Abstract Despite some setbacks in the exploration of this initially highly prospective frontier province, oil and gas discoveries off mid-Norway, and, recently, on the Faroes and western United Kingdom shelf provide continuing encouragement to exploration. Large parts of the area (including the northeast Greenland shelf, a portion of shelf similar in size to the productive area offshore mid-Norway) remain almost totally unexplored. Therefore, numerous basic questions remain unanswered on the region’s geological evolution and petroleum prospectivity. These include provenance and source to sink relationships for the principal reservoir targets (Cretaceous and Paleocene sandstones), and the distribution and relative contribution to the hydrocarbon budget of different source rocks. However, in this paper, we concentrate on the fundamental problems relating to the lithosphere-mantle structure of the margin. Many of these challenges stem from the status of the region as a classic volcanic passive margin.

Abstract Plane-strain, thermo-mechanical, finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional-plastic regime and the strength of the lower crust and mantle lithosphere, respectively, on the style of extension. Crust and mantle lithosphere strength are varied independently. A simple scaling of wet quartz and dry-olivine rheologies is used to examine crust and mantle lithosphere strength variations. Cases are compared where the crust is strong (η wet quartz x 100), weak (η wet quartz ), or very weak (η wet quartz /10), and the mantle lithosphere is either strong (η dry olivine ) or weak (η dry olivine /10). Strain softening takes the form of a reduction in the internal angle of friction with increasing strain. Predicted rift modes belong to three fundamental types: (1) narrow, asymmetric rifting in which the geometry of both the upper and lower lithosphere is approximately asymmetric; (2) narrow, asymmetric, upper lithosphere rifting concomitant with narrow, symmetric, lower lithosphere extension; and (3) wide, symmetric, crustal rifting concomitant with narrow, mantle lithosphere extension. The different styles depend on the relative control of the system by the frictional-plastic and ductile layers, which promote narrow, localized rifting in the plastic layers and wide modes of extension in the viscous layers, respectively. A weak, ductile crust-mantle coupling tends to suppress narrow rifting in the crustal layer. This is because it reduces the coupling between the frictional-plastic upper crust and localized rifting in the frictional-plastic upper mantle lithosphere. The simple strength variation may be taken to represent end-member thermal and/or compositional conditions in natural systems and the relevance for rifting of old, strong, and cold cratonic lithosphere as compared to young, “standard”, and moderately weak Phanerozoic lithosphere is discussed.

Abstract The eastern Canadian margins from Nova Scotia to Newfoundland and Labrador formed during several periods of non-volcanic continental rifting from ~180 Ma in the south, to ~60 Ma in the north. This paper documents the large-scale styles of structural variations across the deep-water regions of the rift-to-drift transition on these margins. We present a series of seismic transects combining deep MCS reflection and wide-angle refraction profiles producing depth sections having similar scales and resolution. The combined profiles exhibit smaller scale structures defined by the reflectivity and larger scale variations defined by the refraction velocity models. These transects show systematic patterns in which changes in crustal velocities can be clearly correlated with changes in reflectivity. All transects include complex variations across broad transitions where the complete sequence from rifted continental crust to oceanic crust occurs over distances of 200–300-km. Within the transition zone, the presence of highly stretched continental crust can exist up to 200-km seaward of the hinge zone. The boundary between continental and oceanic crust is sometimes, but not always, separated by flat basement ~80-km wide probably consisting of highly serpentinized upper mantle. An underlying region 150–200 km-wide of less serpentinized mantle exists where faults cut through the thin crust. Large variations between upper and lower crustal extension occur both between and along transects controlling the deposition of syn-rift sediment; while the deposition of post-rift sediment follows predictions based on the shape of the total crustal extension.

