Abstract The Romanche Fracture Zone was originally a corridor of Aptian-age dextral transtensional rifting along the Equatorial Atlantic margins. Late Albian plate tectonic compression occurred due to a change in plate vectors, when the African and South American continents were still in contact across a 500 km-long section of the Romanche Fracture Zone. This dextral compression produced reactivation of the rift faults to produce asymmetric landward-vergent anticlines and thrusts that trend ENE to NE. Fold-axial planes dip seaward, parallel to the rift faults. Minor asymmetric anticlines were developed on the long seaward-dipping fold limbs and these have subvertical axial planes. The asymmetry of the minor folds is due to the southward stratal dip having been oblique to the horizontal maximum principal stress during the Albian inversion. The folds on the African margin were subsequently tightened by compression in Santonian and Oligo-Miocene times. Aptian-age ENE strike-slip faults were reactivated during the compression phases to produce broad positive flower structures up to 30 km wide that formed topographical ridges along the original strike-slip faults. The intervening and broader flat-bottomed synclines do not appear to be associated with rift faults. The folding and thrust faulting created seabed relief of 1–2 km at the end of the Albian; evidenced by the amount of subsequent erosion that removed the better-quality reservoirs in the upper Albian sequence from the major fold crests. Consequently, there has been a significant number of failed oil exploration wells drilled along the fold crests. The fold ridges would have diverted turbidite channels in the onlapping Cenomanian–Campanian sequence and these will be preferentially located on the landward side of the anticlinal crests. Late Cretaceous stratigraphic and structural traps located between the major anticlines have not yet been explored for hydrocarbons along the Romanche Fracture Zone margins.

Abstract The clastic fill of the rifted grabens in the central South Atlantic includes several fields containing over 1 billion barrels of oil in place. The largest fields are mainly hosted by continental fluvial and aeolian sandstones deposited near the base of the rift, before rift topography was fully developed. Footwall erosion progressively eroded back the main fault scarps, and normal faults propagated through the footwall to create terraces along the rift borders. This increased the potential drainage area on the footwall, so eventually, large alluvial fans developed in the hanging wall along the main rift border faults. The basement clast-supported conglomerates scatter seismic energy; and on seismic data, the top of the fan can appear to be the intact crystalline basement. Even when drilled, the top of the fan can still be mistaken for true basement due to the large size of the clasts. Some important oil fields are hidden below these marginal alluvial fans, within prerift and early rift fluvial and eolian sandstones and in lacustrine turbidite sandstone reservoirs. It is suggested that more fields may be found in West Africa using the model of the sub-fan terrace play. The large amount of footwall denudation of dense basement rocks can lead to unloading of the adjacent basin, as well as the footwall, when the crust has a finite flexural strength. The resulting uplift and erosion produce an ‘End of Rift’ unconformity, and it is suggested this process is likely to have caused the so called ‘Breakup’ unconformity on continental margins, rather than this being due to initiation of ocean spreading.

