Seafloor Spreading Axis Research Articles

The mechanism of formation, development and deformation of sedimentary basins in Viet Nam continental shelf

The sedimentary basins in area of Việt Nam continental shelf are located along the deep fault systems between the folded Indochinese block and Việt Băc-Hoa Nam platform and with the transitional zone. Is means that the zone attenuated continental crust. Due to that extruction of the Indochinese block toward the SoustEast which wrenched in right, in addition, due to the appearance of the thermal anomaly, producing the activity of Bien Dong seafloor spreading axis and drift of Australian–New Guinea plate toward Nord-East, induced some geodinamic factors to form many sedimentary basins in margins of Biển Dong Sea, such as: rift, pressure, extension, vertical slide cliff, horizontal displacement and wrench. These geodinamic factors created favourable conditions to form, develop and deform the sedimentary basins in Việt Nam continental shelf, followed the pull- apart type mechanism. But each sedimentary basin had its type of mechanism which depended on the concrete place of its basin from the Indochinese block and the thermal anomaly in Bien Dong Sea. Beside, itsformed condition for gas hydrate accumulations in some basins.

South China Sea crustal thickness and oceanic lithosphere distribution from satellite gravity inversion

Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.

Depositional characteristics of the northern South China Sea in response to the evolution of the Pearl River

Abstract Geochemical data from South China Sea sedimentary rocks show the effects of both source composition and depositional environments. This enables us to link tectonic trends with erosion in the Pearl River region since c. 32 Ma. In particular, a shift in the geochemistry appears to signal a response to a well-recorded regional tectonic event at c. 23–25 Ma, probably corresponding to a jump in the seafloor spreading axis from the west to the SW within the South China Sea. This may correlate with the uplift of the West Yunnan Plateau and possibly also the eastern Tibetan Plateau. Clay mineralogy, sand–mud ratio, and major and rare earth element concentrations, also varied in response to the environment in the drainage areas of the palaeo-Pearl River. By comparing data from the modern sources and the sedimentary record from the northern South China Sea, especially the erosion–transportation–deposition patterns, three groups of index minerals (Ati, GZi, ZTR), as well as rare earth elements can be recognized. These are used to characterize the Pearl River from the east to the west, representing three different parent rock sources. The evolution of the palaeo-Pearl River can be tracked by variations of heavy minerals and key elements that are indicative of provenance. Supplementary material: Supplementary material, including Tables S-1 and S-2 listing the sampling and analytical data, is available at http://www.geolsoc.org.uk/SUP18855

The breakup of the South Atlantic Ocean: formation of failed spreading axes and blocks of thinned continental crust in the Santos Basin, Brazil and its consequences for petroleum system development

Abstract The occurrence of failed breakup basins and deepwater blocks of thinned continental crust is commonplace in the rifting and breakup of continents, as part of passive margin development. This paper examines the rifting of Pangaea–Gondwanaland and subsequent breakup to form the South Atlantic Ocean, with development of a failed breakup basin and seafloor spreading axis (the deepwater Santos Basin) and an adjacent deepwater block of thinned continental crust (the Sao Paulo Plateau) using a combination of 2D flexural backstripping and gravity inversion modelling. The effects of the varying amounts of continental crustal thinning on the contrasting depositional and petroleum systems in the Santos Basin and on the São Paulo Plateau are discussed, the former having a predominant post-breakup petroleum system compared with a pre-breakup system in the latter. An analogy is also made to a potentially similar failed breakup basin/thinned continental crustal block pairing in the Faroes region in the NE Atlantic Ocean.

Structural architecture of the sheeted dike complex and extensional tectonics of the Jurassic Mirdita ophiolite, Albania

