip” the sedimentary beds successively and calculate the depth of the stratigraphic units during the sedimentation. It is the first time that the evolution of sedimentation which is nonuniform over the basin area has been analyzed for the Vilyui basin. The rift origin of the basin is proven. We estimate the spatial distribution of the parameters of crustal and mantle-lithosphere extension as well as expansion due to dike intrusion. According to the reconstructions, the type of subsidence curves for the sedimentary rocks of the basin depends on the tectonic regime of sedimentation in individual basins. The backstripping analysis revealed two stages of extension (sediments 4–5 km thick) and a foreland stage (sediments > 2 km thick). With the two-layered lithosphere model, we conclude that the subcrustal layer underwent predominant extension (by a factor of 1.2–2.0 vs. 1.1–1.4 in the crust). The goal of numerical experiments is to demonstrate that deep troughs can form in the continental crust under its finite extension. Unlike the oceanic rifting models, this modeling shows no complete destruction or rupture of the continental crust during the extension. The 2D numerical simulation shows the possibility of considerable basement subsidence near the central axis and explains why mafic dikes are concentrated on the basin periphery.     

Mechanism of continental crust subsidence in the Alpine Belt

Formation of deep basins on continental crust in fold belts is often explained by stretching. This mechanism inevitably produces large deformations in the upper crust. No deformations typical of significant stretching were revealed in the predominant part of deep basins on continental crust in the Alpine Belt. This means that these basins were not produced by stretching. Most basins were formed during a short period of time of a few million years. The short duration of the subsidences eliminates thermal relaxation as the mechanism. The space and time relationships between the subsidence and orogeny and the profile of the basin floor exclude thrust loading as a cause of formation for practically all large basins. Gabbro to eclogite transformation is suggested as a mechanism of rapid subsidence. This occurs under the upwelling of hydrous asthenosphere at moderate temperature to the base of the crust. Eclogite sinking into the mantle results in a strong attenuation of the crust and lithosphere, which permits intense subsequent folding. The major part of deep basins in continental crust that formed by rapid subsidence was intensely shortened in the Alpine Belt. Significant crustal shortening did not spread over the cratonic lithosphere.     

Numerical modelling of depositional sequences in half-graben rift basins

ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.     

二维沉积层序计算机模拟研究

本项研究建立了综合性的二维沉积层序模拟系统SSMS。该模拟系统由盆地的沉降过程模拟与盆地充填过程模拟两个子系统构成。盆地的沉降模拟结合了反演的沉降回剥和盆地形成的正演模型;盆地充填过程模拟综合考虑了盆地沉降、重力均衡作用、海（湖）平面升降、沉积物供给、侵蚀作用、沉积物分布和压实等因素。结合实例分析表明,该模拟系统可用于定量分析盆地构造、海（湖）平面、沉积物供给等变化对沉积层序的形成过程、几何形态及其沉积体系分布的控制作用,检验地质模型和进行预测。     

A thermodynamic model for silicate melt in CaO–MgO–Al2O3–SiO2 to 50 kbar and 1800 °C

A fully thermodynamic model for mafic melt in CaO–MgO–Al₂O₃–SiO₂ (CMAS) has been calibrated, for calculation of melting equilibria in the pressure range 0–50 kbar. It is intended as a preliminary step towards a large‐system melt model, suitable for exploring melting, melt loss and crystallization processes in a wide range of natural rock compositions. Calibration was performed with attention to the model's behaviour in its compositional subsystems, as a rigorous test of model structure and parameterization. The model is consistent with the latest Holland & Powell thermodynamic data set, and can therefore be used to calculate phase relations in conjunction with the many solid‐phase activity–composition models written for the data set. Model calculations successfully reproduce experimental melting reactions in CMAS spinel lherzolite and garnet lherzolite assemblages, as well as sapphirine‐ and kyanite‐bearing assemblages, at moderate to high pressure. Thermodynamically sensitive features, such as thermal divides are also recovered. However, some changes to the model structure will be required before the model can describe the full range of mafic and ultramafic melt compositions known from experiment at low pressures.     

Petroleum Geology in Deepwater Settings in a Passive Continental Margin of a Marginal Sea: A Case Study from the South China Sea

Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area.The South China Sea(SCS)is a typical marginal sea that includes Paleo SCS and New SCS tectonic cycles.The latter includes continental marginal rifting,intercontinental oceanic expansion and oceanic shrinking,which controlled the evolution of basins,and the generation,migration and accumulation of hydrocarbons in the deepwater basins on the continental margin of the northern SCS.In the Paleogene,the basins rifted along the margin of the continent and were filled mainly with sediments in marine-continental transitional environments.In the Neogene–Quaternary,due to thermal subsidence,neritic-abyssal facies sediments from the passive continental margin of the SCS mainly filled the basins.The source rocks include mainly Oligocene coal-bearing deltaic and marine mudstones,which were heated by multiple events with high geothermal temperature and terrestrial heat flow,resulting in the generation of gas and oil.The faults,diapirs and sandstones controlled the migration of hydrocarbons that accumulated principally in a large canyon channel,a continental deepwater fan,and a shelf-margin delta.     