Abstract Extensional rifts and their overlying sag basins host prolific hydrocarbon provinces in many parts of the world. This chapter reviews the development of rifts and the controls of tectonics on sedimentation patterns and hydrocarbon prospectivity. Rift tectonics exert the most important control on sedimentation and trap formation, and subsidence rate controls the geometry and facies of the rift fill. Rifting also controls the heat flow and burial history that determine the source rock maturation. Many rifts commence with evenly distributed small extensional faults when they are commonly characterized by closed drainage and continental sedimentation with localized lacustrine facies. As continental rifts develop, source rocks commonly accumulate in deep lakes, especially when the rifting is rapid, and organic shales are commonly located in the bottom third of the rift. Fault displacements increase and faults grow laterally to produce linked normal fault arrays. Marine deposits commonly replace the early rift continental deposition (e.g., North Sea) as the rift propagates to reach the world ocean system. Extremely prolific source rocks may be produced during rapid rifting in the marine phase, especially if half-graben depocenters are starved and oceanographic circulation is poor (e.g., Kimmeridge Clay Formation of northwestern Europe). As rifting wanes the rift fills, and fluvial sedimentation predominates, or in a passive margin, the basins enter into an extensional sag and/or postrift thermal sag phases when shallow-marine to deep-marine sediments infill and bury the former half grabens as sedimentation catches up and exceeds basin subsidence. Recent studies in western Australia, Brazil, and west Africa indicate that thick (unfaulted) sag basins can develop very rapidly above rifted continental crust. The subsidence is too rapid to be produced by normal thermal conductive cooling and is believed to be caused by stretching of the lower/middle crust with no observable faulting (depth-dependent stretching; Driscoll and Karner, 1998). The stretching process can produce a broad passive margin (100-500 km [62-311 mi]) where the continental crust is only 5 km (3.1 mi) thick ( β = 7). The sag basin in southern Brazil reaches about 1 to 2 km (0.6–1.2 mi) in thickness and drapes over the earlier rifted blocks to produce some of the largest oil fields discovered in the last 30 yr (e.g., Lula with 20 to 30 billion bbl of oil in place). These fields are trapped in lacustrine algal-bacterial carbonates that pinch out onto the fault block crests. Many rift basins remain unexploredin remote or inaccessible areas, and new future hydrocarbon provinces are anticipated in the African, Southeast Asian, Arctic, and Atlantic margin rift basins.

Abstract The southern Brazilian salt basin, comprising the three sub-basins Santos, Campos and Espirito Santo, was deposited over a pre-existing rifted basin with c. 1–2 km of relief bordered by an outer basin high that separated the basin from the conjugate African margin. The evaporites are interpreted to have been deposited very rapidly (<1 Ma) during the waning of extension. Deposition of salt caused rapid loading of the basin, so that further basin subsidence occurred and mobile salt drained from structurally higher zones into the subsiding basins. Seismic evidence indicates that downslope salt drainage occurred before any sediment overburden accumulated. Withdrawal synclines within salt units developed adjacent to diapirs, which have intruded the evaporite sequence, and salt extrusions are observed which were buried by later salts. The early movement of the salt probably contributed to significant fault reactivation and redistribution of salt load, so that the final half-graben salt fill reached up to 4.5 km thick where only 1–2 km of salt was originally deposited.

Abstract This paper first reviews the salt basins and depositional ages in the South Atlantic salt province. This comprises a series of salt basins separated by basement highs, deep graben (that never dried up), later volcanic highs and subaerial ocean spreading ridges. Initial halite and anhydrite deposition occurred first in the Sergipe-Alagoas Basin of NE Brazil at c. 124.8 Ma, and was closely followed by deposition in the Kwanza Basin, Angola between 124.5 and 121 Ma. The later potassium-magnesium-rich salts were deposited in the Sergipe-Alagoas and Gabon-Congo basins before 114.5 Ma. The age of the main Santos-Campos salt is not known precisely, but the latest anhydrites deposited on the southern margin of the Santos Basin post-date volcanic rocks dated at 113.2 Ma. The paper then compares the salt tectonics of the wide Campos-Santos Basin segment with the narrow South Bahia basins segment. Sediment loading in the Santos Basin produced a landward-dipping base salt, which led to the development of counter-regional faults, and inhibited downslope sliding, and enhanced later contractional effects caused by either gravity spreading or regional tectonic compression. Folding occurred in simultaneous pulses across the Santos Basin, suggesting that regional tectonic compression occurred. The narrow salt basins of South Bahia have a steeply dipping base salt horizon (4°) and pronounced folding, which initiates at the oceanward pinch-out of the salt and propagates back up the slope. The topographic highs, above fold anticlines, are rapidly eroded on narrow margin slopes, which allows the folds to grow more easily to large amplitudes at the top salt horizon.