The Jurassic Mirdita ophiolite in northern Albania occurs in a ∼ 40 km-wide zone bounded by the passive margins of Apulia (W) and Pelagonia (E). The ∼ 3 km-thick Western Mirdita ophiolite (WMO) contains lherzolitic peridotites and gabbros overlain by basaltic pillow lavas, whereas the ∼ 12 km-thick Eastern Mirdita ophiolite (EMO) represents a Penrose-type oceanic crust with suprasubduction zone affinities. The 1 km-thick sheeted dike complex (SDC) in the EMO shows mutually intrusive relations with the underlying gabbros, is overlain by a 1.1-km-thick extrusive sequence, and records a history of complex seafloor spreading and rift tectonics in a suprasubduction zone environment. Crosscutting relations within the SDC indicate four generations of dike intrusions becoming progressively younger to the east. Early D1 and D2 dikes have basalt and basaltic andesite compositions and NNE and NNW attitudes, respectively, with moderate to gentle dips to the east. They are cut by mineralized, dike-parallel normal faults defining local grabens. Younger D3 dikes display andesitic and boninitic compositions and have WNW strikes with steep dips. These dikes are cut by both dike-parallel and dike-orthogonal faults that form locally well-developed graben structures with extensive epidosite and chalcopyrite mineralization. The youngest D4 dikes with 240°–290°orientations occur as isolated swarms intruding earlier dike generations in the eastern part of the SDC, range in composition from quartz–microdiorite to rhyodacite and rhyolite, and are cut by ∼ NW-oriented faults and shear zones. Changes in dike compositions from basalt and basaltic andesite to andesite, boninite, rhyodacite and rhyolite through time in the SDC are consistent with changes in the lava chemistry stratigraphically upward and eastward in the extrusive sequence, indicating significant chemical variations in melt compositions as the EMO evolved. Development of the SDC and its extensive normal faulting were a result of upper plate extension and rifting in the protoarc–forearc crust caused by rapid slab rollback in the Mirdita basin, reminiscent of the late Cenozoic extensional tectonics of the Izu–Bonin–Mariana system in the Western Pacific. Shifting of dike attitudes to more easterly orientations in later stages of SSZ magmatism signals a gradual change in the spreading direction likely caused by impingement of the clockwise rotating Pelagonia on the arc–trench rollback system in the south. The SSZ Mirdita basin evolved similarly to the last 100 km of recorded spreading in the West Philippine Basin, where the rotation of the seafloor spreading axis was caused by the incipient collision between the northern part of the Philippine arc with rifted fragments of Eurasia.

Late Oligocene sedimentary environments and provenance abrupt change event in the northern South China Sea

A significant change in composition was recorded in late Oligocene sediments from the northern South China Sea. This abrupt event coincided with the seafloor spreading axis jump across the Oligocene/Miocene boundary, leading to sedimentation breaks and slumps as well as obvious changes in sediment geochemical composition, and representing the greatest tectonic activity in the South China Sea region since the Oligocene. Through this tectonic event, the sedimentary environment in the Baiyun sag area transformed from a continental shelf in the late Oligocene to a continental slope since the early Miocene, the provenance of the sediments changed from neighboring areas to the hinterland of the South China block, and the sea level rose since the early Miocene in the area. Therefore, this abrupt change event has a profound influence on the evolution of petroleum offshore in the northern South China Sea.

Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge–mantle plume interaction?

According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian‐Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge‐driven convection, or by asthenosphere flow–lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid‐ocean ridge basalt–capture model, in which this magmatism can be understood through plume–spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low‐degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low‐degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

Rates of continental breakup magmatism and seafloor spreading in the Norway Basin–Iceland plume interaction

In year 2000, an ocean bottom seismometer (OBS) profile was acquired across the Møre margin to the Aegir Ridge, an extinct seafloor spreading axis. The margin is an early Eocene volcanic passive margin, located between the Faeroe‐Iceland Ridge (FIR) and the East Jan Mayen Fracture Zone (EJMFZ). The P wave data were modeled by ray tracing to give a crustal transect showing a 10–11 km thick igneous crust created by breakup magmatism, tapering off to magma‐starved seafloor spreading by C23 time (51.4 Ma). The location of the EJMFZ was reinterpreted from a satellite derived gravity map, and spreading direction in the Norway Basin reevaluated. No other fracture zones were confirmed, and both thin oceanic crust (4–5 km) and lack of fracture zones resemble ultraslow spreading on the Arctic Gakkel Ridge. Magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the OBS profile, and half spreading rates were derived. Early seafloor spreading was slow (15–32 mm yr−1), approaching ultraslow (6–8 mm yr−1) by C20 time (42.7 Ma). A V‐shaped pattern seen in the gravity field located only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the seismic model, recording northeast transport (3–6 mm yr−1) of more melt‐fertile asthenosphere zones. The magma‐starved character of the Norwegian Basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the Iceland plume constructed the FIR to the south, as the asthenosphere is subsequently transported into the Norway Basin.

