Mohamed Sahabi

Morphological reorganization within the Pacific-Antarctic Discordance

Abstract Two domains with different satellite gravity signatures [1] appear along the axis and on the adjacent basins of the Pacific-Antarctic Ridge (PAR) between Udintsev FZ and 180°W. One of these signatures is of rough and faulted seafloor, with a high density of apparent, well-marked fracture zones; the other is of smooth seafloor that is comparable with that of oceanic basins that are generally formed at fast-spreading centres. Between Udintsev FZ and 157°W these two domains are separated by a V-shaped structure that extends for more than 1000 km along the rise axis, whereas west of 157°W the boundary is more diffuse. The satellite gravity also reveals an abrupt change in the axial morphology of the PAR across FZ XII, despite the fact that the current spreading rate [2] is the same on both sides of the fracture zone (about 60 mm/yr, full rate). To interpret these features, we postulate that the domains with an apparently rough seafloor morphology have been created at a spreading centre with an axial valley, and that smooth morphology testifies to a spreading centre which was with or evolving into an axial high at the time the crust was formed. With this hypothesis, we show that, since An 21o time (ca. 48 Ma), the ridge morphology changed from an axial valley to an axial high wherever and whenever the spreading rate exceeded a given threshold value. We also show that there is no unique threshold value. Geophysical evidence suggests that the differences in spreading rate threshold values that we observe are probably related to upper mantle temperature heterogeneities below the axis of the PAR. Therefore, we conclude that changes in spreading rates, combined with changes in the upper mantle temperature, constitute the key process that has governed the morphological reorganization of the PAR between Udintsev FZ and 180°W since An 21o time. The cause of upper mantle temperature heterogeneities, however, remains an open question. The 1000 km long ‘V’ south of Udintsev FZ reflects a change in axial morphology that progressively propagated southwards during the last 30 m.y., at a velocity of about 30 mm/yr. Thus, one tentative explanation for mantle heterogeneities which would also help understand the ‘V’ consists in postulating that the asthenosphere propagated below the PAR axis for the last 30 m.y., from a relatively ‘hot’ mantle province north of Udintsev FZ to a relatively ‘cold’ province south of the fracture zone. This flow model (originally proposed by Marks and Stock [3]) needs to be tested through further investigation.

Opening of the central Atlantic Ocean: Implications for geometric rifting and asymmetric initial seafloor spreading after continental breakup

Study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the central Atlantic conjugate margins are compared based on compilation of wide-angle seismic profiles from NW Africa Nova Scotian and U.S. passive margins. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern central Atlantic image differences in the continental crustal thickness. While profiles on the eastern U.S. margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the African continental margin. In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Canadian side, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. After isochron M25, a large-scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides.

Chapter 9 The Gulf of Cádiz continental shelves

Abstract The continental shelves fringing the Gulf of Cádiz are shallow-water environments of high scientific interest as they record the complex interplay between competing sedimentary, tectonic and oceanographic processes. These shelves receive the contributions of fluvial sources of varying significance, evolving laterally to shelf segments dominated by a vigorous oceanographic regime. In the long term, the Gulf of Cádiz margins show lateral physiographical compartmentalization related to the structural imprint. Recent approaches on the Iberian Shelf have focused on the study of shelf sedimentary processes and long-term evolutionary patterns through the application of high-resolution seismic-sequence stratigraphy concepts. The Late Quaternary record exhibits extensive regressive-lowstand shelf-margin deltas mostly comprising nested sequences disposed in a hierarchical pattern. This complex architecture has been attributed to the leading control of orbitally driven, climatic–eustatic changes. As a result, the Iberian Shelf constitutes an excellent location to study the interaction between sedimentary processes, climatic and sea-level changes. Recently collected data in the Moroccan Shelf allow a high-resolution seismic-sequence stratigraphy scheme to be established. Significant latitudinal changes on depositional sequence architecture are driven by differential accommodation led by the reactivation of deformational fronts during the Pleistocene. As a consequence, this southern shelf constitutes a good example of tectonic control on sequence stratigraphy.