Guinea Margin Research Articles

Paleogene to Neogene sequence stratigraphy and deepwater depositional systems in Kribi-Campo/Rio-Muni margin (Offshore Cameroon-Equatorial Guinea Margin)

2D seismic data and Gamma Ray logs were interpreted to document the Tertiary deepwater depositional system observed in the southern part of the Kribi-Campo sub-basin and the northern part of the Rio Muni basin, both basins separated by the Campo high. The sedimentary filling of both basins occurred with cyclicity during the Tertiary. The Paleogene filling exhibits at least three Seismic Unit Packages (SUP1 to SUP3) interpreted as some Feeder Channels-Lobe Complexes (FCLC). The units with Mass Transport Complexes (MTCs) are low amplitude and chaotic to transparent seismic facies. They are interpreted as mud-rich complexes. The units related to FCLC, features high to moderate amplitude with shingled oblique progradational clinoforms. The Neogene filling is dominated by some submarine Leveed-Channels Complexes associated to Lobes (LCCL) with low to moderate amplitude and concave-up mounded configuration. The well log analysis revealed three main electrofacies namely, shale, siltstone, and sandstone that were deposited in various depositional environments. Shale lithology are underlined by irregular, bell and bow gamma ray log motifs which may be deposited in inner shelf, tidal channels and channel-levees systems that are associated with transgressive to regressive depositional sequences. Sandstone and siltstone lithology are typified by funnel and cylindrical gamma ray log motifs that are deposited in prograding marine shelf fans and slope channels. Influenced by the high slope of paleotopography around the Campo High, the submarine channel-levee are confined to a western part of Kribi-Campo sub-basin. The FCLC shows large planar distribution, while the MTC are poorly structured.

Deep structure of the Demerara Plateau and its two-fold tectonic evolution: from a volcanic margin to a transform marginal plateau, insights from the Conjugate Guinea Plateau

Abstract Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but their origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea plateaus and (2) the relationship between the Demerara Plateau and the adjacent oceanic domains, and finally, (3) throws light on the formation of TMPs. We analyse two existing wide-angle seismic-derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into three layers, including a high-velocity lower crust. The velocities and velocity gradients do not fit the values of typical continental crust but instead correspond to volcanic margin- or large igneous province-type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is made from extrusive volcanic rocks of the same magmatic origin, forming thick seaward (westward)-dipping reflectors (SDRs) sequences. This SDR complex extends to the Guinea Plateau as well and was emplaced during hotspot (Sierra Leone)-related volcanic rifting preceding the Jurassic opening of the Central Atlantic and forming the western margin of the plateau. North–south composite lines linking the Demerara and Guinea plateaus reveal the spatial extent of the SDR complex but also a pre-existing basement ridge separating the two plateaus. The entire Demerara–Guinea margin would therefore be an inherited Jurassic volcanic margin bordering the Central Atlantic Ocean to the east, with a possible conjugate being the Bahamas Plateau on the other side of the ocean. This margin was then reworked during a non-coaxial Cretaceous second phase of rifting potentially accompanied by a magmatic event. Opening of the northern margin occurred in a transform mode splitting the Jurassic volcanic margin into two parts (the Guinea and Demerara TMPs), conceivably along a pre-existing basement ridge. Rifting of the eastern part of the Demerara Plateau occurred surprisingly along the eastern limit of the Jurassic SDR complex, forming the present-day eastern divergent margin of the Demerara Plateau. After that stage, the Demerara and Guinea plateaus are individualized on each side of the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies related to hotspots and superposed tectonic phases in the case of other TMPs that share numerous characteristics with the Demerara Plateau.

Geodynamic and Surface Process Evolution of New Guinea Since the Jurassic

Regional scale flooding of New Guinea has occurred episodically since the Jurassic. The most recent flooding event during the Miocene occurred despite falling long-term eustatic sea levels. Recent work has suggested dynamic topography, the long-wavelength low-amplitude topographic response to mantle flow, as a factor in the emergence and flooding of this region, and therefore influencing the depositional history of New Guinea basins. The link between deep Earth and surface processes has not yet been explored for this region. We use forward numerical models coupling plate kinematics, mantle convection, paleogeography and eustasty to investigate the time-dependent topographic response of the New Guinea margin. Dynamic topography estimates derived from mantle convection models are then coupled with surface process modelling code Badlands to study the landscape evolution of New Guinea and the adjacent Australian continent. Reproducing the inundation history of New Guinea, our models show that continental scale dynamic topography plays a significant role in the development of drainage systems, and erosion-deposition regimes. Our work demonstrates the necessity in linking geological processes that operate across wide spatial and temporal scales to better understand how the interplay between deep Earth and surface processes control the source to sink evolution of basins.

