Deep penetrating MCS imaging of the rift-to-drift transition, offshore Douala and North Gabon basins, West Africa

Abstract

We describe a regional grid of West African PROBE Study (WAPS) deep penetrating multi-channel seismic reflection (MCS) and potential field data crossing the transition from rifted continental crust (RCC) to normal oceanic crust (OC) in the offshore Douala and North Gabon Basins. In profiles oriented sub-parallel to oceanic fracture zones RCC is shown to terminate seaward into a very different form of crust interpreted to be composed of highly attenuated blocks of RCC, wedges of seaward-dipping reflectors representing intermixed mafic volcanics and sediments, mafic lower crust, and possibly exhumed mantle rocks, all of which form a complex faulted terrain. This heterogenous type of crust is referred to here as ‘proto-oceanic crust’ (POC). POC grades seaward into normal to thick OC. In the Douala Basin, sedimentary units deposited in an Early Cretaceous continental rift now form marginal basin highs overlain by < 1 km younger of sediment beneath the shelf. RCC here has undergone little attenuation, as more highly attenuated RCC is believed to occur below the conjugate Sergipe-Alagoas Basin in eastern Brazil. In the North Gabon Basin, RCC has been markedly thinned and resulting rift structures are deeply buried by post-rift deposits. Reflective lower crust within RCC forms discontinuous bands, where normal dip-slip faults sole into it and transform related faults crossthrough it. Interpreted Moho reflections atthe base of RCC form a discontinuous horizon that is sometimes offset at crustal deformation zones; elsewhere it transects the reflective grain of crustal deformation zones. Interpreted WAPS MCS data show that the margin is composed of rift-units separated at ca 40 km intervals by NE trending fracture zones and transfer fault zones of variable length, causing the margin to step progressively southwestward by alternating between normal and transformfaulted segments. Correlation of rift structures between these West African basins and the conjugate Sergipe-Alagoas Basin show that the two margins formed a connected rift-branch that underwent NE-SW oriented extension, ca 35° oblique to the rift axis. This orientation favoured occupation of NE-SW trending preexisting shear fabrics situated in the Late Proterozoic Pan-African/Braziliano mobile belt. At the RCC-POC boundary, wedges of seaward-dipping reflectors abut against RCC and overlie non-reflective lower crust and reflection Moho. They correlate with pronounced negative magnetic anomalies (−200 to −600 nT), probably caused by the juxtaposition of weakly magnetised RCC and more magnetic, mafic volcanic rocks of the POC. Steeply-dipping faults are interpreted to occur at transform-faulted segments, where they separate blocks of RCC from POC, or in some places OC. These NE trending transfer fault zones propagate into oceanic fracture zones (FZs) that compartmentalise and isolate both continental and oceanic rift-units. For example, the Ascension FZ is shown to intersect the African continent south of Bata, Equatorial Guinea, where it forms a 200 km long transform-faulted boundary between RCC and POC. The thickness of POC from the top of the seaward-dipping reflectors to reflection Moho ranges between 2–5 s two-way travel time (twtt). This POC probably formed by volcanic outpouring of partial melt at the time of continental rupture, and then during initial seafloor spreading. Phases of magmatic and amagmatic extension formed within rift-unit spreading cells, owing to variable widths of POC (20–200 km wide) trending parallel to flow lines before evolving seaward into normal OC. Aptial salt extends over RCC and POC, and in some places, salt has been mobilised farther seaward (<10 km) as horizontal sills emanating from salt diapirs. The northernmost known occurrence of this Aptial salt lies 100 km south of Douala, Cameroon. There is no evidence in the MCS data that salt was deposited behind structural highs rimming the margin of the salt basin. Instead, salt overlies sub-horizontal reflectors (0.2–1 s twtt thick) believed to represent sedimentary rocks that drape POC and OC of presumably Aptial age. This interpretation implies that salt in the part of West Africa was deposited in nearshore to deeper basin margin environments without restriction from central basin waters.