The structure of the Atlantic–Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia–Africa plate boundary)

The diffusive plate boundary between Iberia and Africa has been analyzed by means of already published and new 28 multichannel seismic profiles covering most of the Atlantic margins of SW Spain and NW Morocco. This region includes the prolongation of the Betic–Rifean tectonic units along the Gibraltar Arc. One of the most characteristic units in the study area is the Miocene seismically chaotic unit that has been divided in a tectonic and a gravitational domains in accordance to previous interpretation by Torelli et al. [Torelli, L., Sartori, R., Zitellini, N., 1997. The giant chaotic body in the Atlantic Ocean off Gibraltar: new results from a deep seismic reflection survey. Marine and Petroleum Geology 14, 125–138.]. In this work we redefine the limits of both domains and rename them as the Gulf of Cadiz Imbricate Wedge and the Horseshoe Gravitational Unit. We interpret the Gulf of Cadiz Imbricate Wedge as a west-migrating thick thrust system that build up in a relatively short period of time. This unit constitutes the continuation of the Miocene chaotic units observed onshore to the front of the External Units in the Betics and the Rif (the Guadalquivir Allochthonous Unit and the Rides Prerifaines). The Horseshoe Gravitational Unit, covering an approximate area of 18 000 km2 with a volume of around 23 000 km3, is interpreted as giant submarine debris flows sourced from the up-building of the Gulf of Cadiz Imbricate Wedge, surrounding basement highs and the continental Iberian Slope. The fast westward migration of the Flysch units, the External Units and the Gulf of Cadiz Imbricate Wedge, from the Early Miocene to the Late Tortonian corresponds to a different tectonic process than the protracted but slow NNW to NW convergence of Africa. Our interpretation favours the westward slab retreat theory to form the arcuate shape of the Atlantic side of the Iberia–Africa plate boundary. The subduction mechanism as well as potential roll-over was only active from late Early Miocene to Late Miocene spanning about 12 Myr.