Most peridotite outcrops, such as mantle tectonically exhumed rocks, constitute major markers in the orogenic edifices where they are found. Peridotites are relatively common in considerably thinned continental margins (Beslier et al., 1990) or associated to ophiolite sequences derived from oceanic lithosphere (see, for instance, Kusky et al., 2001). Peridotite bodies among high temperature and pressure rocks, such as Ronda (Balanyá et al., 1997; Tubía et al., 1997) and Beni Bousera (Bouybaouene et al., 1998) (Figs. 1 and 2) are less common, and represent continental collision involving deep thrusts through the mantle (Tubía and Cuevas, 1986; Tubía, 1990). The Ronda and Beni Bousera peridotites, which constitute the largest outcrops of subcontinental mantle worldwide (Dickey et al., 1979), are intercalated in the Alpujarride complex, a group of tectonic units with a common record of high-pressure metamorphism (Azañón et al., 1998).
Figure 1. Main tectonic elements of the Gibraltar arc
Main tectonic elements of the Gibraltar arc: deformed foreland areas (Subbetic, Prebetic and Intra-Rif), Flysch Trough Complex, and the Alboran Domain (hinterland) Complexes. Peridotite units (green) belong to the Alpujarride Complex, which occupies an intermediate position within the Alboran Domain stack.
The Alpujarride peridotites are mostly concentrated around the Gibraltar arc, in the westernmost Mediterranean (Fig. 1). In addition to the outcropping bodies, several elliptical gravity highs (Fig. 2) displaced from the major peridotite outcrops suggest the existence of buried ultramafic slabs in the basement of the Alboran basin (Torné et al., 1992). The current geographic Gibraltar Arc and the rest of the Betic and Rif mountain chains were formed during the Miocene by the superposition of a crustal terrane called the Alboran Domain (made up, in ascending order, of the Nevado-Filabride, Alpujarride and Malaguide complexes) on the South Iberian and Maghrebian margins. During the collisional process, the mountain front migrated outward, imbricating the margin cover, while the Alboran domain underwent considerable tectonic thinning (García-Dueñas et al., 1992; Martínez-Martínez and Azañón, 1997). Several tectonic models have been proposed for the Alpujarride Peridotite evolution (Tubía and Cuevas, 1986; Van der Wal and Wissers, 1993; Tubía et al., 1994; Zeck, 1997), the Alboran Domain evolution (Platt and Vissers, 1989; Vissers et al., 1995; Martínez-Martínez and Azañón, 1997), and the entire Gibraltar Arc/western Mediterranean evolution (e.g. Lonergan and White, 1997; Doglioni et al., 1997; Gelabert et al., 2002). However, no consensus has been reached and relevant questions, such as the timing, kinetic vectors or number of tectonic events, remain a subject of discussion.
Figure 2. Main peridotite bodies and western Alpujarride outcrops in the Gibraltar arc.
Main peridotite bodies (green) and western Alpujarride (pink) outcrops in the Gibraltar arc. At the ODP-976 and DSDP-121 Sites marked on the figure, basement rocks attributed to the Alpujarride have been also recovered (Sánchez-Gómez et al., 1999). Geological sections of Figure 5 are indicated in red. Blue dashed lines and triangles are respectively Bouguer anomaly contours and maximums in mgal. Note the good correlation of some of the gravity anomaly highs with the Alpujata and Beni-Bousera massifs. The displacement of the gravity highs with the outcropping peridotites has been interpreted as due to the main part of the peridotite slabs remaining buried (Torné et al., 1992). Thus, gravity highs east of Málaga or Cabo Negro could correspond to hidden bodies of an intermediate size. The right bar roughly represents the projected horizontal length of the peridotite bodies, including the buried part, in a section from Carratraca to Beni-Bousera massifs (green lined bars). Supposing a single former peridotite slab, total N-S calculated extension is 2, but a more conservative value in the same section, considering the loss of undetectable serpentinized material, could be 1.4.
Alpujarride peridotite bodies constitute, due to their large volume and outcrop context, a key piece of the western Mediterranean puzzle, and their tectonic evolution should be carefully taken into account for the general models. Most peridote emplacement models include a former diapiric uplift through the mantle lithosphere (e.g. Dickey et al., 1979, Obata, 1980). Field geology evidence (e.g. Westerhof, 1977; Tubía and Cuevas 1987; Sánchez-Gómez et al., 1995) and geophysical data (Barranco et al., 1990, Torné et al., 1992) nevertheless show that peridotites form thick slabs among crustal rocks, which can only be tectonically emplaced, in opposition to the initial proposition that the mantle intruded diapirically through the crust (Loomis, 1972; Loomis, 1975).
In this paper we focus on the intracrustal evolution of the ultramafic rocks, discussing the emplacement into the middle-lower crust up to the final mechanism of exhumation, when average highs of more than one thousand metres were reached in the Sierra de Ronda. We will attempt to fit the structural evolution of the peridotite bodies in the tectonometamorphic history recorded by the surrounding crustal rocks and propose a coherent model for both crustal and mantle rocks.