Abstract The Orphan Basin formed during the Mesozoic intra-continental extension, continental breakup, and North Atlantic Ocean opening. New seismic data collected in the Orphan Basin, and regional potential field data, show a wide non-volcanic rift area with two successive rift zones characterized by greatly stretched continental crust. From both tectono-structural and petroleum potential points of views, the Orphan rifted area can be subdivided into an older East Orphan Basin situated in deep water (1,500-3,000m) and a younger West Orphan Basin situated in shallower water (1000–1,500m). Seismic stratigraphic relationships indicate that the first episode of rifting could be as old as the Late Triassic-Early Jurassic in the East Orphan Basin. A petroleum system including Kimmeridgian, marine source rocks is postulated for this basin. A second rifting stage, from latest Late Jurassic to Early Cretaceous, created the eastern part of the West Orphan Basin and re-mobilized basement blocks and older sedimentary features in the east Orphan Basin. A third extensional stage during mid-Cretaceous, probably coupled with Labrador Sea extension and opening, mostly affected the westernmost parts of the West Orphan Basin. A later extensional stage is postulated to have occurred in the early Tertiary, related to the initiation of a new rift between Greenland and northern Canada. The West Orphan Basin petroleum system should be anchored by Cretaceous or early Tertiary source rocks. The two main rift basins, east and West Orphan, are separated by a major crustal fault zone, the White Sail fault dipping eastward and affecting the entire upper crust. The N 020° linear, tilted faults blocks identified in the West Orphan Basin are perpendicular to the flow-lines of the herein proposed Flemish Cap motion during the M25-M0 period, giving an independent evidence of the validity of the Flemish Cap/North America reconstruction at chron M25 derived from magnetic and gravity data. On the M0 reconstruction, the east and West Orphan basins are located in front of the Porcupine basin and Rockall trough respectively. The Flemish Cap behaved as a large, monolithic continental block and rotated clockwise 43°, apparently moving more than 200 km to the southeast, while the upper crust of the east Orphan Basin has a = 2.5 average stretching from Late Triassic to early Tertiary. Regional seismic data suggest that trans-tensional movements, although hard to identify on reflection data, played an important role and continued until the Paleocene. Beyond the Orphan Knoll-Flemish Cap “outer ridge” lineament lies a true divergent margin basin showing little deformation within the sedimentary sequence and which overlies a relatively wide continent-ocean transition zone.

Abstract Reconstructions of the relative positions of the North American and African continents before the opening of the Central Atlantic Ocean have either been based on morphological fits using coastlines/isobaths or seafloor magnetic/fracture data. Additional constraints for the best fit between these continents may come from pre-rift structural elements if they are sufficiently oblique to the line of rifting. None of the existing reconstructions utilize syn-rift structures, which are expected to be preserved on the conjugate passive margins. We report a direct correlation of syn-rift structures across the Central Atlantic based on new reflection seismic and potential field data. The prominent basement high of the Tafelney Plateau, offshore Morocco, is interpreted as a highrelief accommodation zone analogous to many well studied examples in the present-day East African rift system. It developed between two regional-scale, normal fault systems with opposing polarities. The actual Early to Middle Jurassic breakup occurred obliquely across the Tafelney accommodation zone leaving most of it on the Moroccan margin.

Abstract Interpretation of magnetic, gravity, seismic, and geological data shows that the curvilinear Late Paleozoic orogen affected the location of Central Atlantic syn-rift faults. While northeast-southwest striking thrust faults were perpendicular to extension, prominent curvatures, such as the Pennsylvania salient, introduced structural complexities. East-northeast/west-southwest striking, dextral, transpressional strike-slip faults of this salient became reactivated during Carnian-Toarcian rifting. They formed sinistral, transtensional strike-slip “rails” that prevented the Georges Bank–Tarfaya Central Atlantic segment from orthogonal rifting, causing formation of a pull-apart basin system. Central Atlantic segments to the south and north underwent almost orthogonal rifting. “Rails” lost their function after the continental breakup, except for minor younger reactivations. They were not kinematically linked to younger oceanic fracture zones. Atlantic segments initiated by normal rifting differ from the segment initiated by the Georges Bank–Tarfaya strike-slip fault zone. They contain Upper Triassic-Lower Jurassic evaporites having salt-detached gravity glides, while the connecting transfer segment does not. Their structural grain is relatively simple, divided mostly by northeast-southwest striking normal faults. Northwest-southeast striking oceanic fracture zones kinematically link with continental faults in a few places, controlling the sediment transport pathways across the uplifted continental margin. The connecting Georges Bank–Tarfaya Central Atlantic segment, initiated as a sinistral transfer-zone, has a complex structural grain, characterized by numerous small depocenters and culminations. Their boundaries are formed by east-northeast/west-southwest striking, sinistral, strike-slip, north-northeast/south-southwest, striking normal and west-northwest/east-southeast striking, dextral, strike-slip faults. Sediment transport pathways have complex trajectories, weaving through local depocenters.

