Asymmetric Atlantic continental margins

We analyze the gross crustal structure of the Atlantic Ocean passive continental margins from north to the south, comparing eleven sections of the conjugate margins. As a general result, the western margins show a sharper continental-ocean transition with respect to the eastern margins that rather show a wider stretched and thinner margin. The Moho is in average about 5.7°±1° dipping toward the interior of the continent on the western side, whereas it is about 2.7°±1° in the eastern margins. Moreover, the stretched continental crust is on average 244 km wide on the western side, whereas it is up to about 439 km on the eastern side of the Atlantic. This systematic asymmetry reflects the early stages of the diachronous Mesozoic to Cenozoic continental rifting, which is inferred as the result of a polarized westward motion of both western and eastern plates, being Greenland, Northern and Southern Americas plates moving westward faster with respect to Scandinavia, Europe and Africa, relative to the underlying mantle.     

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.     

On sheared passive continental margins

Studies in intra-continental and intra-oceanic shear zones reveal structures that may be developed during the formation of a sheared passive continental margin.During the intra-continental shear stage of margin development, rapid vertical movement of the crust may occur resulting in small, tectonically-active basins containing thick sedimentary sequences. At deeper levels in the continental crust, more plastic deformation may lead to a zone of strongly sheared rocks that widens downwards. The tectonic fabric in this zone may exert some control over the subsequent development of the continent-ocean transition under the influence of regional stresses.The thermal event related to asthenosphere upwelling at sheared margins is a transient one and thus of less effect than the event on rifted margins. Nevertheless, following the event the cooling and contraction of oceanic crust against the continent may throw the oceanic crust into tension and lead to normal, block faulting in the oceanic regions analogous to the faulting seen in oceanic fracture zones. The subsidence of oceanic crust as it ages at the margin will either drag down the adjacent continental crust or, more likely, cause the oceanic crust to slip down by normal faulting along the continent-ocean boundary. The kinds of compressional features observed in oceanic fracture zones may also occur at sheared margins.     

Oil source rocks at the mesozoic and cenozoic continental margins: Communication 1. Oil source rocks at continental margins in the Triassic-Jurassic and Neocomian-Aptian

The presence of rocks capable of generating hydrocarbons (HC) in the section of sedimentaryrock basins is an essential criterion for their qualification as structures with oil and gas pools. Although organic matter (OM) is always present as dissemination in genetically different sediments, it is believed that rocks enriched with OM of the sapropel series (2 to 3% C_org) can generate a significant amount of liquid HC. However, rock sequences with the C_org ranging from 3–5 to 15–20% are considered oil source formations. The rock section of large petroliferous basins usually includes one or two source sequences, which generated liquid and gaseous HCs after submergence to high temperature and pressures zones. In the basin confined to the Arctic slope of Alaska, one of the main producers of liquid HC is represented by the Upper Triassic clays and limestones of the Shublik Formation. In the Barents Sea and North Sea basins, such rocks are represented by the Spekk Formation and the Kimmeridge Clay, respectively; in the West Siberian basin, by the Bazhenovo Formation; in the Persian Gulf, by the Fahlian, Sargelu, and Garau formations; in basins of the Caribbean region, by marls and clayey limestones of the La Luna Formation. In perioceanic basins of the South Atlantic, the major source sequences are represented by the Neocomian and Barremian clays and marls. The source rocks are identified as the Lagoa Feia Formation in the Campos and Santos basins. They are cognized as the Black Marlstone or Bukomazi Formation in the Lower Congo, Kwanzaa-Cameroon, and Angola basins.     

Petroliferous basins at continental margins of paleozoic paleoseas: Communication 2. Petroliferous basins at continental margins in the Rheic, Uralian, and Central Asian paleoseas

The continental block of the Earth’s crust was separated in the Paleozoic into two unequal parts: (i) huge supercontinent Gondwana located at high latitudes of the Southern Hemisphere and (ii) several small continents (Laurentia, Baltica, Siberia, Kazakhstan, South Chinese block, and North Chinese blocks) located at low latitudes south and north of the equator. Morphology of the Paleozoic seas between these blocks was subjected to changes (expansion and contraction) with time. Their closure was provoked by several orogenic (Taconian, Caledonian, Acadian, and Hercynian) phases. At present, relicts of these ancient orogenic structures extend as belts along the boundaries of many petroliferous basins and record the position of past seas. One of the oldest oil-and-gas deposition belts, which appeared in southern Iapetus in the Precambrian/Phanerozoic, was confined to a passive margin of Gondwana. In the Early Paleozoic, small blocks of the continental crust (Avalonia, Armorica, Perunica, Iberica, and others) were successively detached from the passive margin. This process was accompanied by the opening of a new deep basin (Rheic Sea or Paleotethys). The Uralian and Central Asian paleoseas were formed approximately at the same time. Many petroliferous basins existing now were located in the Paleozoic at the margins of these paleoseas.     

