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

摘要

The study of the Gulf of Cadiz on the basis ofmultichannel seismic profiles and wells illustrates the stratigraphyand tectonics. The evolution of the southern Iberian margins wasmore complex than in most North Atlantic margins since itentailed several phases of rifting, convergence and strike-slipmotions. Three main tectonic provinces surround the internalzones of the Gibraltar Arc orogenic belt. These include in theIberian margin of the Gulf of Cadiz the flysch units of the Campode Gibraltar complex, the Betic External Zones, and the Neogenebasins of the Guadalquivir Valley.Fault-bounded blocks of flyschand Subbetic units crops out over large areas of the southeasternIberian shelf. The basement of the northwestern area, in contrast, isrepresented by the Paleozoic rocks of the Hercynian massif ofIberia. Half-graben structures determined the main structuraltrends of the margin during the Mesozoic, which were affected byinversion structures during the Neogene compressional stages. TheMesozoic and lower Cenozoic units are best observed in wells andseismic profiles from the northern area. These units are eitherobscure below a thick olistostrome deposit or are absent in mostof the rest of the Gulf of Cadiz. Seven lithoseismic units fromTriassic to Upper Oligocene and another seven Neogene andQuaternary units are identified based on the relationship to thedepositional 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-grabenextensional structures and carbonate platforms. This evolutioncomprises the Mesozoic and early Cenozoic. Ocean-spreading inthe North Atlantic induced extensional tectonics, which deformedthe Cretaceous syn-tectonic post-Aptian deposits. Increasedamounts of terrigenous materials were supplied to the marginfrom Aptian to Albian times, controlling depositional patterns,while terrigenous siliciclastic facies replaced the Jurassiccarbonate platforms. From Middle Eocene to Early Miocene themargin was influenced by the relative motions of Iberia and Africaand the development of the Alpine orogeny. TheIberian-Africanboundary in the Gulf of Cadiz experienced transpression and theMesozoic basins probably underwent subduction. Theemplacement of an olistostrome took place in the Gulf of Cadiztowards the Central Atlantic basin plains during the Tortonian. Theend of the olistostrome emplacement during the Late Miocenecoincides with accelerated tectonic subsidence, while thickprogradational and aggradational depositional sequences weredeveloped. The occurrence of closely juxtaposed regions ofcompression and extension during the Miocene may reflect thewestwards progression of the Gibraltar Arc mountain front over asubducting thinned Tethys crust. When the motion between Iberianand Africa was N-S to NNW-SSE oriented, the migration of thearcuate mountain front into the eastern Gulf of Cadiz is attributedto a mechanism of collision induced delamination of the mantlelithosphere. In contrast to the Betic-Rif belts, however, there is nogeophysical evidence to postulate that continental collision tookplace in the Gulf of Cadiz during the Cenozoic.