THE GIANT CHAOTIC BODY IN THE ATLANTIC-OCEAN OFF GIBRALTAR - NEW RESULTS FROM A DEEP SEISMIC-REFLECTION SURVEY (VOL 14, PG 125, 1997)

The central-eastern Atlantic continental slope and parts of the Horses hoe and Seine Plains, west of the Gibraltar Are, are occupied by a sei smically 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 main ly gravity deposits (debris flows, olistostromes) in the northern sect or, and tectonic melanges in the southern, where the body forms part o f a south-verging accretionary prism related to Africa-Iberia converge nce (submarine extension of the Rifian system of N-Africa). The northe rn 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 Pl ain and adjacent slope from structural highs generated during the Late Miocene paroxismal phase of Iberia-Africa convergence. Submarine flow s were triggered along regional, continuous slopes up to 100 km in len gth. Their volume largely exceeds previously reported marine occurrenc es of gravity flow deposits, the outstanding examples of which occur i n intraplate settings (hot spots, old passive margins). At the boundar ies (lateral or frontal ramps) of the southern tectonic melange comple x, 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 co mparable to those that can be calculated for huge chaotic units in oro gens, such as the 'Argille scagliose' (=Liguride complex) of the Apenn ines, in which disrupted olistostromes and tectonic melanges coexist. (C) 1997 Elsevier Science Ltd.