Petroliferous basins at continental margins of Paleozoic paleoseas: Communication 1. Petroliferous basins at margins of Iapetus and Panthalassa

Although large marine basins governing the fabric of our planet in the Paleozoic disappeared later (whether or not they were oceans is a debatable issue), sedimentary basins formed at continental margins at that time played a crucial role as depositories of various fossil minerals, including ores, salts, phosphorites, coal, bauxites, and construction materials. Many of these basins are oil- and gas-bearing structures. Their oldest representatives are confined to margins of Proterozoic/Paleozoic paleoseas (Iapetus and Panthalassa), whereas other basins appeared after opening of the Central Asian, Uralian, and Rheic (Paleotethys) deep-marine basins. Study of specific features of the sedimentary cover of such basins, rock composition therein, rocks and associated oil- and gas-bearing systems revealed that the Paleozoic planet was divided into two parts: Gondwana, with the major portion confined to high latitudes of the Southern Hemisphere; and other smaller near-equatorial continents. This pattern significantly governed the composition and mode of post-sedimentary transformations of natural reservoirs, as well as age and spatial distribution of the major hydrocarbon (HC) source sequences. Most Paleozoic oil- and gas-bearing basins make up specific belts because of their confinement to continental margins in paleoseas of that time.     

Seaward dipping reflectors and the continent-ocean boundary at passive continental margins

The definition of the continent-ocean boundary at passive continental margins has proved to be an elusive task. Even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally. Multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors, dipping seaward to form a wedge-shaped structure. Their general characteristics and velocity structure suggest that they may be largely volcanic in nature, but there is no agreed upon model for their origin. Nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean, and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins.     

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.     

Oil source rocks at mesozoic and cenozoic continental margins: Communication 2. Oil source rocks at continental margins in the second half of the cretaceous and in the Cenozoic

Oil source rocks represent sequences with the C_org content ranging from 3–5 to 15–20%. Sedimentary sections of large petroliferous basins usually include one or two such sequences, which generated liquid and gaseous hydrocarbons (HCs) during their long-term subsidence to the elevated temperature zone. The middle episode of the Late Cretaceous was marked by the accumulation of sediments with a high C_org content in different areas of the World Ocean. However, truly unique settings favorable for accumulation of the sapropelic organic matter (OM) appeared at continental margins that primarily faced the Tethys Ocean. The La Luna Formation is one of the best known source rock sequences responsible for the generation of liquid HCs in basins of the Caribbean region. In the Persian Gulf, the Kazhdumi Formation composed of marls and clayey limestones is considered the main oil-generating sequence. In the Paleogene after closure of the Tethys, the Pacific continental margins became the main domains that accumulated source rocks. The maximal deposition of sapropelic OM in this region corresponded to the early-middle Eocene. In the Neogene, the accumulation of source sediments was associated with deltas and submarine fans of large rivers and with upwelling zones. In basins of the Californian borderland, the main oil-generating sequences are represented by siliceous rocks of the Monterey Formation. They were deposited in a regional upwelling zone related to the cold California Current.     

大陆边缘增生造山作用

文章评述了增生造山作用的研究历史和进展,认为增生造山作用贯穿地球历史,是大陆增生的重要方式。用大陆边缘多岛弧盆系构造理解造山带的形成演化,提出巨型造山系的形成与长期发育的大洋岩石圈俯冲制约的两侧或一侧的多岛弧盆系密切相关。在多岛弧盆系演化过程中的弧 弧和弧 陆碰撞,弧前和弧后洋盆的消减冲杂岩的增生,洋底高原、洋岛/海山、外来地块(体)拼贴等一系列碰撞和增生造山作用形成大陆边缘增生造山系。大洋岩石圈最终消亡形成对接消减带,大洋岩石圈两侧的多岛弧盆系转化的造山系对接形成造山系的联合体。拼接完成后往往要继续发生大陆之间的陆 陆碰撞造山作用、陆内汇聚(伸展)作用,后者叠加在增生造山系上,使造山过程更加复杂。对接消减带是认识造山系形成演化的关键。大洋两侧多岛弧盆系经历的各种造山过程可以从广义上理解为一个增生造山过程。多岛弧盆系研究对于划分造山带细结构非常重要,是理解造山系物质组成、结构和构造的基础,并制约了造山后陆内构造演化。大陆碰撞前大洋两侧多岛弧盆系及陆缘系统更完整地记录了威尔逊旋回,记录的信息更加丰富。根据多岛弧盆系的思路对特提斯大洋演化提出新的模式,认为西藏冈底斯带自石炭纪以来受到特提斯大洋俯冲制约,三叠纪发生向洋增生造山作用,特提斯大洋于早白垩世末最终消亡。     

Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a numerical simulation

The evolution of a subduction channel and orogenic wedge is simulated in 2D for an active continental margin, with P-T paths being displayed for selected markers. In our simulation, subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years, with four zones from the trench into the forearc: (i) an accretionary complex of low grade metamorphic sedimentary material, (ii) a wedge of nappes with alternating upper and lower crustal provenance, and minor interleaving of oceanic or hydrated mantle material, (iii) a megascale melange composed of high pressure (HP) and ultra-high pressure (UHP) metamorphic rocks extruded from the subduction channel, and (iv) the upward tilted frontal part of the remaining lid. The P–T paths and time scales correspond to those typically recorded in orogenic belts. The simulation shows that HP/UHP metamorphism of continental crust does not necessarily indicate collision, but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction.     

Ancient continental margins and comparative analysis of their oil-and-gas bearing

Analysis of peculiarities in the distribution of hydrocarbon accumulations within the basins of Phanerozoic continental margins, which had completed their evolution, and complicated peripheral regions of ancient Laurasian and Gondwanian platforms nowadays, has enabled us to reveal certain regularities related to two stages in the evolution of sedimentary basins. The first stage of evolution of sedimentary basins (period of existence of the continental margin proper) is related to large accumulations of fluid and gaseous hydrocarbons in the margins of continents belonging to the Laurasian megablock; for the margins of continents belonging to Gondwana, this period was reflected in the formation of large gas accumulation only (in the Permian). At the second stage of sedimentary basin evolution, large oil and gas accumulations were formed in areas associated with fore deeps, which were laid in the boundary of the Gondwanian platforms and fold belts. In comparison, in fore deeps that emerged in the marginal parts of Laurasian platforms, less significant accumulations of fluid and gaseous hydrocarbons were found (Table 1). The results of comparative analysis in oil-and-gas bearing basins located in the margins of the Laurasian and Gondwanian megablocks would help in purposeful exploratory works for oil and gas.     

Lithospheric extension from rifting to continental breakup at magma-poor margins: rheology,serpentinisation and symmetry

The symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years, with various authors proposing asymmetric simple shear models, others advocating more symmetric, pure shear models and some combinations of the two. The unroofing of vast expanses of sub-continental mantle at non-volcanic margins has led some authors to argue in favour of simple shear models, but supporting evidence is lacking. Subsidence evidence from conjugate margin pairs is equivocal, and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions. In the Porcupine Basin, where the final stages of break-up are preserved, the development of structural asymmetry is demonstrable, and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust. This agrees with theoretical modelling results, which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled. However, whether such asymmetry is maintained during continued exhumation of the mantle is unclear.     

