The basement recovered at Site 976 records high-T conditions (about 700°C) with a clockwise PT path (Fig. 10). The main deformation phase (D2) occurred close to the metamorphic peak, and was associated with partial melting. Later shear zones (D3) developed during decreasing P and T conditions.
The PT evolution of the Alboran Sea basement is quite different from most of the PT paths followed by the Alpujárride nappes of the western Betic chain (Fig. 10). In particular, the nearly isothermal decompression after the PT-peak, frequently shown by the Alpujárrides (Torres-Roldàn, 1981; Monié et al., 1994; García-Casco and Torres-Roldán, 1996), has not been recognized in the present study. Moreover, in the basement of the Alboran Sea the peak P should have occurred before the peak T, which is an uncommon feature for the Alpujárride nappes.
However, the coincidence of the main D2 deformation phase with the peak metamorphism and the occurrence of a D3 phase during decreasing pressures has been recognized elsewhere in the Alpujárride units (Tubía et al., 1992). In addition, a peak T following the peak P has been described by Bakker et al. (1989) for the Alpujárride units of the eastern Betic chain (Fig. 10).
In the western Betics or the Internal Rif (Fig. 1), high P/T metamorphism related to the main nappe-forming event was followed by extension and vertical shortening (Balanyá et al., 1997). This process uplifted hot mantle material to higher crustal levels and triggered low P/T metamorphism. In the Alboran basement crustal thinning, coupled with substantial thinning of the mantle lithosphere (Sandiford and Powell, 1986), can explain contemporaneous heating and decompression between D1 and D2. Moreover, magmatic underplating during extension could take into account the persistence of high-T conditions at very low P during M3. This interpretation is consistent with the shallow moho depth reported for the Alboran sea (Torné and Banda, 1992), and with the protracted volcanic activity during Miocene (Hernandez et al., 1987).
The age of metamorphism of the Alboran basement rocks should be similar to the lower Miocene ages obtained for the Betic cordilleras (Platt et al., 1996). Fast exhumation and cooling rates after the high-grade metamorphism are indicated by the short time difference between these ages and the Serravallian age of the marine sediments that cover the Alboran basement. A further evidence of fast cooling comes from a biotite K-Ar cooling age of 16±1 Ma, obtained by Steiger and Frick (1973) from rocks of the Alboran basement at DSDP Site 121 (Fig. 1).
The presence of widespread cataclastic bands and the scarcity of low-T plastic deformation is in agreement with a short residence time of the Alboran rocks in the middle-upper crust at high-T conditions. On the contrary, they cooled down rapidly after the high-T metamorphism.