ALBERT

Description: Non-volcanic margins such as the West Iberian margin exhibit certain characteristics, such as a deficit of synrift igneous rock, a zone of exhumed subcontinental mantle in the continentocean transition and an apparent extension discrepancy. These observations can be explained as a consequence of the progressive extension of the lithosphere above relatively cool mantle. The evolving rheological stratification of the lithosphere controls the style of extension at different lithospheric levels at different times; extension is probably heterogeneous at all stages, with lower crustal and upper mantle boudinage controlling the patterns of thinning and mantle upwelling early in the rift history, and complete crustal embrittlement and mantle serpentinization controlling the formation of late-stage detachment faults. Extension in the brittle crust is via multiple phases of faulting, with a general focusing of extension towards the incipient ocean. The lack of melt is explained by a combination of heterogeneous extension of the lower lithosphere and a cool subcontinental geotherm. The extension discrepancy may in places be controlled by depth-dependent stretching of the crust through lower crustal boudinage, but may also simply be the result of incomplete recognition of the entire polyphase faulting history. The latter seems to be the case for West Iberia. Evidence for all these processes can be found at the West Iberian rifted margins as well as those preserved and partially exposed in the Alps.

Description: A characteristic of rifted margins is the extension discrepancy: i.e., the amount of extension estimated from fault geometries on seismic images is far too little to explain the observed crustal thinning and subsidence. Either the crust has been thinned in some other way or the amount of extension has been severely underestimated. To investigate the latter, we create a model structural section across a rifted margin by focusing extension in the center of a rift, producing successive phases of crosscutting faults. From one side of this section, a synthetic seismic image is generated and interpreted as if it were a real profile. Just as for real margins, apparent listric faults and eroded fault block crests are seen, but these are not present in the model and instead represent intersecting fault surfaces, and are thus diagnostic of polyphase faulting. Just as for real margins, the amount of extension measured from the seismic is only a fraction of the true extension. Just as for real margins, this extension discrepancy increases markedly oceanward. Demonstrably for the synthetic margin, and by implication for real margins, the extension discrepancy is the failure of the seismic method to image unambiguously the polyphase faulting required to accommodate increasing extension, combined with a general lack of awareness of the features, outlined here, diagnostic of such faulting.

Description: Techniques for detecting faults have been applied to a 3D seismic volume acquired in the outer fold and thrust belt in the deep-water Niger Delta. Firstly, the dip and azimuth of seismic traces in the data were calculated in a volume referred to as the "raw steering" data. The data were further improved by calculating two additional generations of dip volumes representing localized and subregional structural dips referred to as the "detailed" and "background" steering volumes, respectively. A multitrace similarity attribute volume was then calculated with the reflectivity and background dip-steering data as the input. The attribute data detected discrete zones of dip and similarity anomalies, trending WNW-ESE, that represented the location of discontinuities in the area. The anomalies may not have been seen clearly in the reflectivity and similarity data calculated without the application of dip-steering.

Description: Seafloor mapping of the outer fold and thrust belt in the deep-water Niger Delta using high-resolution 3D seismic data has revealed a variety of geomorphic features related to gravity-driven compressional tectonics, submarine sedimentary processes, and fluid migration as evidenced by bathymetric ridges caused by folding of the underlying sedimentary succession, gravity slide scars, submarine canyons and pockmarks all clearly imaged on the seismic-derived seabed bathymetry. The largest canyons, typically 25–35 km in length with widths of up to 5 km, incise an EW-trending arcuate zone of elevated bathymetry across the area. This ridge is the reference point for dividing the seabed topographic pattern into distal and proximal domains. Generally, seabed topography is gentle and less complex in the proximal domain and the major structures in the area include circular clusters of fluid-escape features primarily along channel margins and in places along discontinuities and ridges in the eastern half of the seabed. The large-scale distribution of these structures in the proximal parts of the study area may be related to fluid venting from shallow and or deeper reservoirs.

Description: A region of oceanic core complexes (OCCs) exists at 13°N on the Mid-Atlantic Ridge that is regarded as a type site. This site includes two OCCs at 13°20′N and 13°30′N, thought to be in the active and dying stages of evolution, and two together called the Ashadze Complex (centred at 13°05′N) that are considered to be relict. Here we describe the results of S-wave seismic modelling along an ∼200-km-long 2-D transect traversing, south-to-north, through both the Mercurius and Marathon fracture zones, the southern outside corner of the 13°N segment, the OCCs, the ridge axis deviation in trend centred at 13°35′N, and the youngest oceanic crust of the eastern ridge flank to the north. Our inversion model, and the corresponding Vp/Vs ratio, show that the majority of the crust beneath the 13°30′N OCC comprises metamorphosed lithologies that have been exhumed to the shallowest subseabed level, while basaltic lithologies underlie the 13°20′N OCC. The transition between these contrasting crustal structures occurs over a distance of 〈5 km, and extends to at least ∼2 km depth below seafloor. The northern and southern OCCs of the Ashadze Complex have contrasting structures at shallow depth, with the northern OCC having a faster S-wave velocity in the upper crust. A Vp/Vs ratio of >1.9 (and equivalent Poisson's ratio of >0.3) indicates exhumed and/or metamorphosed lithologies beneath the bathymetric depression between them and within the crust beneath the southern OCC. Between the northern and southern flanks of the Marathon fracture zone and northern flank of Mercurius fracture zone, the lower crust has a relatively low Vp/Vs ratio suggesting that the deformation associated with Marathon fracture zone, which facilitates fluid ingress, extends laterally within the lower crust. Marathon fracture zone itself is underlain by a broad zone of low S-wave velocity (∼2.0 km s−1) up to ∼20 km wide from the seabed to at least the mid-crust, that is mirrored in a high Vp/Vs ratio and lower density, particularly deeper than ∼1 km below seabed within its bathymetric footprint. Volcanic domains are highlighted by a low Vp/Vs ratio of 〈1.6 (and equivalent Poisson's ratio of 〈0.15). Our combined seismic and density models favour the localized model of OCC evolution. They also show a considerable ridge-parallel variability in the amount and distribution of magmatic versus metamorphosed crust. Our results suggest that the current focus of magmatism lies to the north of the 13°20′N OCC, where the magmatic accretion-type seabed morphology observed is mirrored in the pattern of microseismicity, suggesting that its inward-facing median-valley-wall fault may link to the 13°20′N OCC detachment surface. Magmatism and active faulting behind (to the west) the footwall breakaway of the 13°30′N OCC, and the microseismicity concentrated in a band along its southern flank, suggest a readjustment of ridge geometry along axis is underway. As part of this, a transform offset is forming that will ultimately accommodate the 13°30′N OCC in its inside corner on the eastern flank of the ridge axis to the north.