Tectonic and Geodynamic Evolution of the Northern Australian Margin and New Guinea

Rapid convergence between the Indo-Australian, Southeast Asian, and Pacific plates in the Cenozoic has resulted in a complex tectonic evolution of Australia’s northern margin. A lack of available geologic data leads to large uncertainties, such as the timing of the Sepik collision with the New Guinea margin, currently constrained to sometime between 50 and 30 Ma. Previous work suggested a link between the Sepik collision and a voluminous fast seismic anomaly presently in the mantle beneath Lake Eyre. Following from previous work, this study uses coupled plate reconstruction and numerical geodynamic models to test 50 Ma and 30 Ma collision timings of the Sepik terrane, along with an upper extent back-arc basin, to further refine our understanding of the origin and trajectory of the slab beneath Lake Eyre and address uncertainties in the plate reconstructions. The results of mantle flow models indicate that the Eocene collision timing (~50 Ma) is more likely than an Oligocene collision (~30 Ma). In addition, dynamic topography results support previous suggestions that dynamic subsidence relating to the down-going Sepik slab has influenced the evolution of the Eyre Basin, with up to ~100 m of dynamic subsidence since ~20 Ma. However, further work is required to address numerical issues relating to rapid thermal diffusion of slab material, and to investigate reasonable trench retreat velocities for intermediate (~3000 km) subduction zone lengths. This work highlights the role of numerical experiments in understanding transient plate-mantle processes and their effect on basin evolution.

Evolution of the Southern Guinea Plateau: Implications on Guinea-Demerara Plateau formation using insights from seismic, subsidence, and gravity data

The Guinea Plateau (offshore Guinea) and its conjugate, the Demerara Plateau (offshore French Guiana), comprise two of the most prominent passive continental margins in the Atlantic Ocean. The conjugate plateaus formed as a result of two periods of rifting, the Jurassic opening of the Central Atlantic Ocean and the northward-propagating Cretaceous opening of the Southern Atlantic Ocean. Although several studies are published on the Demerara Plateau that explain the evolution of its multi-rift history and the effect of rifting on its distinct geometry, the Guinea Plateau, and in particular its south-eastern margin, remain relatively unexplored in the literature. Here we present interpretations of the structure and evolution of the Guinea Plateau using recent 2-D and 3-D seismic-reflection data collected at the intersection of the southern and eastern margins. We substantiate our study with calculated subsidence curves at four locations along the southern margin, as well as two 2-D gravity forward models along regional seismic-reflection profiles to estimate stretching factors (β) and crustal thicknesses. We combine our results with previous studies concerning the south-western Guinea margin, and compare them to published interpretations regarding the conjugate margins of the Demerara Plateau. The resolved amounts of rift-related volcanism, listric-style normal faults, and moderate stretching factors suggest that a component of upper-crustal asymmetry (simple shear) and depth-dependent stretching may have persisted at the Demerara-Guinea conjugate margins during Cretaceous rifting of the equatorial segment of the Southern Atlantic Ocean.

Revision of Dobodura Darlington (Coleoptera: Carabidae: Odacanthini): Diversification on accreted terranes of northern New Guinea

The Papuan endemic genus Dobodura Darlington is taxonomically revised, with five newly described species — Dobodura alildablldooya sp. n., D. hexaspina sp. n., D. obtusa sp. n., D. svensoni sp. n., and D. toxopei sp. n. — complementing the type species, D. armata Darlington. The sympatric Dobodura alildablldooya and D. svensoni are described from Chimbu Province, Papua New Guinea. Known distributions of the other three new species are: D. hexaspina, Madang Province, P.N.G.; D. obtusa, Olsobip, Fly River, Western Province, P.N.G.; and D. toxopei, Bernhard Camp, Papua, Indonesia. Dobodura is the sole precinctive Papuan genus in an Australian-Papuan clade also including Clarencia Sloane, Dicraspeda Chaudoir, and Eudalia Laporte. Phylogenetic analysis of Dobodura places its known earliest divergence event on the northern New Guinea margin of the Australian craton. Later divergence events result in species occupying island-arc terranes progressively incorporated into present-day northern New Guinea, commencing in the Miocene.