Abstract Field, seismic, and drill-hole data provide a wealth of information about the tectonic processes associated with rifting, breakup, and the early stages of seafloor spreading for the passive margin of eastern North America. The onset of rifting, from Florida to the Canadian Grand Banks, was approximately synchronous, occurring by Late Triassic time. The cessation of rifting (and presumably the onset of drifting) was diachronous, occurring first in the southeastern United States (latest Triassic), then in the northeastern United States and southeastern Canada (Early Jurassic), and finally in the Grand Banks (Early Cretaceous). The Central Atlantic Magmatic Province developed simultaneously (earliest Jurassic, ~200 Ma) throughout eastern North America. This magmatic activity occurred after rifting in the southeastern United States, and during rifting in the northeastern United States and maritime Canada. The passive margin, from Florida to southern Nova Scotia, is volcanic, characterized by seaward-dipping reflectors ( SDRs ) near the continent-ocean boundary. The remainder of the passive margin lacks SDRs and is, thus, non-volcanic. In the continental crust, most rift-related structures parallel preexisting zones of weakness created by Paleozoic and older orogenies. Few transfer zones exist, and these also parallel the pre-existing fabric. In the oceanic crust, fracture zones parallel the direction of relative plate motion. Thus, the trends of the fracture zones in the oceanic crust differ from the trends of the rift-related structures in the continental crust. The deformational regime changed substantially after rifting throughout eastern North America: post-rift shortening (inversion) replaced syn-rift extension. Detached structures associated with salt movement also developed after rifting, especially on the Scotian shelf and Grand Banks.

Abstract We used a reprocessed 2D seismic line across the ultra-deep water area offshore Angola to map a thick pre-salt sequence divided by a prominent angular unconformity into two main packages. The seismic line shows a strong reflector at ca 9 seconds two-way time (twt), possibly the crust-mantle boundary (Moho). We interpret that the pre-salt strata records major crustal extension, followed by rapid sedimentation. Presalt decompacted sedimentation rates were in excess of 200–600 m/My, in contrast to a maximum post-salt deposition rate of 50 m/My. This suggests that the transition from pre-salt to post-salt deposition coincided with a dramatic change in climate and/or drainage pattern. This is also indicated by the change from mainly lacustrine clastics in the pre-salt sequence to mainly carbonates in the immediate post-salt sequence. The total post-rift subsidence in the study area indicates a crustal extension factor between 2.3 and 3.4, increasing from east to west. An alternative estimate of crustal extension, ranging from 3.6 to 4.9, is obtained by assuming that the 9 seconds twt reflector is the Moho. These differing estimates may be reconciled if it is assumed that the study area is uplifted by ca. 750 m relative to the McKenzie (1978) subsidence model predictions. It is noteworthy that an uplift of this magnitude corresponds to the current uplift of continental southern Africa.

