Structures and metamorphic effects of peridotite emplacement.

Different structures contribute to the emplacement and exhumation of the peridotite bodies. Many are extensional shear zones and normal faults that were superposed on previous contacts with an originally contractive nature. Because this type of structure cannot be distinguished on the basis of geometric criteria alone, the metamorphic record becomes a necessary reference for establishing its character and deformation conditions.

As mentioned above, the upper boundary of the Ronda peridotites is located in a thick shear zone that represents the paleo-Moho. This shear zone developed in extensional conditions before the Miocene (Balanyá et al., 1993, Tubía, 1994), although previous works associated it with the thrusting of peridotites (Tubía and Cuevas, 1986, 1987). During this extensional event, the main Alpujarride foliation (Sp) was generated after a high pressure – low gradient event (Fig. 4, Azañón et al., 1997, 1998). The P-T paths of garnet gneiss and lherzolite in the Bermeja-Jubrique shear zone indicate that general flattening, local simple shear and the omission of crustal rocks took place at the same time as the uplift of the peridotites through the mantle, to the base of the crust (Fig. 4). Whereas the pressure in garnet gneiss drops from >14 kbar at syn-kinematic conditions to ca. 6 kbar at post-kinematic conditions (T=850-700°C; Balanyá et al., 1993), in the garnet lherzolite pressure ranges around 20 kbar (Van der Wal and Vissers, 1993) to 4-8 kbar (Obata, 1994) for a comparable temperature range.

The lower part of the peridotite massifs contains two structural devices. In the Bermeja massif, peridotite layers of varying thickness are intercalated among layers of fragment-bearing granitoids (Figs. 3 and 4). These can be interpreted as mantle imbrications overthrusting crustal rocks that were later strongly modified by superimposed faulting. On the other hand, under the peridotites of the Alpujata massif, a large overturned limb of the Blanca Unit (Fig.5) exhibits crenulation cleavage and microfolding. Moreover, the lithostratigraphic sequence and metamorphic zones are inverted (Figs. 3 and 5). This would appear to confirm that the emplacement of the peridotite slabs is related to tectonic inversion, crenulation cleavage and large folds, as previous authors have suggested (Buntfuss, 1970, Westerhof, 1975, Navarro-Vilá and Tubía, 1983). The presence of microfolds and crenulation cleavage in enclaves in the granitoids at the bottom of some peridotite bodies also support this hypothesis.

Figure 5. Shear zones beneath the peridotites

Shear zones beneath the peridotites

Shear zones beneath the peridotites that have traditionally been attributed to "cold emplacement" (Westerhof, 1977) or to a late emplacement stage (Tubía and Cuevas, 1986) correspond to ductile N-S extension described in greater detail below.

The P-T conditions of the contractive emplacement of the peridotites must have been similar to those reached at the end of the uplift through the mantle peridotites, that is, <800°C and 6 kbar (Obata, 1994). These conditions comprise the stability field of the porphyroclastic plagioclase-bearing peridotites (recrystallized granular peridotites, Lenoir et al., 2001). There is no evidence of a significant pressure increase in the rocks of the Blanca Unit. However, the ubiquitous presence of sillimanite, which developed simultaneously to the crenulation cleavage, indicates T>500°C and P<9 kbar in all the pelitic formations of the Blanca Unit (Fig. 4), later resulting in extensive migmatization and generation of granitoids. The main consequences of the emplacement of the peridotites would therefore be the tectonic inversion of part of the Ojén unit, and the nearly complete thermal homogenization of the unit. Eclogite remnants near the bottom of the Alpujata peridotites (stratigraphic bottom of the Blanca Unit) have been attributed to the emplacement of the peridotite slab (Tubía and Gil Ibarguchi, 1991), but similar eclogites and HP granulites have been found above the peridotite bodies, at the bottom of the Jubrique Unit (Balanyá et al., 1997; Michard et al., 1997). Moreover, assemblages of HP-LT metamorphism have been preserved at the top of the Jubrique Unit (Michard et al., 1993; Azañon et al., 1995; Bouybaouene, 1995), at conditions in agreement with the gradient suggested by the eclogites (Fig. 4). In this context, eclogites and granulites below the peridotite slab represent the P-T conditions reached during a contractive episode prior to its emplacement; thus, peridotites were contractively emplaced after they reached extensionally mid-crustal levels after the previous HP event.

In short, the emplacement of the mantle slab was the result of a composite thrust after lithosphere-wide extension. It is possible that several peridotite imbrications originated in a complex suture, while at the same time the overthrust crustal units were intensely folded. Recumbent folds (Simancas and Campos, 1993) and crenulation foliation (Tubía et al., 1992, Azañón et al., 1996) are common in all the units of the Alpujarride complex, and together with the peridotite emplacement, they mark a major organizational episode in the Alpujarride complex.