Quantitative Meso-/Cenozoic development of the eastern Central Atlantic continental shelf, western High Atlas, Morocco

The well exposed Late Paleozoic to Cenozoic succession in the western High Atlas (Morocco) documents the early rift to mature drift development of the eastern Central Atlantic continental shelf basin. Vertical sections, depositional geometries and unconformities have been used to reconstruct the basin architecture prior to Atlasian inversion. Two-dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest Paleozoic to Early Cenozoic. Basin evolution stages include (i) early rift, Late Permian to Anisian; (ii) rift climax, Ladinian to Carnian; (iii) sag, Norian to Early Pliensbachian; (iv) early drift, Late Pliensbachian to Tithonian; (v) mature drift, Berriasian to Cenomanian; (vi) mature drift with initial Atlasian deformation, Turonian to Late Eocene; (vii) Atlasian deformation; Late Eocene to Early Miocene; (viii) Atlasian uplift and basin inversion, Early Miocene to Recent. The Late Permian to Late Cretaceous basin development comprises eight subsidence trends of 10–35 Myr duration. Trends were initiated by changes in thermo-tectonic subsidence, which in turn triggered positive and negative feedback processes between sediment flux, flexural and compaction-induced subsidence. Plate-tectonic reconfigurations in the Atlantic domain controlled the thermo-tectonic subsidence history and the basin development of the Agadir segment of the northwest African passive continental margin: (1) major shifts in the sea-floor spreading axis; (2) significant decreases in sea-floor spreading rates; (3) the stepwise migration of crustal separation and sea-floor spreading in and beyond the Central Atlantic; (4) African–Eurasian relative plate motions and convergence rates. During the Early Pliensbachian/Toarcian to Cenomanian the first three plate-tectonic reconfigurations triggered changes in ridge-push forces and modulated the extensional stress field of Central Atlantic plate drifting. Since the Turonian, African–Eurasian relative plate motions and convergence rates represented the dominant control on the thermo-tectonic subsidence history in the Agadir Basin. Major variations in sediment flux and total subsidence characterize the development of the northwest African passive continental margin. The explanation of typical stratigraphic sequences as caused predominantly by sea-level fluctuations, and rough assumptions on sediment input/production and subsidence, is not necessarily applicable to passive continental margins. The methodology applied in this study, including the newly developed tool of ‘Compositional Accommodation Analysis’, allows to develop more rigorous genetic models for the development of continental shelf basins.

Macquarie Island mapping reveals three tectonic phases

The first detailed mapping of Macquarie Island, perhaps the world's quintessential ophiolite has revealed three tectonic phases—extension, transtension, and transpression. Nearly all of the crust and most deformational structures formed during the first phase (D1), which involved extension along the paleo‐Macquarie seafloor spreading axis. Magmatism, block tilting, and large‐scale differential block uplift were synchronous, resulting in juxtaposition of disparate rock associations characteristic of different crustal levels. As D1 waned, the second phase (D2) developed. Involving transtension with a north‐south extension axis, it produced minor late‐stage dolerite dikes and seafloor volcanic centers.The third phase (D3), still in progresses dextral transpression. Its initiation along the Australia‐Pacific plate boundary terminated igneous activity and extension on the island.The D3 shortening axis trends east‐west to northeast‐southwest.

Geophysical and morpho-tectonic study of the transition between seafloor spreading and continental rifting, western Woodlark Basin, Papua New Guinea

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and continental rifting in the western Woodlark Basin, off-shore eastern Papua New Guinea. The oceanic domain comprises new oceanic crust formed during the Bruhnes Epoch, older transitional crust and the rifted continental margins. Two rift branches are recognized within the continental domain. The southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crustal accretion. Based on magnetic data, seafloor spreading in the western part of the Woodlark Basin commenced between 3.5 and 2.5 Ma along the northern margin of the basin and maintained its position adjacent to this margin up to 0.8 Ma (Jaramillo event). Just prior to the Bruhnes Epoch, the seafloor spreading axis jumped southward to its current position. Frequent jumps of seafloor spreading centers and relocation of paleoaxes indicate the instability of the Woodlark extensional system. The transition between the two domains is characterized by differential localization of extensional strain. Variations in the localization can result from progressive change of extensional mode when the initial rifting evolves into seafloor spreading and/or from differences in lithospheric rheology as the rift propagates into the continental margin. A consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced across the transfer zone by broadly distributed deformation within continental lithosphere.