Abstract A combination of seismic reflection and gravimetric imagery has been used to map four sectors of proto-oceanic crust along conjugate segments of the West African and Brazilian margins. These form corridors isolating oceanic crust, produced about the post-118 Ma pole of rotation, from continental crust. Seaward of the proto-oceanic crust/oceanic crust boundary, relatively uniform, thin oceanic crust (4.2–6.5 km thick) has been generated at the paleo-Mid-Atlantic Ridge. Structural variability is limited largely to fracture zones. Proto-oceanic crust in the northern sectors ( i.e. , Kribi, Mbini, and Ogooue) is up to 10 km thick, block-faulted, compartmentalized, and seismically layered. These sectors of proto-oceanic crust likely were generated by slow spreading, as the relative plate motions evolved from left-lateral dislocation along the Sergipe-Alagoas transform to full-fledged spreading. Thus, proto-oceanic crust in the north is the product of a leaky transform fault and records the evolution from disorganized to organized spreading under a changing stress regime. Proto-oceanic crust in the southern sector, the Gabon sector, may consist of slivers of lower crustal or upper mantle rocks emplaced along detachments and unroofed as the Gabon upper plate detached from the Brazil lower plate. The ocean-continent boundary marks the transition from deeply-subsided proto-oceanic crust to relatively elevated continental crust. Merging the mapped oceanic crust/proto-oceanic crust boundaries from the conjugate margins results in a rigid closure model at about 118 Ma for this part of the Atlantic. Merging the mapped ocean-continent boundaries ( i.e. , removing proto-oceanic crust) produces a Neocomian rigid closure fitting the continental sectors of the African and South American plates. Prior to this stage, and beginning in earliest Neocomian, continental deformation was dominated by sinistral shear along the Sergipe-Alagoas transform, parallel to the West African margin between 3° north–1° south (or parallel to the Brazilian margin between 8°–13° south). Shear along the transform produced a complex swath of transcurrent fault branches, relays, pull-apart basins, and transpressional ridges. Conjugate fits of paired seismic lines from the two margins indicate the South American plate moved more than 100 km southwest relative to the African plate, prior to a 40° change in plate motion direction leading to genesis of proto-oceanic crust. Dislocation along the transform obliquely extended both the Gabon rift zone to the south of the transform, and the Jatobá-Tucano-Recôncavo rift zone marking the western boundary of the Sergipe microplate. Conjugate seismic pairings across the adjoined Brazil and Gabon rift zone margins show that a simple shear mode of extension developed in this area as dislocation along the transform progressed. The low-angle main décollement dips toward the Gabonese side and deepens beneath it, dividing a narrow band of abruptly extended São Francisco cratonic crust (lower plate) from a broad zone of extended Congo fold belt rocks (upper plate). With the 40° change in plate motion direction and the onset of seafloor spreading, extension of the Gabon rift zone ceased and the Jatobá-Tucano-Recôncavo rift zone became an aulacogen. The plate closure scenarios presented here have an important bearing on matching rock units and basins from the two margins, particularly in an exploration context. The scenarios also explain why previous attempts to pair apparent conjugate seismic lines from offshore Brazil and West Africa have been unable to consistently match both continental and oceanic crustal sectors.

Abstract Extensional tectonics in the Gulf of California region occurred during middle- to early-late Miocene time. One of the best examples is found at Bahía Concepciόn, which is the largest fault-bound bay on the peninsular gulf coast of Baja California. The extensional basin confined to Bahía Concepciόn developed within an accommodation zone related to the late Miocene east-west extension in the broader Gulf of California region. This contribution is focused on the large, central part of the gulf region along the peninsular axis, sometimes referred to as the Baja California Central Domain ( BCCD ). The mainly volcanic Comondú Group is extensively exposed throughout the BCCD , but on a local basis it occurs as huge andesite blocks associated with stratified units that characteristically dip in opposite directions. This is a direct result of a main east-west extensional episode that generated normal faults on the surface and listric detachment faults at depth. Half grabens are the most common result of such extensional episodes in the BCCD . This event has produced subsidence, creating depocenters for mostly nearshore marine basins. The oldest marine sedimentary units present in the BCCD are the late Miocene–early Pliocene Tirabuzón Formation. Less typically, on Peninsula Concepciόn and at Punta San Antonio, a late Miocene extensional episode has resulted in up-thrown granodiorite basement along bounding faults. Extension on the Bahía Concepciόn zone was responsible for development of a half-graben structure first fully flooded in late Pliocene time. The tectono-sedimentary evolution of the adjacent Santa Rosalía and Bahía Concepción areas is recorded by three stratigraphic stages: (1) pre-rift strata represented by the Comondú Group; (2) a syn-rift stage containing syntectonic siliciclastic deposits; and (3) post-rift strata, represented by the late Miocene to Pliocene flat-lying to low-angle marine sedimentary units. The three stratigraphic stages are interpreted from outcrops in this region.