The Palu Metamorphic Complex, NW Sulawesi, Indonesia: Origin and evolution of a young metamorphic terrane with links to Gondwana and Sundaland

The Palu Metamorphic Complex (PMC) is exposed in a late Cenozoic orogenic belt in NW Sulawesi, Indonesia. It is a composite terrane comprising a gneiss unit of Gondwana origin, a schist unit composed of meta-sediments deposited along the SE Sundaland margin in the Late Cretaceous and Early Tertiary, and one or more slivers of amphibolite with oceanic crust characteristics. The gneiss unit forms part of the West Sulawesi block underlying the northern and central sections of the Western Sulawesi Province. The presence of Late Triassic granitoids and recycled Proterozoic zircons in this unit combined with its isotopic signature suggests that the West Sulawesi block has its origin in the New Guinea margin from which it rifted in the late Mesozoic. It docked with Sundaland sometime during the Late Cretaceous. U–Th–Pb dating results for monazite suggest that another continental fragment may have collided with the Sundaland margin in the earliest Miocene. High-pressure (HP) and ultrahigh-pressure (UHP) rocks (granulite, peridotite, eclogite) are found as tectonic slices within the PMC, mostly along the Palu–Koro Fault Zone, a major strike-slip fault that cuts the complex. Mineralogical and textural features suggest that some of these rocks resided at depths of 60–120km during a part of their histories.Thermochronological data (U–Th–Pb zircon and 40Ar/39Ar) from the metamorphic rocks indicate a latest Miocene to mid-Pliocene metamorphic event, which was accompanied by widespread granitoid magmatism and took place in an extensional tectonic setting. It caused recrystallization of, and new overgrowths on, pre-existing zircon crystals, and produced andalusite–cordierite–sillimanite–staurolite assemblages in pelitic protoliths, indicating HT–LP (Buchan-type) metamorphism. The PMC was exhumed as a core complex at moderate rates (c. 0.7–1.0mm/yr) accompanied by rapid cooling in the Plio-Pleistocene. Some of the UHP rocks were transported to the surface at significantly higher rates (⩾16mm/yr).The results of our study do not support recent plate tectonic reconstructions that propose a NW Australia margin origin for the West Sulawesi block (e.g. Hall et al., 2009).

Structure and composition of the ocean–continent transition at an obliquely divergent transform margin, Gulf of Guinea, West Africa

ABSTRACT Deep-imaging reflection seismic profiles offshore the Gulf of Guinea, West Africa constrain the structure and composition of a major fracture zone accommodating obliquely divergent intra-continental rifting and eventual break-up along the Gulf of Guinea margin. Interpretation of the seismic data reveals a c . 70 km wide fracture zone comprising fault-bounded blocks of hybrid ‘proto-oceanic’ crust. Gravity modelling of the seismic profiles allows us to propose a testable interpretation in which the fracture zone is composed of a central block of oceanic crust between blocks of partly serpentinized mantle. A synoptic model for the middle Cretaceous break-up of this margin suggests it was accommodated by a left-lateral transtensile shear zone in which fault-bounded blocks underwent continuous counter-clockwise rotation. Serpentinization of the upper mantle was promoted by the embrittlement and fracturing that resulted from progressive stretching and thinning. Domino-style back-rotation of early-formed faults within the shear zone meant that they attained progressively gentler dips before eventually locking up. Consequently, overprinting of faulting led to later faults dissecting – and translating in their hanging walls – a mixed assemblage of stretched continental crustal material and partly serpentinized mantle peridotite. With progressive strain, oceanic crust was produced at releasing bends within the shear zone and a leaky transform evolved.