Complex structure along a Mesozoic sea-floor spreading ridge: BIRPS deep seismic reflection, Cape Verde abyssal plain

SUMMARY As part of an intensive study of a small area of oceanic lithosphere, the British Institutions Reflection Profiling Syndicate (BIRPS) acquired closely spaced deepseismic-reflection profiles over the Early Cretaceous crust of the Cape Verde abyssal plain off West Africa. The survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea-floor spreading axis (‘isochron’ profiles) and orthogonal dip lines oriented in the original direction of spreading (‘flow’ profiles). A large-capacity, well-tuned airgun source and very quiet shooting conditions ensured a high signal-to-noise ratio for deep reflection. Devising a strategy for mitigating contamination from ‘wrap-around’ multiples arriving from previous shots enabled us to use the minimum possible shot-point interval (50 m) allowed for collecting long (18 s) records. Data processing was oriented towards a medium with low root-mean-square velocity, steeply dipping structure, and pervasive low apparent velocity noise from diffraction at the top of the igneous crust. The contrast between the isochron and flow profiles is striking. Isochron profiles are typically highly reflective throughout the igneous crust, consisting of bright, bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted Moho reflection. These reflections do not offset intracrustal or top-basement structure and thus are not interpreted as faults: an igneous intrusive origin seems more likely. Flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust, continuing down in places to the Moho. Dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust. The dominant dip of these reflections is to the west towards the spreading ridge axis. Reflections from the vicinity of the Moho, while well developed in some places, are not particularly prominent across the survey area. Moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles: on isochron profiles, dipping reflections occasionally flatten out into, and may merge with, the Moho reflection; on flow profiles, as dipping crustal reflections approach the Moho reflection, they are usually abruptly cut off by it without extending deeper. This survey shows how oceanic crustal structure can vary rapidly over relatively small areas, provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust, and gives a conclusive observation of faults that penetrate the entire oceanic crust.

Hydrothermal circulation through mid-ocean ridge flanks: Fluxes of heat and magnesium

Thermally driven convection of seawater occurs through oceanic crust of all ages, at the seafloor spreading axis, on mid-ocean ridge flanks, and in the ocean basins. At the ridge axis and on the flanks, circulating seawater produces a major chemical exchange between the oceans and the crust. Based on heat budget constraints and the composition of hot springs, between 10 and 40% of the river flux of Mg can be taken up during high-temperature alteration of the basaltic crust along the ridge axis. Most of the hydrothermal heat loss, however, occurs on the mid-ocean ridge flanks, where the temperatures are lower and the seawater flux correspondingly larger. The estimated heat loss on the flanks is so large that upwelling must occur over a large fraction (5–30%) of the seafloor less than 65 Ma in age, if temperatures are < 20°C and seepage velocities are on the order of 10 to 100 cm/y. The circulating seawater needs to lose on average less than 1–2% of its Mg content in order to solve the Mg mass balance for the oceans. Chemical fluxes through mid-ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date. The most critical parameter for characterizing crustal conditions is temperature in basement, which is a function of crustal age, basement topography, and sediment thickness and permeability. Basement temperature, which can usually be inferred from heat flow and seismic reflection surveys, largely determines the change in the composition of seawater circulating through basement. This change can be inferred, in turn, from profiles of sediment porewater chemistry. All sites studied to date with temperatures at the sediment-basement interface ≤ 25°C have a large component of advective heat loss and show only a small (< 10%) loss of Mg from the circulating seawater, whereas all sites with basement temperatures ≥ 45°C have a small advective component and show a large (> 80%) Mg loss. Both types of sites may be important for chemical fluxes. Whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust; only small changes are needed. Whether the warmer sites are important depends on how much heat is lost by advection in this type of setting. The warmer sites could produce significant chemical fluxes even if they are scarce: they could account for the entire river input of Mg to the oceans even if they represent only 8–20% of the total advective heat loss on ridge flanks.

