ALBERT

Description: 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.

Description: The Loulé salt diapir grew through the Jurassic and Early Cretaceous, when tight subvertical curtain folds were developed. It was then squeezed during the Alpine orogeny in the Oligocene and Miocene. This produced an intense array of brittle faults, veins and brittle–ductile shear zones through most of the diapir. The diapir exhibits more localized shear and brittle structures than any other previously described diapir. The most unusual deformation structures are ‘streaky shear vein’ zones (a new term), with an average spacing of 1 – 5 m, and usually less than 30 cm wide. They consist of a central coarse-grained halite vein surrounded by parallel zones of wispy discontinuous shale streaks and recrystallized halite. A new mechanism is proposed for the development of these zones, where repeated pulses of fluid flow have caused extensive halite recrystallization and streaking-out of unlithified mud intercalated with crystalline halite. This explains the formation of wide zones of well-aligned mud streaks (now lithified to shale), but with only small amounts of simple shear offset observed across these zones (〈0.2 m). The lower tips of the streaky shear vein zones exhibit splaying mud injection veins, which indicate that fluid overpressures preserved liquid mud from salt deposition at 200 Ma to the Alpine collision at 20 Ma.

Description: The Loulé salt diapir grew through the Jurassic and Early Cretaceous, when tight subvertical curtain folds were developed. It was then squeezed during the Alpine orogeny in the Oligocene and Miocene. This produced an intense array of brittle faults, veins and brittle–ductile shear zones through most of the diapir. The diapir exhibits more localized shear and brittle structures than any other previously described diapir. The most unusual deformation structures are ‘streaky shear vein’ zones (a new term), with an average spacing of 1 – 5 m, and usually less than 30 cm wide. They consist of a central coarse-grained halite vein surrounded by parallel zones of wispy discontinuous shale streaks and recrystallized halite. A new mechanism is proposed for the development of these zones, where repeated pulses of fluid flow have caused extensive halite recrystallization and streaking-out of unlithified mud intercalated with crystalline halite. This explains the formation of wide zones of well-aligned mud streaks (now lithified to shale), but with only small amounts of simple shear offset observed across these zones (〈0.2 m). The lower tips of the streaky shear vein zones exhibit splaying mud injection veins, which indicate that fluid overpressures preserved liquid mud from salt deposition at 200 Ma to the Alpine collision at 20 Ma.