Western Pacific modulation of large phytoplankton blooms in the central and eastern equatorial Pacific

Satellite observations show that large‐scale phytoplankton blooms (increases in chlorophyll) occurred in the equatorial Pacific in 1998, 2003, and 2005, following termination of the three most recent El Niño events. The occurrence of blooms following successive El Niño events cannot be explained by local enhancement of vertical nutrient flux, as evidenced by observations of equatorial winds, thermocline depth, and the depth and strength of the Equatorial Undercurrent (EUC, which supplies the limiting nutrient iron to the euphotic zone). However, near the peak of each El Niño event (late in 1997, 2002, and 2004), while the thermocline of the western equatorial Pacific was anomalously shallow, the flow of the New Guinea Coastal Undercurrent (NGCUC, which is the primary source of iron‐enriched waters to the EUC) intensified, and its core shoaled from >200 m to ∼100 m depth. Analysis of NGCUC variability using a high‐resolution, terrain‐following three‐dimensional ocean circulation model simulation indicates that as the NGCUC shoals and intensifies, it develops meanders and eddies that augment coupling of the New Guinea shelf and upper slope to the EUC. We hypothesize that these changes in NGCUC circulation during El Niño enhance iron transport from the New Guinea margin into the EUC and thereby trigger large‐scale blooms when iron‐enriched waters subsequently reach the euphotic zone along the equator. The threefold to fourfold chlorophyll increases over large regions, up to ∼5 × 105 km2, must have profound impacts on the equatorial ecosystem and biogeochemical cycles.

Evidence for multiple paleomagnetic intensity lows between 30 and 50 ka BP from a western Equatorial Pacific sedimentary sequence

A paleomagnetic study was carried out on the radiocarbon dated MD97-2134 core located in the western Equatorial Pacific (Southern Papua New Guinea margin). Rock magnetic investigations revealed changes of the magnetic mineralogy along the hemi-pelagic sedimentary sequence but the reconstruction of past direction and relative paleointensity variations of the geomagnetic field remained feasible. Four successive paleointensity drops are recorded between 30 and 50 ka BP. The largest one is associated with an abrupt swing of declination and inclination interpreted as a smoothed signature of the Laschamp excursion (∼41 ka BP). The succession of four events of weak intensity between 30 and 50 ka BP introduces a complex behaviour of the geomagnetic field in the time interval spanning over the Laschamp and the Mono Lake excursions.

Copper‐gold mineralisation in New Guinea: Tectonics, lineaments, thermochronology and structure

In New Guinea, the upper crust is rich in Late Miocene and Pliocene copper‐gold deposits, yet the host intrusives are mainly in the New Guinea Fold Belt and are of mantle origin and not directly subduction‐related. Structural, thermochronological and geodynamic analyses of the Grasberg, Porgera, Ok Tedi and Frieda River deposits show that the richest deposits occur along the eastern edge of the intersections between long‐lived crustal transfers perpendicular to strike and strike‐parallel crustal extensional faults that were strongly inverted during Late Miocene ‐ Pliocene orogenesis. The deposits are all associated with north‐northeast‐trending transfers, parallel to the aeromagnetic grain in basement, across which the continent‐ocean suture shows >50 km horizontal separation, as identified by the southern limit of the central New Guinea ophiolites. In the fold belt, the transfers coincide with the termination of regional anticlines or uplifts that are 150–200 km long and 30–60 km wide. Balanced sections reveal that the southern limit of these regional anticlines is commonly fault‐bound and coincides with major facies and thickness changes, indicating long‐lived, crustal extensional faults that were inverted. Fission track and 40Ar/39Ar cooling ages show that mineralisation occurred during inversion of these faults and, hence, correlates with propagation of orogenesis from northeast to southwest. It is proposed that the pre‐compression New Guinea margin comprised step‐like promontories and embayments delineated by long‐lived crustal fracture zones, as on Australia's North West Shelf. During Late Miocene ‐ Pliocene compression the crust was thickened, accompanying melting of the underlying mantle, and the crustal fracture zones were reactivated as transfers. Where the transfers intersected crustal extensional faults that were being inverted, local zones of dilation occurred, allowing emplacement of mantle magmas and associated mineralisation. When the deformation propagated southwards, so did the crustal thickening and the reactivation of major faults, allowing emplacement of younger magmas and mineralisation.

