Most western Mediterranean Neogene basins overlie attenuated continental crust belonging to an Alpine collisional orogen (Late Cretaceous-Paleogene) formed by several tectonometamorphic complexes. Significant segments of this orogen crop out onshore: the Betic and Rif internal zones (Alboran Domain), surrounding the Alboran Basin; the Kabylies, to the south of the Balearic-Algerian Basin; and some massifs of Sicily and Calabria, around the Tyrrhenian Basin (Boullin et al., 1986). The above-mentioned crustal segments overthrust peripheral arcuate thrust sheets of detached covers; the nappe stacking is concomitant with the rifting processes in the western Mediterranean basins.
Drilling in the Alboran Sea at Ocean Drilling Program Leg 161 Site 976 has confirmed the existence of continental crust beneath the western Alboran basin (Fig. 1). In fact, Holes 976B and 976E at Site 976 recovered 259 m and 84 m of high-grade metamorphic rocks, respectively (Shipboard Scientific Party, 1996). These rocks are similar to others on land that form part of one of the three main tectonometamorphic complexes comprising the Alboran Domain (Fig. 1): the Nevado-Filabride, Alpujárride, and Malaguide, from bottom to top. Only the Alpujárride Complex contains high-grade metamorphic rocks; thus, the Shipboard Scientific Party (1996) proposed that the basement rocks drilled at Site 976 belong to this complex.
Nevertheless, the Alpujárride Complex is formed of many crustal tectonic units; several mantle slabs (Ronda peridotites) are emplaced between them. Generally, the former thrust-nappe boundaries of the Alpujárride units are not preserved, having been reused or obliterated by brittle, low-angle, normal faults and extensional detachments related to the Miocene Alboran rifting. Structural position, pressure-temperature (PT) metamorphic conditions, and lithology are used to differentiate, correlate, and assemble the Alpujárride units over extended areas of the Alboran Domain (Tubía et al., 1992; Azañón et al., 1994; Balanyá et al., 1997).
Preliminary estimates of PT conditions from the high-grade metamorphic basement rocks at Site 976 (Shipboard Scientific Party, 1996) justify their being attributed to the Alpujárride units surrounding the Ronda peridotite massifs (west of Málaga; Fig. 1) described by Torres-Roldán (1979), Tubía et al. (1992), Balanyá et al. (1993), and Balanyá et al. (1997), among others.
The metamorphic evolution and the downward increase in metamorphic grade of basement at Site 976 is similar to the high-grade metamorphic rocks overlying the peridotite massifs; however, the lithologic assemblage is rather different because of the presence of marble, calc-silicate rocks, and granitic rocks (Shipboard Scientific Party, 1996). In fact, all these lithologies, together with high-grade schist, are characteristic of the units underlying the Ronda peridotite slabs.
We compared selected lithologic, metamorphic, and structural features common to both the basement rocks beneath Site 976 and to the onshore Alpujárride lithologic sequences surrounding the Ronda peridotite massifs. Our aim was to establish a more accurate structural position for the basement segment recovered at Site 976. Finally, we discuss some implications of our proposal for the correlation of these rocks.