Data from holes drilled during Leg 161 (Comas, Zahn, Klaus, et al., 1996) confirmed the geologic assumption that the Alboran Sea basement is continental, consisting primarily of rocks belonging to the Alboran Domain (Balanyá and García-Dueñas, 1987; Comas et al., 1992; García-Dueñas et al., 1992; Vissers et al., 1995). This crustal domain includes the Internal Zones of the Betic and Rif Cordilleras.
The Alboran Crustal Domain, hinterland of the Gibraltar Arc, consists of several tectonic complexes that extend on land, thrusting over the detached covers of the Iberian and Maghrebian Margins. The seismostratigraphic sedimentary units identified in the Alboran Sea basin (Comas et al., 1992) also continue onshore, unconformably overlying the Alboran Domain.
The three main nappe complexes of the Alboran Domain can be differentiated by their tectonometamorphic record (Fig. 1). The Alpujárride Complex, tectonically placed over the Nevado-Filabride Complex and below the Malaguide Complex, characteristically includes in its highest tectonic units rocks that reached temperatures of 550°-700°C under conditions of low-pressure, high-temperature metamorphism; that is, the same values as the basement rocks at Site 976 (Shipboard Scientific Party, 1996; Platt et al., 1996).
Summarizing Tubía et al. (1992), Azañón et al. (1994), and Balanyá et al. (1997), four groups of Alpujárride tectonic units have been distinguished in the Betics according to structural and metamorphic criteria, particularly taking into account conditions reached by the rocks from each unit in high-pressure, low-temperature, and low-pressure, high-temperature metamorphic events. From top to bottom, these groups of units are termed the following:
The high-pressure event detected in Triassic, Permian-Triassic, and Paleozoic rocks belonging to the Jubrique, Blanca, and Escalate groups of units represents the former Alpine record of the Alpujárride Complex (Azañón et al., 1994). The present-day relative structural position of these groups of units implies extensive modification of the initial Alpujárride nappe stack, because high-grade metamorphic rocks from the Jubrique and Blanca Groups now systematically overlie the low-grade metamorphic rocks from the Escalate and Lujar-Gador Groups.
The tectonometamorphic evolution of the Alpujárride Complex was established on the basis of the metamorphic events and related structures observed in the Jubrique-Group tectonic units of the eastern and western Betics (Azañón et al., 1997; Balanyá et al., 1997). The proposed succession of events was contrasted with available structural and metamorphic data for the remaining groups of Alpujárride units, and reflects the main orogenic events in the Alboran Domain.
Throughout event 1, crustal thickening occurred in a collisional setting, as suggested by the presence of high-pressure mineral assemblage relics and incipient foliation. Event 2 is related to the pervasive flattening foliation (Sp) that formed in nearly isothermal decompression conditions; significant crustal thinning is obvious in this event, as indicated by the PT paths and the approximation of metamorphic isograds. Event 3 coincides with the development of major north-vergent overturned and large-scale recumbent folds that deformed the Sp foliation and metamorphic-isograde geometries. These folds and associated thrusts contributed to the general reorganization of the former nappe stack; development of crenulation foliation (Sc) during this event was accompanied by low-pressure mineral growth. The Alpujárride units were once again heavily attenuated during the Miocene rifting (event 4) that culminated in the development of the Alboran basin over the continental crust collisional belt (e.g., Galindo et al., 1989; García-Dueñas et al., 1992; Crespo-Blanc et al., 1994); outward migration of the Gibraltar Arc mountain front was concomitant with the Alboran rifting (Platt and Vissers, 1989; García-Dueñas et al., 1992). Ductile and brittle extensional detachments and listric normal faults associated with tectonic event 4 altered the boundaries of the Alpujárride units. Finally, folds and faults that developed during a late contraction event 5 (Weijermars et al., 1985; de Larouzière et al., 1988) modified the regional physiography and reduced the Miocene Alboran basin to the boundaries of the present Alboran Sea.
In short, the orogenic evolution of the Alpujárride Complex, and therefore of the Alboran Domain, is characterized by alternating contractional and extensional events (Balanyá et al., 1997). The evolution started with continental collision around the Upper Cretaceous or Paleocene (DeJong, 1991; Azañón et al., 1994) and ended in the Miocene with crustal thinning (Alboran rifting), finally dominated by roughly isobaric cooling (Zeck et al., 1992; Monié et al., 1994).