ABSTRACT: Play Types Of The Deepwater Northern Gulf of Guinea Margin

ABSTRACT: Play Types Of The Deepwater Northern Gulf of Guinea Margin

Reconstructing Cenozoic SE Asia

Abstract Reconstructions of SE Asia at 5 Ma intervals for the past 50 Ma are presented. They are constrained by new data from the Philippine Sea plate, which forms the eastern boundary of the region, by recent interpretations of the South China Sea and Eurasian continental margin, forming the western boundary, and by the known motions of the Indian-Australian plate to the south. An attempt is made to satisfy geological and palaeomagnetic data from the region. The implications of these reconstructions for the Tertiary evolution of SE Asia are discussed in the light of other new data from the region. There are two regionally important periods of change during the past 50 Ma. Both appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc caused major effects which propagated westwards through the region. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan. This appears to be a key to the recent tectonics of the region. Principal features of the model include the following interpretations. Middle Tertiary counter-clockwise rotation of Borneo closed a large proto-South China Sea and led to the development and destruction of marginal basins north of the Celebes Sea. The rotation implies that much of the north Borneo margin was not a subduction, but a strike-slip boundary for most of this period. It also suggests that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin which opened between late Eocene and mid Oligocene, and narrowed westwards like the present South China Sea. Luzon is suggested to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin, whereas most of the other Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the Early Miocene. Arc-continent collision in the early Miocene caused plate boundaries to change and initiated the clockwise rotation of the Philippine Sea plate. Since then the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, with significant strike-slip motion of fragments at the plate margin. Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Molucca Sea was a very wide area which formed part of the Philippine Sea plate before c. 15 Ma and originated as trapped Indian ocean lithosphere. It has been eliminated by subduction on its east and west sides. The present-day double subduction system never extended north of the present Molucca Sea into the Philippines. The Sulawesi ophiolite has an Indian ocean origin and was emplaced on the west Sulawesi continental margin at the end of the Oligocene. The major change in plate boundaries at the beginning of the Miocene following arc-continent collision of the Australian margin with the Philippine Sea plate arc caused initiation of the Sorong Fault system and led to westward movement of continental fragments which were accreted to Sulawesi during the late Neogene. The Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird’s Head before c. 15 Ma. They moved to their present positions after slicing of fragments from this microcontinent at different times and each was attached to the Philippine Sea plate for a few million years before collision. Most of the Banda Sea is interpreted to have an extensional origin and to have opened during the late Neogene. The reconstructions imply that there has been little convergence at the north Australian margin in Irian Jaya since the early Miocene and most convergence has occurred during the last c. 5 Ma. Movement of Philippine Sea arc fragments within the northern New Guinea margin along strikeslip zones probably accounts for the terrane character of this orogenic belt.

The Guinea continental margin: an example of a structurally complex transform margin

The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E-W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). Local tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.

Current-controlled sedimentation in the Equatorial Atlantic: examples from the southern margin of the Guinea Plateau and the Romanche Fracture Zone

3.5 kHz and seismic reflection data were used in a study area of bottom-current control of sedimentation in two areas of the Equatorial Atlantic: a 100 km long segment of the Romanche Fracture Zone (RFZ) and a 30 km wide sector of the southern margin of the Guinea Plateau. The RFZ is the most important conduit for Antarctic Bottom Water (AABW) into the eastern Atlantic. At mid depths (ca. 1500–4500 m), North Atlantic Deep Water (NADW) spreads southwards and eastwards, whereas at the surface, directly overlying the RFZ, is the Equatorial Divergence, a zone of high biological production. On the Guinea margin, a mixture of AABW/NADW moving north washes the foot of the continental rise below 4250 m and the overlying NADW hugs the slope contours. Another divergence zone lies just to the north of this area. On the southern margin of the Guinea Plateau, sediment accumulation is controlled by contour currents within the NADW. Deposits are concentrated into a series of slope-parallel sediment drift, the surfaces of which are moulded into standing sediment waves in the slack region between areas of eroded or condensed sequences marking the loci of higher velocity current cores. The outer edge and upper slope of the Plateau, under the influence of high velocities in the Equatorial Surface Water, are swept bare of unconsolidated sediment. The lower-rise sediments, washed by the weak, mixed AABW/NADW water mass are generally flat-surfaced. In the RFZ, restricted sediment deposition and accumulation, as well as the reworking of sediment surfaces, document the influence of bottom currents. Flat-surfaced sediments deposited on the floor of the axial valley (below the ca. 4500 m CCD), are ponded into basins at the foot of the valley walls by a weak AABW. On the northern flank of the North Ridge (Pillsbury Seamount) and on the South Ridge, the overlying NADW has moulded the thin sediment cover into sediment waves. A thicker accumulation of generally flat-surfaced, ponded sediments in the suspended valley were apparently deposited under a weaker current regime in the lee of the Pillsbury Seamount. Stronger bottom currents, as well as steepness of slope, appear to prevent sediment deposition on both the upper slope and crest of the North Ridge.