Abstract The Red Sea—Gulf of Aden rift System provides a superb example of the formation of passive continental margins. Three phases are well represented: (1) continental rifting (Gulf of Suez); (2) rift-to-drift transition (northern Red Sea); and (3) sea-floor spreading (Gulf of Aden and southern Red Sea). Recently published radiometric and biostratigraphic ages, outcrop studies, and reflection seismic profiles more tightly constrain the evolution of this rift system. The principal driving force for separation of Arabia from Africa was slab-pull beneath the approaching Urumieh-Dokhtar volcanic arc on the north side of Neotethys. However, the rifting trigger was impingement of the Afar plume beneath northeast Africa at ~31 Ma. Rifting followed quickly thereafter, initiating in the Gulf of Aden, perhaps in the area between Socotra Island and southern Oman. Extension occurred in the central Gulf of Aden by ~29 Ma. Shortly thereafter, at ~27 Ma, rifting jumped to Eritrea, east of the Danakil region. Rifting then spread from Eritrea to Egypt at ~24 Ma, accompanied by a major dike-emplacement event that covered more than 2,000 km in possibly less than 1 Ma. At ~14 Ma, the Levant transform boundary formed, largely isolating the Gulf of Suez from later extension. Constriction of the Suez-Mediterranean and Red Sea-Aden marine connections resulted in widespread evaporite deposition at this time. Sea-floor spreading began in the eastern Gulf of Aden at ~19 Ma, the western Gulf of Aden at ~10 Ma, and in the south-central Red Sea at ~5 Ma. Propagation of the oceanic ridge has taken much longer than the propagation of its continental rift predecessor. Therefore, the rift-to-drift transition is diachronous and is not marked by a specific “breakup” unconformity. The Red Sea sub-basins are each structurally asymmetric during the syn-rift phase and this is seen in the geometries obtained when its present paired conjugate margins are palinspastically restored. During the Late Miocene and Pliocene, regional-scale, intrasalt detachment faulting, salt flowage, and mass-movement of the post-Miocene salt section toward the basin axis masked the deeper fault block geometry of most of the Red Sea basin. This young halokinesis has enormous consequences for hydrocarbon exploration.

Abstract The tectonic province of the western Indian Ocean is defined by the East Africa Rift Zone to the west and by the Ninety-East Ridge to the east. The area is bounded to the north by the Arabian Peninsula and to the south by the southern Indian Ocean spreading center. The topography-bathymetry is dominated by the triple-junction Indian Ocean spreading center, the mantle plume extrusions forming the Laccadives-Maldives-Chagos and Mascarene Plateau-Mauritius-Reunion chains of volcanic archipelagos and islands, and the mantle plume extrusion of the Ninety-East Ridge. Initial breakup of ancestral Gondwana, sea floor spreading, and appearance of oceanic crust was preceded by continental sag and development of the Late Carboniferous–Early Jurassic Karoo basins. The first oceanic crust appeared in the Middle Jurassic as the Africa-Arabia plate moved northward relative to the India-Seychelles-Madagascar-Australia-Antarctica plate. This north-south separation continued through the Neocomian. A major jump in the spreading center occurred in earliest Barremian with Antarctica-Australia separating from India-Seychelles-Madagascar. Madagascar separated from India-Seychelles via a transform fault along the east coast of Madagascar. The trans-tensional transform evolved into a spreading center during the middle Cretaceous Barremian-Aptian-Albian as oceanic crust appeared. The mantle plume Rajmahal Traps first appeared in eastern India during the Aptian-Albian, and as the Indian plate continued to migrate northward, evolved into the Ninety-East Ridge. The mantle plume-derived volcanic rocks of the Deccan Traps first appeared in western India near the Cretaceous-Tertiary boundary. The Seychelles began to separate from India in the early Paleocene. By the close of the Paleocene, a broad expanse of oceanic crust separated the Seychelles and western India. The mantle plume formed an extensive oceanic ridge that became the Laccadives-Maldives-Mascarene Plateau. Beginning in the Eocene and continuing through the Oligocene, the ongoing spreading center split the oceanic ridge. North of the spreading center, mantle activity extended the Laccadives-Maldives to include the Oligocene-age Chagos Archipelago, while south of the spreading center, the Mascarene Plateau basalts continued as the Saya de Malha and Nazareth Banks. Mantle plume extrusion continued to the south as the plate moved northward, creating Mauritius Island during the Miocene and Reunion Island during Pliocene-Recent. To the northwest, Red Sea separation of Egypt from Arabia began during the Oligocene. Extension of the Indian Ocean spreading center into the Gulf of Aden between Somalia and Yemen-Oman did not occur until the Miocene. To the north in Ethiopia-Eritrea, the East Africa Rift Zone originated during the early Miocene and has extended southward through Uganda-Kenya-Tanzania-Mozambique into the southern Indian Ocean.