含油气盆地构造分析的认识与实践

摘 要　 从我国含油气盆地研究与勘探实践出发‚结合当今盆地构造分析在理论上的重大进 展‚重点讨论盆地类型与地球动力学的关系‚盆地与造山带的耦合关系‚盆地内构造变形与 动力学背景的关系‚盆地构造活动与油气分布的关系等。     

Sequence Stratigraphic Modeling of Sedimentary Basin: Conceptual Model and Application to Erlian and Yinggehai Basins, China

INTRODUCTIONQuantitativebasinmodelinghasbecomeoneofthefrontiersinsedimentarybasinanalySis.Alargevarietyofnumericalmodelsconcerningbasinformation,basinfillingprocess,thermalhistory,hydrocarbongenerationandaCCUmulationhavebeenformulatedoverthelastdecadetondAnctheprocessesandevollltionofsedimentarybasins(Mckenzie,1978,signeretal.,1990;Ungereretal.,1990).Thebasinmodeling,utilizingcomputersimulationtechnique,isapowerfultoolforbasinanalySisandhasincreasinglybeenintegratedintoconventionalbasi…     

Differentiating the effect of episodic tectonism and eustatic sea-level fluctuations in foreland basins filled by alluvial fans and axial deltaic systems: insights from a three-dimensional stratigraphic forward model

Many studies of foreland basins have recognized a hierarchical organization in the stacking of sequences deposited by axial‐deltaic and alluvial fan systems. The hierarchy is often explained in terms of the competing control of eustasy and pulsed tectonic subsidence and the different frequencies at which these processes operate. Unravelling the relative contributions of tectonic and eustatic controls on the sequence stacking pattern is a fundamental question in foreland basin analysis, yet this is difficult because of the lack of independent stratigraphic evidence. In this study, a three‐dimensional numerical model is presented, which aids in the interpretation of alluvial successions in foreland basins filled by transverse and axial depositional systems, under conditions of variable tectonism and eustatic sea‐level change. The tectono‐sedimentary model is capable of simulating the hierarchical stratigraphic response to both eustatic and tectonic forcing, and is of higher resolution than previous models of foreland basin filling. Numerical results indicate that the onset of tectonic activity is reflected by rapid retrogradation of both depositional systems and by widespread flooding and onlap of carbonate sediments. Syntectonic fluvial patterns on the axial‐deltaic plain are dominated by bifurcating channels, swiftly relocating in response to the general rise in relative sea level induced by flexural subsidence. The resulting surface morphology of the axial delta is convex upwards. Syntectonic eustatic sea‐level fluctuations result in parasequence‐scale packages of retrograding and prograding fan and delta sediments bounded by minor flooding surfaces and type 2 sequence boundaries. Incised channels are rare within the syntectonic parasequences and are formed only during phases of tectonic quiescence when eustatic falls are no longer compensated by the subsidence component in the rise in relative sea level. Suites of amalgamating, axial channels corresponding to multiple eustatic falls delineate the resulting type 1 unconformities. Coarse‐grained, incised‐channel fills are found in the zone between the alluvial fan fringes and the convex‐upward body of the axial delta, as the axial streams tend to migrate towards this zone of maximum accommodation.     

Evidence for a synchronous circum-Iberian subsidence event and its relation to the African-Iberian plate convergence in the Late Cretaceous

Subsidence curves from Mesozoic sedimentary basins at the southern Iberian margin (Betic Cordilleras) display pronounced changes in subsidence rates around 85 Ma (chron 34, Late Cretaceous, Santonian to earliest Campanian). The subsidence events correlate with changes in the bulk and clay mineral composition in these basins, as well as with an Eoalpine high-pressure metamorphic event in the western Mediterranean region. The synchroneity with subsidence events observed in basins around the Iberian microplate suggests a causal relationship with the regional plate tectonic setting. We propose that the circum-Iberian subsidence event was largely controlled by the convergence and incipient collision of the Iberian microplate with Africa.     

前陆盆地沉降机理和地层模型

前陆盆地形成的主要原因是造山带负载导致的岩石圈挠曲。逆冲作用造成地壳增厚,造山带的巨大质量又导致下部岩石圈的区域均衡沉降,从而临近和平行于造山带发育了凹陷。另外,前陆盆地的演化也受到沉积物供应、盆地内沉积物扩散能力、岩石圈强度、造山带逆冲速率、全球海平面变化、和俯冲有关的动力沉降及俯冲负载等众多其它因素的影响。本文阐述了这些因素与前陆盆地沉降的关系,介绍了与幕式逆冲有关的地层模型和欠补偿-过补偿地层模型。希望本文能够对中国西北地区前陆盆地的研究起到一定的借鉴意义。