Structure and evolution of the ‘‘Olistostrome’’ complex of the Gibraltar Arc in the Gulf of Cádiz (eastern Central Atlantic): evidence from two long seismic cross-sections

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Reflection profiles characterize the structure and the upper Mesozoic to Cenozoic deposits of the Gulf of Ca´diz region. Two long ENE–WSW multichannel seismic lines (ca. 400–500 km long) are analyzed to study the evolution of the area from the continental shelf to the Horseshoe and Seine abyssal plains. The huge allochthonous deposits emplaced in this region (the socalled ‘‘Olistostrome’’ of the Gulf of Cadiz) are described in terms of three different domains on the basis of the seismic architecture, the main tectonic features and the nature of the basement, oceanic or continental. The eastern domain extends along the continental shelf and upper and middle slope and corresponds to the offshore extension of the Betic–Rifean external front. It is characterized by salt and shale nappes later affected by extensional collapses. The central domain develops along the lower slope between the Betic–Rifean front and the abyssal plains and is characterized by a change in dip of the allochthonous basal surface and the basement. The allochthonous masses were emplaced by a combined gravitational and tectonic mechanism. The northern boundary of this domain is marked by the occurrence of an outstanding WNW–ESE-trending thrust fault with a strike-slip component, termed here as the Gorringe–Horseshoe fault. The westernmost domain corresponds to the abyssal plains, where the distal emplacement of the allochthonous body takes place; it is characterized by thrust faults affecting both the sedimentary cover and the oceanic basement. The allochthonous masses show a less chaotic character and the thickness decreases notably. These domains represent different evolutionary steps in the mechanisms of emplacement of the allochthonous units. The eastern domain of the allochthonous units was emplaced as part of the pre-Messinian orogenic wedge related to the collision that gave rise to the Betic–Rifean Belt, whereas the allochthonous wedge of the central and western domains were emplaced later as a consequence of the NE–SW late Miocene compression that continues in present times.
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