Abstract The South American divergent continental margin extends from eastern Brazil towards the continental margin off Argentina. This segment is limited, both to the north and south, by transcurrent movements associated with oceanic fracture zones and by the subduction zone north of Antarctica. Within the extensional margin, the transitional phase salt basins are also controlled by transform faults in the eastern Brazilian and West African margins. The evaporite basin is associated with siliciclastic and carbonate sediments deposited above a regional unconformity (breakup unconformity) that marks the beginning of the continental drift phase. This was followed by Aptian evaporite sedimentation between the Sergipe-Alagoas and Santos basins on the Brazilian side, and from the Rio Muni to Benguela basin in West Africa. The evaporitic conditions seem to extend up to early Albian in some regions, as evidenced by extremely thick layers of stratified evaporites, indicating several depositional cycles. A highly mobile evaporite layer resulted in the development of a characteristic tectonic style marked by salt diapirs, and extensional and compressional structures affecting the post-salt sedimentary successions. The regional deep-penetration seismic profiles acquired in the South Atlantic provide a unique dataset allowing identification of salt tectonic domains from the platform towards the oceanic crust boundary. These prolific hydrocarbon-bearing salt basins constitute a framework for the interpretation of the less-explored salt basins of the North Atlantic continental margins, particularly along the Iberian and North American continental margins. Examples of analogue autochthonous and allochthonous salt structures, and their geodynamic evolution, have important implications for petroleum exploration in the deep-water frontier regions.

Abstract Salt tectonics has attracted considerable attention in the last few decades because of its importance in controlling the location of hydrocarbon reserves. As a consequence of its low density and viscous nature, salt is can be deformed by buoyant flow over geological time, deforming and penetrating overlying sedimentary sequences. Salt can act directly as a hydrocarbon seal, and salt-related deformation may totally change the stratigraphic and structural interpretation through the geohistory of the basin. Timing of the salt tectonic evolution may be crucial for hydrocarbon exploration, not only because the structural traps indicated on presentday seismic data may not have been in-place while hydrocarbons were migrating into the area, but also for the direct effect that salt geometries have on temperature distribution and therefore on hydrocarbon generation and expulsion. The economic consequences of increased understanding of salt movements are thus significant. A 2D finite element numerical code has been developed to allow study of salt motion impact on the structural evolution of a passive margin. In the numerical code, salt and the other sediments are considered as a continuous media possessing variable properties in space and time. Salt is modeled with viscous Newtonian rheology and the overburden with a pseudo-plastic non-Newtonian rheology. The spatial mesh used in the numerical model is triangular, unstructured, and nonuniform. The interfaces between layers are tracked (during evolution) using a lagrangian approach, and, in order to improve the resolution of the tracked interfaces, refinement and de-refinement techniques have been implemented. A real case study was developed focusing attention on the impact of: sediment density, rheological parameters, and progradation rate of the pre-/sin-kinematic sequences on the evolution of passive margins. This offered a better understanding of salt behavior in a complex structural context.

Abstract This paper compares and contrasts salt tectonics on two different types of continental margins in Brazil. Narrow margins, such as the Jequitinhonha and Camamu basins, are 50–100 km wide and have a steep, (up to 5°) seaward-dipping base of salt seismic horizon. These margins are sediment starved due to their steepness. Consequently, sediment has bypassed the salt basin and been deposited on the abyssal plain. Pronounced contractional folding of the salt overburden is present on these margins. This commenced at the oceanward pinch-out of the salt and propagated back up the continental slope into areas, which are expected to be in extension due to gravity sliding. This is the opposite sense of fold and thrust propagation compared to ‘normal’ mountain fold and thrust belts. The bathymetric highs above pre-existing diapirs and fold anticline crests were rapidly eroded on narrow margins, which allowed the folds to grow more easily to large amplitudes (1.5 km) at the top salt seismic horizon. Folds continued to unroof until the salt reached the seabed and produced a duck-head shaped diapir due to downslope flow of a salt glacier. This was followed by collapse of the salt structure producing an unconformity-bounded graben. Wide margins, such as the Campos and Santos basins, are >100 km and <650 km wide and have a subhorizontal to landward-dipping base of salt seismic horizon. Salt basins on wide margins are effective sediment traps; e.g. , sediment loading in the Santos basin has produced a 2° landward-dipping base salt seismic horizon across the outer portion of the basin. Landward dip at the base salt seismic horizon has promoted development of counter-regional faults and enhanced later folding, which appear to develop approximately simultaneously across the whole basin. The folds are limited in amplitude to a maximum of 1 km, as little or no erosion has taken place over the crests and the thick sediment lid above the salt structures produces more competent rocks, causing a high vertical confining pressure, which inhibits fold growth. This compression is probably due to both downslope gravity sliding, and regional tectonic compression is due to ridge push or Andean collisional events.

