The accretionary complex of the Mediterranean Ridge: tectonics, fluid flow and the formation of brine lakes – an introduction to the special issue of Marine Geology

Abstract

The Mediterranean Ridge is an accretionary complex that is largely composed of sediments that have been accreted from the African plate, which is subducted northeastwards beneath the Aegean at about 37 mm/yr (Le Pichon et al., 1995). The trend of the western part of the ridge is nearly orthogonal to the direction of convergence, whereas the eastern branch of the ridge is subject to very oblique convergence (Fig. 1). Beneath most of the accretionary complex, the subducting crust of the African plate is oceanic in nature, but along its southern margin in its central sector, the accretionary complex is colliding with the continental margin of Africa, oi Libya (Chaumillon and Mascle, 1997). In the 1970s, the linear deeps of the Hellenic trench south of the Cretan arc were interpreted as a trench system similar to trenches at other subduction zones, but with the realisation that the Mediterranean Ridge is an accretionary complex (Biju-Duval et al., 1978; Kastens et al., 1992; Le Pichon et al., 1982; Ryan et al., 1982), it became apparent that the Hellenic trench represents a forearc basin with little sediment ¢ll. The former bathymetric trench of the subduction zone has been ¢lled by the accretionary complex of the Mediterranean Ridge, which is very wide, over 200 km across its western branch, comparable to Alaska (220 km), southern Barbados (270 km) and the Makran (230 km), all of which have very thick sequences of sediment on the subducting plate. The deformation front of the accretionary complex, along its southern margin, now lies beyond the crest of the £exural bulge in the subducting plate (Fig. 2), shown by a positive gravity anomaly (see ¢gs. 4 and 5 of Le Pichon et al., 2002), that lies seaward of the trench at most subduction zones. The slope of the surface of the southern £ank of the Mediterranean Ridge is low, about 1‡, which, from the dynamics of accretionary wedges (Chapple, 1978; Davis et al., 1983), is indicative of very low shear stress on the decollement at the base of the wedge. One feature of the Mediterranean Ridge that distinguishes it from other accretionary complexes is the presence of a sequence of evaporites of Messinian age (Ryan et al., 1973; Hsu et al., 1978), which lie within the sediments at a depth of a few hundred metres beneath the sea bed. These evaporites have the potential to in£uence the deformational behaviour of the accretionary complex, as their mechanical properties are intrinsically diierent from those of siliciclastic sediments, and because of their eiect upon £uid £ow and £uid pressure, which control eiective stress. The discovery of a lake of concentrated brine in the Bannock Basin, a small basin in the