The stratigraphic sequences recovered from the Granada-D1 and Andalucía-A1 commercial wells and from Hole 976B were used to determine subsidence and uplift episodes in the basement of the Alboran Sea basin. We used the backstripping computer program of Allen and Allen (1990), which calculates total subsidence and basement subsidence by decompaction of the sedimentary sequences. The program assumes an "Airy-type" isostatic model, and tectonic subsidence is calculated by removal of the sediment load effect. Although backstripping analysis for the Alboran Basin was also performed by Docherty and Banda (1992), Cloetingh et al. (1992), and Watts et al. (1993), our analysis employs new biostratigraphic data that allow us to better determine the main hiatuses detected in the stratigraphic record. This improved determination of periods with sedimentation has enabled us to distinguish, in the Andalucía-A1 well, two episodes of subsidence during the middle Miocene, and two during the upper Miocene (Fig. 5C).
For the Granada-D1 and Andalucía-A1 wells, we adopted the standard porosity and density values from Sclater and Christie (1980). We introduced paleobathymetric corrections on the basis of the new sedimentary and micropaleontological data from our study. For Hole 976B, we used porosity, density, sedimentary, and biostratigraphic data from Leg 161 (Shipboard Scientific Party, 1996b). These data were processed using the decompaction-porosity computer program of Stam et al. (1987). Eustatic sea-level changes were not incorporated in this backstripping analysis because their effect on the final results is negligible.
Described below are the main subsidence and uplift episodes common to all the studied wells that were determined by backstripping analysis. Subsidence diagrams (Fig. 5) show the total and tectonic subsidence uplift at the three holes. In Andalucía-A1 well (Fig. 5C), two periods of heavy subsidence (from ~15.5 to 14.5 Ma and from 13 to 10.7 Ma) can be distinguished. These episodes could be related to coeval rifting events during the middle Miocene, which are depicted in onshore basement units (e.g., García-Dueñas et al., 1992). Subsidence analysis at Andalucía-A1 well revealed a third subsidence episode located in the middle Tortonian (9.2-8.5 Ma). This episode was also detected in the Granada-D1 well, although it seems to have had a minor importance in the subsidence history of the sector in which this well is located (<200 m of tectonic subsidence). The significance of this subsidence episode in the tectonic evolution of the Betic Cordillera should be analyzed in future studies.
Sediments from Hole 976B record a significant sudden period of subsidence that occurred in a short period of time (11-10.7 Ma; Fig. 5B) and may well be correlated with the youngest rifting event recognized in the Andalucía-A1 well. Another period of subsidence (2.5-0 Ma) is recorded in this same hole in the upper Pliocene-Pleistocene, which can be correlated with the Pliocene-Pleistocene subsidence episode described by Comas et al. (1992) in the Alboran Basin.
The Granada-D1 well (Fig. 5A) shows a sudden period of uplift at the Tortonian/Messinian boundary. This event could be correlated with the continentalization of the Granada Basin, coincident with a late Tortonian compressional event that affected the entire Betic Cordillera and Rif (e.g., García-Dueñas et al., 1992; Cloetingh et al., 1992; Rodríguez-Fernández and Martín-Penela, 1993; Crespo-Blanc et al., 1994; and Martínez-Martínez et al., 1997).
The subsidence history during the Pliocene is different in all of the wells studied, which probably indicates different evolutions controlled by local tectonic processes that determined local and coeval uplift and subsidence in the Betic Cordillera and Alboran Sea. The significance of this observation must be integrated in the tectonic evolution of the region in future studies.