Plate-moving mechanisms: their relative importance

The lithosphere forms the upper boundary layer of the Earth's mantle convection system which facilitates escape of thermal energy from the Earth's interior. Motion and interaction of lithospheric plates is probably governed by the combination of drag-forces excerted on the base of the lithosphere by the convecting mantle and by plate boundary forces. Phanerozoic movements and interaction of major continental blocks are difficult to explain in terms of conventional plate moving mechanisms, such as slab-forces, ridge-push and deviatoric tensional stresses developing over upwellings of the asthenosphere and in response to lithospheric over-thickening in orogenic belts. Circumstantial evidence suggests that shear-traction exerted by the convecting asthenosphere on the base of the lithosphere plays an important, and at times even a dominant role as a plate moving mechanism. The relative importance of the different processes contributing to the motion of lithospheric plates probably varies during the assembly and break-up of Pangaea-type megacontinents. Continents assemble in areas of downwelling branches of the asthenospheric convection system. Development of major orogens along the trailing edges of drifting continents and subduction-progradation from the megasuture of colliding continents to their distal margins suggests that plate convergence is controlled, apart from slab-forces and ridge-push, also by shear-traction. Assembly of a Pangaea has an insulating effect on the downwelling convection cells, governing its suturing, causes their decay and a reorganization of the global upper-mantle convection system. Development of new upwelling and outflowing asthenospheric cells under mega-continents gives rise to tensional stresses in the lithosphere, causing its extension. Following crustal separation, the asthenosphere advects passively into the space opening between diverging plates. Progressive opening of oceanic basins is coupled with the development of ridge-push forces, contributing to plate divergence. Activity along sea-floor spreading axes can terminate abruptly if far-field stresses, resulting from plate interaction, impede further plate divergence. The nearly contemporaneous decay of sea-floor spreading axes in often distant areas reflects changes in plate interaction. Assuming a finite globe, generation of new lithosphere at sea-floor spreading axes has to be compensated for elsewhere by subduction of commensurate amounts of oceanic lithosphere and/or shortening and subduction of continental lithosphere. Plate interaction, driven largely by shear-traction of the mantle convection systems and their changes, ridge-push and slab forces, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception of and activity along subduction zones. A two-layered mantle convection system is envisaged, that may be coupled to a greater degree during the break-up of mega-continents than during periods of dispersed continents.

Pattern of mantle convection and Pangaea break-up, as revealed by the evolution of the African plate

A tensional stress regime has governed the tectonic evolution of most of the African continent. The long history of crustal extension is documented by the widespread occurrence of continued intraplate rifting and volcanism since Triassic times and by the large-scale plate movements following the break-up of Pangaea. A remarkable result of the present investigation is the recognition that the extension displays a continent-wide and plate-wide radial pattern, centred in equatorial Africa near 10°E/0°N, the African Centre A. The radial pattern is evident from the diverging block movements, documented by the intra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragments away from Africa, entailing a radial growth of the African plate. All these phenomena are manifestations of what may be referred to as the ‘African lithospheric divergence’. The African plate is surrounded, to 85%, by active oceanic ridges. Ridge-push forces exerted on the African plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large parts of the African continent . Shear traction exerted by the convecting asthenosphere on the base of the African lithosphere offers an explanation for the observed phenomena. It is proposed that relatively warm and less dense mantle material rises from the deep mantle below the African plate. In the upper mantle and at the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent. Upper mantle flow below the sea-floor spreading axes is unidirectional and horizontal and is directed away from the African centre A. It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth's mantle. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such large-scale convection systems.

Plate tectonics, plate moving mechanisms and rifting

Abstract Analysis of a sequence of palaeo-reconstructions of the distribution of continents in the Western Hemisphere suggests that frictional forces exerted on the base of the lithosphere by the slowly convecting sub-lithospheric upper mantle play an important role as a driving mechanism of plate movements. With such a scenario, slab-pull and roll-back, ridge-push and deviatoric tensional stresses, related to upwellings of the asthenosphere as well as to lithospheric over-thickening in orogenic belts, can be considered as secondary, albeit important, plate moving forces. The present stress state of the globe and circumstantial geological evidence suggest that major compressional stresses can be transmitted over great distances through continental and oceanic lithosphere. Assembly of major continental masses (like Pangea) probably has an insulating effect on upper-mantle convection systems, causing their decay and reorganization. Development of new asthenospheric upwelling systems under mega-continents causes their break-up by development of deviatoric tensional stresses in the lithosphere and by exerting drag forces on its base. Extension of the lithosphere, culminating in its failure, is followed by passive advection of asthenospheric material into the space opening between the diverging plates to which it is accreted as oceanic lithosphere. At the same time developing ridge-push forces contribute to plate divergence. However, activity along seafloor spreading axes can terminate abruptly if far-field compressional stresses impede further divergence of the respective plates. This may explain the nearly contemporaneous decay of seafloor spreading axes in often distant areas during periods of plate boundary reorganization. Assuming a finite globe, the generation of new oceanic lithosphere at seafloor spreading axes has to be compensated for elsewhere by the subduction of commensurate amounts of oceanic lithosphere and/or shortening of continental crust and subduction of lower crustal material and the subcrustal lithosphere. Plate interaction, driven by mantle convection systems and their changes, ridge-push and slab-pull and roll-back, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception and development of subduction zones. Mantle plumes rising from the deep mantle do not appear to play a significant role in the development of intra-continental rifts though they probably contribute to a weakening of the lithosphere. The relative importance of the various processes contributing to the movement of lithospheric plates differs probably during the assembly and break up of Pangea-type mega-continents.

