Ocean Drilling Program Leg 161 drilled four sites (976 to 979) in the Alboran Sea to investigate the formation and tectonic behavior of the Alboran Basin. The highest tectonic priority of Leg 161 was to sample a basement high in the West Alboran Basin at Site 976. Drilling at Sites 977, 978, and 979 was aimed at constraining the timing of the postrift deformation of the basin. The integration of drilling results with regional geological and geophysical data and the interpretations of deep and shallow seismic-reflection profiles aid in constraining the structure of the crust beneath the Alboran Sea and the origin and tectonic history of the basin.
The principal conclusions of this synthesis paper are summarized as follows:
1. Site 976 was drilled on a basement high and was cored 343.19 m of high-grade metamorphic rocks in two holes. Sample results demonstrate that the Alboran Sea basin is underlain by rocks of continental origin formed of high-grade schist, gneiss, migmatitic gneiss, marble, calc-silicate rock, and granite from the Alpujarride Complex of the Alboran Crustal Domain.
2. The metamorphic basement at Site 976 has undergone high-temperature metamorphism and melting during exhumation. P-T data show high and increasing temperature in the high-grade schist (from about 550°-600°C at 10.5-8 kbar to 650-700°C at 3-4 kbar), fast exhumation (rate of 4.5 km/m.y. in the interval 20.5-18 Ma) during crustal and lithospheric stretching, and rapid cooling (from 450°C to 60°C between 20.5-18 Ma).
3. MCS profiles combined with drilling results indicate that rifting structures—major intracrustal extensional detachments—accounted for the crustal thinning beneath the basin. Effective constraints for the extension-related exhumation history at Site 976 are given by the cooling history of the basement. Thermal modeling of the P-T path indicates that the exhumation (from a burial depth of around 40 km) and extensional denuding of the Alboran Crustal Domain started during the late Oligocene (at about 27 Ma), somewhat before the marine transgression that created the Alboran Sea took place. Drilling data demonstrated that the Site 976 High was exposed to the seafloor by the late Serravallian, but near-site MCS profiles suggest that the basement high has been a submarine, or occasionally emergent relief since the early Miocene.
4. Site 976 reveals two periods of subsidence, in at 11-10.7 Ma (rate of 3 km/m.y.) and at 2.5-0 M (a rate of 0.5 km /m.y), and uplift at 5-2.5 Ma in the West Alboran Basin. Subsidence analyses across the Alboran Basin (data from commercial wells) indicate a conspicuous lateral variation in rift-related subsidence phases, suggesting migration of the locus of extension, and probably also changes in the direction of extension during the Miocene rifting. The almost generalized gentle phase of post-Messinian subsidence, contemporaneous with the contractive reorganization of the basin and the general uplift at the basin margins, could be interpreted as thermal subsidence.
5. Sites 977 and 978 were drilled in the East Alboran Basin and sampled 598.5 m and 485 m, respectively, of equivalent sedimentary sequences ranging in age from latest Miocene?-early Pliocene to Holocene. A gravel interval containing pebbles of volcanic, metamorphic, and sedimentary rocks, sampled at both Sites 977 and 978, is associated with the base of the Pliocene-Pleistocene sequence in the Alboran Basin (the M-reflector)
6. Drilling results and seismic images in the East and South Alboran Basins (Sites 977, 978, and 979) constrain the timing and mode of the contractive reorganization of the basin. The post-Messinian to Holocene deformation mostly resulted in wrench-tectonics giving way to both transtensive (generating new depocenters; e.g. the Yusuf Basin) and transpressive conditions (producing local uplift and positive flower-structures; e.g., the Alboran Ridge), that involve the present structural organization of the basin. The post-Messinian reorganization is consistent with the north-northwest/north present-day convergence of Iberia and Africa.
7. The volcanic pebbles sampled at Sites 977 and 978 constrain several periods of middle-to-late Miocene volcanic activity in the eastern Alboran Domain that occurred at about 12 Ma, 9 Ma, and 6 Ma. Incompatible/immobile trace- element systematics in the tholeiitic to calc-alkaline rocks indicate magmatic sources containing both depleted (MORB-type) component and an ocean-island or plume-type component ("arc signature"). On the basis of their geochemical signature, an alternative model envisaging oceanic-crust subduction beneath the eastern Alboran from 6 to at least 12 Ma has been suggested. However, the geochemistry of the magmatism does not solely provide a forceful constraint to discriminate among genetic models, as similar magmatic signatures can support different geodynamic models.
8. The age, rate, and thermal behavior of extension determined at basement Site 976 provide constraints on the initial thermal structure, the exhumation, denudation history, and the processes that have modified the crust beneath the Alboran Sea since the initiation of extension. These results support mechanic models that invoke the removal of the continental lithosphere mantle for the origin of the Alboran Basin.