Reconstruction (continued)

North Africa

During the Early-Middle Miocene, intense tectonic activity took place in North Africa due to the opening of the Provençal, Algerian and Alboran basins, and the subsequent emplacement of the Kabylies block and the Internal Rif onto the African margin. Extension in the Provençal Basin commenced after rifting in Valencia Trough had failed. In early Burdigalian (~21 Ma), continental breakup occurred between the Balearic Islands and the Kabylies blocks, and a new basin developed. Extension was probably governed by a rapid southward rollback and subsequently led to sea floor spreading and formation of a new oceanic crust in the Algerian-Provençal Basin (Rehault et al. 1985)(Figure 13 & Figure 14).

Figure 14 Late Burdigalian reconstruction (18 Ma).

Evidence for extensional fabrics associated with the opening of the Algerian-Provençal Basin are found in the Kabylies metamorphic core complexes, which are now accreted to the African margin. These complexes are characterised by the occurrence of low-angle extensional detachments, which juxtaposed upper crustal rocks on top of high-grade metamorphic rocks (Caby & Hammor 1992, Tricart et al. 1994, Saadallah & Caby 1996). The occurrence of core complexes implies that a considerable amount of crustal thinning was made possible by non-coaxial shearing along extensional detachments. Based on ⁴⁰Ar/³⁹Ar dating, it has been suggested that extensional deformation occurred at 25-16 Ma (Monié et al. 1984, 1988, 1992).

The Kabylies blocks drifted southward in response to the southward rollback of the subduction zone until they collided and accreted to the African margin (Cohen 1980, Tricart et al. 1994)(Figure 15). Collision occurred when the Mesozoic oceanic lithosphere, which had previously separated the Kabylies from the African margin, was totally consumed by subduction. The collision occurred between 18-15 Ma, based on the cessation of extensional tectonics in the Algerian-Provençal Basin and the Kabylies core complexes, and the commencement of thrusting in the External Maghrebides (Frizon de Lamotte et al. 2000). The accretion of the Kabylies block and the final consumption of old oceanic lithosphere in this region permanently terminated southward subduction rollback. South-directed thrust systems propagated southward after accretion (Frizon de Lamotte et al. 2000), but subduction processes were impeded and eventually halted with the presence of the relatively buoyant continental crust at the subduction zone. The Middle Miocene cessation of subduction processes in North Africa resulted in the segmentation of the western Mediterranean subduction system, with an eastward dipping subduction in the Alboran region, and a westward dipping subduction in the Tyrrhenian region (Lonergan & White 1997)(Figure 15).

Figure 15 Middle Miocene reconstruction (15 Ma).

The Alboran Domain

The tectonic evolution of the Alboran Sea is a matter of controversy, and several different models have been proposed. This reconstruction follows Royden (1993a) and Lonergan and White (1997), who suggested a slab rollback model for the opening of the Alboran Sea. Alternatively, it has been suggested that extension in the Alboran Sea developed as a result or an extensional collapse of a thickened continental crust and its underlying lithospheric mantle (e.g. Platt & Vissers 1989, Housman 1996). These models are not entirely supported by field observations from the Rif-Betic cordillera, which imply episodic alterations from crustal shortening to extension (Azañón et al. 1997, Balanya et al. 1997, Martínez-Martínez & Azañón, 2002). Neither are such models supported by the block rotations inferred from palaeomagnetic data.

According to our model, the origin of the Internal Zone of the Rif-Betic cordillera is similar to the origin of Corsica, Sardinia, Calabria and the Kabylies (Figure 11). Thus, the Rif-Betic in its Oligocene position formed a continuous orogenic belt together with the Kabylies, Calabria, Corsica and the western Alps that underwent high-pressure metamorphism during Alpine orogeny (Figure 11). As mentioned before, these terranes (excluding the western Alps) were part of an overriding continental slab above a northwest dipping subduction zone, which started to retreat oceanward during the Oligocene. Once rollback of the subduction hinge commenced, rocks of the Rif-Betic Internal Zone were subjected to an extensional regime leading to the formation of metamorphic core complexes and exhumation of high-pressure metamorphic rocks below extensional detachments (Platt et al. 1983, Azañón et al. 1997, Balanya et al. 1997).

Westward rollback of the subduction hinge in the westernmost Mediterranean was probably triggered by an original curvature of the subduction zone in its western terminus. Rollback, therefore, led to southwestward migration of the subduction hinge accompanied by southwestward drifting of the extended continental fragments of the overriding plate. Southerly migration, however, could not proceed after the subduction zone reached the passive margin of Africa (see previous section). Rollback continued only in areas where the existence of oceanic lithosphere still permitted oceanward retreat of the subducting lithosphere (Figure 15). Thus, in the Middle Miocene (16-15 Ma), the western east-dipping segment of the subduction zone rolled back in the direction of the oceanic lithosphere.

The formation of the Alboran Sea occurred during the westward migration of the subduction hinge. Rapid rollback was compensated by wholesale extension in the overriding continental crust, which was thinned to ~15 km between 23-10 Ma (Lonergan & White 1997). Contemporaneously, fragments of continental crust were thrust onto the passive margin of Africa and Iberia (the External Zone), forming rotation patterns consistent with oblique thrusting derived by the westward rollback of the subduction zone. Final accretion of the Rif-Betic Cordillera occurred at ~10 Ma, when the subduction zone rolled back as far as Gibraltar (Figure 16). Subduction rollback then ceased, together with the cessation back-arc extension in the Alboran Sea (Lonergan & White 1997).

Figure 16 Tortonian reconstruction (10 Ma).