Rifting of Africa and pattern of mantle convection beneath the African plate

Abstract The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident from the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a “centre” in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African “spreading centre”, referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80–90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50–60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

SeaMARC II evidence for the locus of seafloor spreading in the southern Mariana Trough

Two surveys near 13°45′N across the Mariana Trough identify the axis of backarc seafloor spreading at 143°53′E. The spreading center is identified on the basis of its high acoustic backscatter in SeaMARC II side-scan images and by the presence of young volcanic features. Neither asymmetric spreading nor discrete jumps in the spreading center location are required to explain the observed morphology and backscatter characteristics. Changes in the orientation of the seafloor spreading fabric suggest that an early phase of arc rifting may have been followed by northward propagation of the backarc spreading center to widen this portion of the basin. This could have produced the current situation, with the rift axis offset about 35 km east of the center of the Mariana Trough. The off-center position of the spreading axis may also the result of overgrowth of backarc crust by the volcanic arc.

Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America

Research Article| March 01, 1992 Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America Dennis L. Harry; Dennis L. Harry 1Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar Dale S. Sawyer Dale S. Sawyer 1Department of Geology and Geophysics, Rice University, P.O. Box 1892, Houston, Texas 77251 Search for other works by this author on: GSW Google Scholar Geology (1992) 20 (3): 207–210. https://doi.org/10.1130/0091-7613(1992)020<0207:BVMPAT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dennis L. Harry, Dale S. Sawyer; Basaltic volcanism, mantle plumes, and the mechanics of rifting: The Paraná flood basalt province of South America. Geology 1992;; 20 (3): 207–210. doi: https://doi.org/10.1130/0091-7613(1992)020<0207:BVMPAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Dynamic modeling of continental extension between South America and Africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the Paraná and Etendeka flood-basalt provinces on the Brazilian and Namibian margins. The key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension, which provided a mechanism for transporting magma generated beneath the incipient sea-floor spreading axis into the Paraná province, 100-200 km distant. The horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting. The model accounts for the large quantity of basalt, the asymmetric distribution of basalt on the conjugate margins, and the northward migration of the eruptive center with time. The rapidity of magma genesis is in agreement with models of decompression melting during rifting. The model indicates that although elevated asthenosphere temperatures associated with the Tristan plume account for the volume of melt generated, the mechanics of rifting control the location and style of emplacement. The model suggests that extension in the region began ca. 150-155 Ma, and lasted about 25 m.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Melt extraction from partially molten regions beneath mid-ocean ridges

A complete set of two-dimensional equations for conservation of mass, momentum and energy of a two-phase system of melt in a deformable matrix is used to derive analytically a model for the vertical distribution of the melt fraction. Also, a set of asymptotic relations for parameters in the velocity field of the matrix and the melt in the neighbourhood of the symmetry axis of sea-floor spreading is obtained. These relations have been used to derive solutions appropriate to the melt extraction from the partially molten regions beneath mid-ocean ridges and the conditions for their existence. The results show the convergence of the melt streamlines towards the spreading center. It is caused by: (1) a horizontal gradient of the piezometric pressure associated with the deformations of the matrix, and (2) an upward increase of the porosity due to melting. For accepted values of the relevant parameters, a focusing effect of melt towards spreading centers exists, thereby providing an explanation for the mid-ocean “paradox” of a narrow region of ridge axis volcanism overlying a broad melt production zone.