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 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 North Sea Central Graben salt diapirs grew by both passive downbuilding and active compressional reactivation during deposition of large Paleocene age turbidite fans derived from the uplifted Scottish platform. Once downbuilding diapirs were buried by more than approximately 200 m of overburden, including some Late Cretaceous chalk, very little bathymetric relief (<20 m) was present over the salt domes, and turbidites could flow across the crests of the salt diapirs. Diapirs having less, or no, overburden present during the Paleocene created more bathymetric relief (ranging between approximately 100 to 300 m), which diverted turbidite flows around the diapirs. Consequently, sandstones are absent on the crest of these salt domes, and high angle onlap reflectors are present on the diapir flanks. The turbidite sandstones close to the diapirs show large amounts of slumping and soft sediment deformation where the paleo-topographic relief was high during deposition ( e.g. , South Pierce, Merganser diapirs). This can have a slightly detrimental effect on oil recovery, but apparently not productivity, if the affected sequence contains shales. Some diapirs ( e.g. Jenny and Kyle, Fig. 1 ) are flanked by Paleocene debris flows containing lithified chalk fragments and occasionally reworked Zechstein evaporites indicating that the salt diapirs were emergent, or had high local sea bed relief. These flows can constitute high quality reservoirs due to the large amount of inter-clast porosity and later fracturing. Figure 1. Regional map of the Central North Sea, showing approximate limit of Paleocene sandstone (yellow), salt diapirs (pink) and oilfields (green on inset map). The inset scan with black background shows the Paleocene reflector amplitudes outlining the fans, with orange shading indicating the brightest amplitude reflectors. Compliled from Ahmadi et al. (2003) , Zanella and Coward (2003) , and PGS (2004). This study indicates the importance of analyzing the thickness and nature of the overburden present above salt crests, and the sediment onlap patterns, when turbidite deposition took place. These observations can be useful in predicting the presence, or absence, of turbidites over the crests of salt diapirs.

Abstract Numerical models are used to investigate the effects of bathymetric obstacles and sinks on turbidite flow and sediment deposition. Modeling results suggest that bathymetric obstructions such as salt domes produce a thick sedimentary apron on the landward side of the dome and a heart-shaped thin area over the dome crest. The maximum thickness of sediment is located where the flow impinges on the dome. Seaward-dipping fault scarps accelerate flow and produce local elongate depocenters both landward and seaward of the fault scarp. The depocenter is located seaward of the scarp as flow accelerates over the scarp, and there is no deposition immediately in the hanging wall. Counter-regional faults are very effective barriers to sedimentation and produce an elongate depocenter in the hanging wall of the fault. Oblique counter-regional faults produce an asymmetric depocenter, in which the thickest sediment is located in the opposite direction to the flow deflection. Depressions caused by salt withdrawal are the most effective sediment traps and produce depocenters twice the thickness of those produced by fault scarps or salt domes.

ABSTRACT The Al Salif and Jabal al Milh salt diapirs of Miocene age cut through a 4 km thick overburden of Miocene to Recent sedimentary rocks in the southern Red Sea. The A1 Salif diapir is a north-south oriented diapiric wall which has caused updoming of the overburden and active extensional and minor reverse faulting. Carbonate reefs deposited at or below mean sea level, with 14 C ages of 3,700 a have been raised up to an elevation of 17 m above present day sea level giving an average surface uplift rate of 4.6 mm a -1 . There is no evidence for large-scale stoping or injection of salt into overlying faults and fractures, and the upward movement of the salt dome is probably caused by the overburden being forced aside and sliding off the dome. Adjacent to the Jabal Al-Milh diapir recumbent folding and thrusting are the main deformation features observed in the siltstones and gypsum layers of the overburden. The overburden surrounding the Jabal al Milh salt neck has been rotated to the vertical at least 200 meters from the diapir walls. The folding and thrusting deformation is interpreted to be caused by flow of salt in an overlying and since eroded namakier.