Transects of the ancient and modern continental margins of eastern Canada

Rocks of the west flank of the northern Appalachian Orogen (miogeocline) record the history of the late Precambrian-early Paleozoic passive continental margin of Eastern North America. The ancient margin was destroyed by ophiolite obduction and arc collision during the Ordovician Taconic Orogeny. The present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re-entrants of a previous orthogonal margin bounded by rifts and transforms.Four major terranes are recognized east of the miogeocline in Newfoundland and Nova Scotia. From west to east, these are the Dunnage, Gander, Avalon and Meguma. The Dunnage and Gander terranes were linked to the miogeocline during the Middle Ordovician Taconian Orogeny. The Avalon terrane arrived later, possibly during the mid-Paleozoic Acadian Orogeny. The Meguma terrane of southern Nova Scotia had docked with the Avalon terrane by Carboniferous time. The Dunnage terrane contains arc volcanics which lie above an ophiolitic substrate. The Gander terrane comprises a thick sequence of clastic sedimentary rocks, underlain by basement rocks with continental affinities. It has been interpreted as a continental margin, perhaps once on the eastern side of the Paleozoic Iapetus ocean. The Avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by Grenvillian basement. Its tectonic affinities are unclear. The Meguma terrane comprises a thick sequence of sediments, derived from the south-east. It is found only in southeastern Atlantic Canada. The boundaries between terranes are compressional in the west and steep, transcurrent faults in the east.The surface extent of the geological terranes is grossly correlative with deep structural zones, although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth. A striking feature of the deep crustal structure is a lower, high velocity crustal layer beneath the Dunnage and Gander terranes.The modern margin of Atlantic Canada developed by rifting and by transform motion between adjacent continents. Stretching and thinning of the lithosphere, and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust, are the most likely processes responsible for the evolution of the modern margin. These processes predict the observed deep sedimentary basins along the margin, the thinning of continental crust, and the high seismic velocities found within the ocean-continent transition zones.Rifting adjacent to Nova Scotia began in Late Triassic-Early Jurassic time between the present African and North American plates. These plate motions are also responsible for the major transform margin south of the Grand Banks. Separation between Iberia and the eastern Grand Banks occurred in mid-Cretaceous time, before the Late Cretaceous opening of the Labrador Sea. While the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust, the Grand Banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover. Numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment.Mimicry of Paleozoic promontories and re-entrants by modern rift and transform margin segments, the location of Mesozoic sedimentary basins on ancestral Appalachian structures, and the reactivation and propagation of major Precambrian and Paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins.The rift phase of both the ancient and modern passive margins is characterized by volcanism, mafic dike intrusion and by the development of basins filled with clastic sediments. The drift phase of both the ancient margin and the present Nova Scotia margin is marked by a change in sedimentary environment, such that carbonates replaced the rift phase clastic sediments. Two of the markers used to delineate the ancient ocean-continent transition zone; carbonate banks and steep gravity anomaly gradients, should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone. Furthermore, it is naive to view the ancient transition as simple and narrow, for the modern margins exhibits complex transition zones between 30 and 300 km wide.In general, the evolution of the ancient and modern passive margins appear to be remarkably similar. Predictably, closing the present Atlantic will mimic the evolution of the Appalachian Orogen.     

Oil and gas potential of continental margins of the Pacific Ocean

Oil and gas basins (OGB) of active and transform margins of the Pacific Ocean are discussed. Their western and eastern parts differ substantially in the evolution, tectonic pattern, and scope of resources. In the west, marginal seas incorporated into the Cenozoic geodynamic system of deep-water basins (marginal seas) and conjugate island arcs exhibit a greater oil and gas potential (hereafter, petroleum potential) as compared to the eastern margin bounded by a deep-water trench and transformed into the framing with OGBs only in separate sectors. The abundance of siliceous rocks influenced the formation and accumulation of oil and gas in the Pacific region. The most part of hydrocarbon accumulations is related to organogenic edifices and channels of shelf fans. Oil and gas fields confined to fans on slopes of deep-water troughs of active and transform margins are also known. Proceeding from the global practice, significant petroleum potential in Russia is associated with back-arc seas of the Pacific. The poorly studied deep-water basins on slopes are worthy of special attention.     

Rheologic properties of the lithosphere and applications to passive continental margins

Thermal and petrologic models of the crust and upper mantle are used for calculating effective viscosities on the basis of constant creep rates. Viscosity—depth models together with pressure—depth models are calculated for continental and oceanic blocks facing each other at continental margins. It is found from these “static models” that the overburden pressure in the lower crust and uppermost mantle causes a stress which is directed from the ocean to the continent. The generally low viscosity of 10²⁰–10²³ poise in this region should permit a creep process which could finally lead to a “silent” subduction. In the upper crust static stresses act in the opposite direction, i.e. from the continent to the ocean, favouring tension which could produce normal faulting in the continent. Differences between observations and the results obtained from the static models are attributed to dynamical forces.     

Kinematics and timing of continental block deformation from margins to interiors

During the deformation of continental blocks, the magnitude of tectonic stress generally decreases with increasing distance from the margin of the block. However, the timing and kinematics of stress transmission from the margins to the interiors of continents are poorly resolved, even though this information is critical to our understanding of the dynamics of continental deformation. Here, we present a case study of Mesozoic deformation of the North China Craton (NCC). Field investigations of Mesozoic thrust faults and folds, granitic intrusions and dykes, combined with zircon SHRIMP and LA–ICP–MS dating and muscovite ⁴⁰Ar/³⁹Ar plateau ages, reveal the age of the NE–SW‐trending tectonic belts as ~180–155 Ma, where the deformation of the craton margin occurred 10–20 Ma earlier than that of the craton's interior. Although the kinematics of deformation are similar for the interior and the margin of the NCC, strain decreases with increasing distance from the margin. Notably, the bulk of the strain in the cratonic interior was focused in zones of pre‐existing weakness. Overall, we determined that the NCC deformed under conditions of uniaxial compression, a conclusion that is compatible with simple rheological models, and that the stress magnitude attained in the cratonic interior was much less than that along its margin.