Abstract The Algarve basin developed as an extensional basin during the Mesozoic, associated with northwest- southeast transtension in ending at the Azores-Gibraltar plate boundary. Several main episodes could be identified in the evolution of this plate boundary: Triassic rifting and mid-Atlantic extension until the Early Cretaceous; fracturing and strike-slip movement along the Azores-Gibraltar Fracture Zone, from Early Jurassic to lower Eocene; continental convergence between Europe and Africa, beginning in the Late Cretaceous, reaching its culmination during the Neogene. The study of the Algarve basin began with the interpretation of 2D MCS seismic profiles generating structural and isopach maps. These maps show the geometry of the basin to be asymmetric, associated with a depocenter striking northeast-southwest and located very close to Guadalquivir ridge. Two salt units are interpreted in the Algarve basin: an older one of Triassic age and another of as Late Jurassic age. The latter salt formation is described initially as an evaporitic layer deposited during one of the Jurassic uplift episodes. However, seismic data character suggests that it might not be autochthonous salt and could be allochthonous salt sourced from the Triassic. Salt is also distributed differently throughout the basin: the Triassic salt extends over nearly the entire basin while the Jurassic evaporites are limited to the central and eastern area. Jurassic salt structures are associated either with extensional tectonics, such as salt-rollers with associated listric faults, or compressional geometries resulting from the reactivation of listric faults as reverse faults during the compressive episodes of the Tertiary. Morphologically, the Triassic salt is characterized mainly by a number of salt piercement features occurring as circular salt diapirs or elongate salt ridges. Although the time of initiation of salt movement cannot be determined, final movement occurred during Miocene time.

Abstract Deep-water geophysical data acquisition and drilling technologies have had an impact on every aspect of the offshore petroleum industry in recent years. Massive, non-exclusive, speculative 2D and 3D surveys have been conducted offshore Brazil and West Africa and in the Gulf of Mexico at a record pace in the last few years and are now being followed by diverse deep-water, drilling programs. The last frontier, ultradeep water, is being tackled, and understanding of salt architecture is fundamental for developing exploration plays. This study makes a comparison of three, salt-constrained, deep-water areas: the Sigsbee Escarpment in the Gulf of Mexico, the Angolan Escarpment along the West Africa Congo Basin, and the São Paulo Plateau Escarpment along the eastern Brazilian margin. Autochthonous salt tectonics shapes the architecture of the mirror image basins of the South Atlantic, such as the producing Campos basin off eastern Brazil and Congo basin off West Africa. However, at the continent-ocean boundaries, major fault scarps affect these basins with possible allochthonous salt movements around the transition area. Deep-water, oil and gas discoveries have been made in a single oil play, the slope-constrained play. The major questions in both the Campos and Congo basins are oil generation in the thick, syn-rift sections beneath salt, and the conduits for hydrocarbon migration through windows in the overlying salt. In the ultra-deep water realm, additional variables are imposed by overburden thickness. Hence, a clear understanding of salt architecture around fault scarps is fundamental. Oil generation in the area of the Sigsbee Escarpment in the Gulf of Mexico, occurs in sediments that overlie oceanic crust. Hydrocarbon migration is related to allochthonous salt emplacement. This is a distinct phenomenon when compared with basins of the South Atlantic. Models of the oil plays near the Sigsbee Escarpment, compared with the escarpments of the continent-ocean boundaries in the South Atlantic, improve the understanding of salt behavior and provide exploration possibilities.