The geomorphology of the Southwest Guinea Margin: tectonic, volcanic, mass movement and bottom current influences

Slope morphology controls were examined in two topographically different areas on the southern transform margin of the Guinea Plateau (Sectors G1 and G2). Tectonic control is manifested by steep, faulted slopes in both study areas, and by basement warping in Sector G1 and magmatic emplacements in Sector G2. Sediment deposition and the formation of bedforms such as sediment waves on the margin are influenced by topographically intensified bottom currents within the intermediate-deep water mass, the North Atlantic Deep Water (NADW), as well as in the shallower Equatorial Surface Water (ESW)/ Antarctic Intermediate Water (AAIW) at the uppermost slope/plateau-edge level. Thus, unconsolidated sediments are eroded from steep slopes (the upper and lower slopes, canyon walls and the slopes of the volcanic cones) and concentrated into slope-parallel depocentres in Sector G1, and ponded in moats between the volcanic cones in Sector G2. Surficial sediment waves, which are apparently oriented at an angle to the slope, range from 0.5 to 2 km in wavelength and up to 100 m in height. Mass movement of the sediment cover is significant on the lower slopes of both sectors and is the result of slope steepness and instability caused by the continuing tectonic activity.

Evolution of the southern margin of Tethys (North Australian region) from early Permian to late Cretaceous

Abstract A mid-Permian rifting episode appears to have removed continental blocks (now forming parts of Asia) from Australian Gondwana, but the present continental margin of Northern Australia-central New Guinea was formed by a later episode of rifting during the Jurassic. With the exception of the north-west Australian offshore region west of the Scott Plateau, this rifted continental margin has been greatly modified during the Cainozoic by collisions with arc-trench systems. Collision resulted in fold and thrust mountains, locally with ophiolite emplacements, that now characterize most of northern New Guinea and the Banda Arc. The identification of the Australia-New Guinea rifted continental margin in what is now the fold and thrust mountain belt of New Guinea, islands of the Outer Banda Arc, East Sulawesi, and Buton, rests largely on stratigraphic correlation and the recognition of the pre-rift sequence, breakup unconformity, and the post-breakup marine transgression and prolonged subsidence in these now highly deformed rocks. One implication of the Jurassic continental rifting is that the pre-rift rocks of the region accumulated in an intracratonic basin of eastern Gondwana. Another important implication is that the continental blocks rifted from the Northern Australia-central New Guinea margin must have drifted northwards towards Asia as the Mesozoic Tethys ocean spread across the tropics and equatorial region. These continental fragments of Gondwana collided with parts of south-east Asia that had been derived from Gondwana in an earlier phase of rifting.

EVOLUÇÃO ESTRUTURAL DO PLATÔ MARGINAL DA GUINÉ E SUA RELAÇÃO COM A FORMAÇÃO DO OCEANO ATLÂNTICO CENTRO-EQUATORIAL

Offshore West Africa the Guinean Marginal Plateau facing the Caribbean area lies between the Central Atlantic and the South Atlantic. It constitutes a very important piece in order to fit continents around the Atlantic. In 1983 an oceanographic cruise conducted off Guinea brought news geological and geophysical data; they have been used in this study to propose a detailed stratigraphy and to construct a structural sketch of the region. The evolution of the area can be summarized in two phases: I. A first one (in Jurassic times) corresponds to the creation of the western Guinea margin, resulting from the separation of North America from Africa. II. During a second phase (in Cretaceous times) part of the former margin was submitted to a tectonic reactivation due to shear stresses along Guinea Fracture Zone, when the South Atlantic started to open. The results of this last phase created, south of the Guinea Plateau, a complex structural system including emplacement of volcanic piles.

