The giant chaotic body in the Atlantic Ocean off Gibraltar: New results from a deep seismic reflection survey

The central-eastern Atlantic continental slope and parts of the Horseshoe and Seine Plains, west of the Gibraltar Arc, are occupied by a seismically chaotic body. This paper reports new deep seismic reflection data that allows a first assessment of the three-dimensional geometry of the body, and discussion of its evolution. The seismically chaotic body is huge in area and volume and complex in origin, comprising mainly gravity deposits (debris flows, olistostromes) in the northern sector, and tectonic melanges in the southern, where the body forms part of a south-verging accretionary prism related to Africa-Iberia convergence (submarine extension of the Rifian system of N-Africa). The northern part, up to several tens of thousands of cubic kilometers in volume, results from the superposition of a few endoolistostromes, probably of Miocene age, centripetally discharged onto the Eastern Horseshoe Plain and adjacent slope from structural highs generated during the Late Miocene paroxismal phase of Iberia-Africa convergence. Submarine flows were triggered along regional, continuous slopes up to 100 km in length. Their volume largely exceeds previously reported marine occurrences of gravity flow deposits, the outstanding examples of which occur in intraplate settings (hot spots, old passive margins). At the boundaries (lateral or frontal ramps) of the southern tectonic melange complex, small-scale olistostromes of Pliocene age also occur, recalling the so-called precursory olistostromes of collisional chains. The gravity and tectonic structures occur in a convergent plate setting, with low shortening rates, and their eventual fate will be incorporation, with disruption, in a collisional chain. Their volume and extent appear comparable to those that can be calculated for huge chaotic units in orogens, such as the 'Argille scagliose' (=Liguride complex) of the Apennines, in which disrupted olistostromes and tectonic melanges coexist. © 1997 Elsevier Science Ltd.