The Betic orogen and the Iberian–African boundary in the Gulf of Cadiz: geological evolution (central North Atlantic)

Abstract

The study of the Gulf of Cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics. The evolution of the southern Iberian margins was more complex than in most North Atlantic margins since it entailed several phases of rifting, convergence and strike-slip motions. Three main tectonic provinces surround the internal zones of the Gibraltar Arc orogenic belt. These include in the Iberian margin of the Gulf of Cádiz the flysch units of the Campo de Gibraltar complex, the Betic External Zones, and the Neogene basins of the Guadalquivir Valley. Fault-bounded blocks of flysch and Subbetic units crops out over large areas of the southeastern Iberian shelf. The basement of the northwestern area, in contrast, is represented by the Paleozoic rocks of the Hercynian massif of Iberia. Half-graben structures determined the main structural trends of the margin during the Mesozoic, which were affected by inversion structures during the Neogene compressional stages. The Mesozoic and lower Cenozoic units are best observed in wells and seismic profiles from the northern area. These units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the Gulf of Cadiz. Seven lithoseismic units from Triassic to Upper Oligocene and another seven Neogene and Quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome. The first tectonic phase was characterized by a passive margin, which was controlled by the development of half-graben extensional structures and carbonate platforms. This evolution comprises the Mesozoic and early Cenozoic. Ocean-spreading in the North Atlantic induced extensional tectonics, which deformed the Cretaceous syn-tectonic post-Aptian deposits. Increased amounts of terrigenous materials were supplied to the margin from Aptian to Albian times, controlling depositional patterns, while terrigenous siliciclastic facies replaced the Jurassic carbonate platforms. From Middle Eocene to Early Miocene the margin was influenced by the relative motions of Iberia and Africa and the development of the Alpine orogeny. The Iberian–African boundary in the Gulf of Cadiz experienced transpression and the Mesozoic basins probably underwent subduction. The emplacement of an olistostrome took place in the Gulf of Cadiz towards the Central Atlantic basin plains during the Tortonian. The end of the olistostrome emplacement during the Late Miocene coincides with accelerated tectonic subsidence, while thick progradational and aggradational depositional sequences were developed. The occurrence of closely juxtaposed regions of compression and extension during the Miocene may reflect the westwards progression of the Gibraltar Arc mountain front over a subducting thinned Tethys crust. When the motion between Iberian and Africa was N–S to NNW–SSE oriented, the migration of the arcuate mountain front into the eastern Gulf of Cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere. In contrast to the Betic–Rif belts, however, there is no geophysical evidence to postulate that continental collision took place in the Gulf of Cadiz during the Cenozoic.