Seismotectonics of New Guinea: A model for arc reversal following arc‐continent collision

The structure and evolution of the northern New Guinea collision zone is deduced from International Seismological Center (ISC) seismicity (1964–1985), new and previously published focal mechanisms and a reexamination of pertinent geological data. A tectonic model for the New Guinea margin is derived which illustrates the sequential stages in the collision and suturing of the Bewani‐Toricelli‐Adelbert‐Finisterre‐Huon‐New Britain arc to central New Guinea followed by subduction polarity reversal in the west. East of 149°E, the Solomon plate is being subducted both to the north and south; bringing the New Britain and Trobriand forearcs toward collision. West of 149°E the forearcs have collided, and together they override a fold in the doubly subducted Solomon plate lithosphere, which has an axis that is parallel to the strike of the Ramu‐Markham suture and that plunges westward at an angle of 5° beneath the coast ranges of northern New Guinea. Active volcanism off the north coast of New Guinea is related to subduction of the Solomon plate beneath the Bismarck plate. Active volcanism of the Papuan peninsula and Quaternary volcanism of the New Guinea highlands are related to slow subduction of the Solomon plate beneath the Indo‐Australian plate along the Trobriand Trough and the trough’s former extension to the west, respectively. From 144°–148°E, seismicity and focal mechanisms reveal that convergence between the sutured Bismarck and Indo‐Australian plates is accommodated by thrusting within the Finisterre and Adelbert ranges and compression of the New Guinea orogenic belt, together with basement‐involved foreland folding and thrusting to the south. The Finisterre block overthrusts the New Guinea orogenic belt, whereas the Adelbert block is sutured to New Guinea and overthrusts the oceanic lithosphere of the Bismarck Sea. Along the New Guinea Trench, west of 144°E, seismicity defines a southward dipping Wadati‐Benioif zone, and focal mechanisms indicate oblique subduction. Only this oldest, westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity.

African transform continental margins: Examples from Guinea, the Ivory Coast and Mozambique

Marine Studies of selected sheared-type margins have been undertaken along both West and East African coastal regions in order to determine the geological structure and the evolution of this specific type of passive margin. Geological and geophysical data have been obtained in the coastal waters off Guinea, Ivory Coast, and Ghana (Western Equatorial Africa) and off Mozambique (Eastern Africa). The continental margin off the Guinea coastline can be divided into three distinct sectors. To the Northwest, the observed seismic stratigraphy and structures are typical of a rifted Central Atlantic margin of Jurassic age. To the south, several features such as steep scarps, basement ridges, volcanic piles, magnetic and gravity lineaments, etc., are interpreted in relation to a transcurrent motion. A southwestern marginal area contains lower tectonized sequences of Early Cretaceous age covered by post-Albian detrital sediments. The overall structure is interpreted as a Jurassic rifted margin reactivated by shearing during the Lower Cretaceous. Off shore from the Ivory Coast and Ghana, two main marginal areas are also distinguished. To the east, south of Ghana, the upper margin is fractured by dissymetric and sedimented grabens, with the continental slope representing the unsedimented truncated African craton. South of the Ivory Coast, thick sedimentary basins, containing a Lower Cretaceous tectonized sequence developed between the continent and the Ivory Coast–Ghana Ridge, correspond to a composite feature consisting of deformed sedimentary wedges and basement structures. Off the Mozambique coast, the strike-slip motion of Madagascar with respect to Africa, during the Upper Jurassic, has generated a series of en echelon structures. The Davie Ridge includes a series of sedimentary shear folds and basement structures. Later, in Cretaceous times. throughout the Cenozoic, and since Miocene times, the continental margin has been submitted to different tectonic stresses generating diversely-trending extensional features. When shearing commences between two continental margins, it seems to generate either tensional features (e.g. Ivory Coast) or transpressive features (e.g. Mozambique) depending on the area. Later on, basement-involved structures and sedimentary wedges develop at the boundary between a thinned continental crust and a thick continental crust before connecting laterally with a typical oceanic fracture zone. The southern Guinea margin represents an unusual example where a rifted margin has been reactivated by transcurrent motion.