western Mediterranean since the Oligocene
The geology of the western Mediterranean region
In this section, we briefly discuss the main characteristics of geological terranes that were incorporated in the post-Oligocene evolution of the western Mediterranean. Time relationships between the different terranes are presented in Figure 4.
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| Figure 4 Time-space diagram of the tectonic activity in the western Mediterranean (time scale after Palmer 1983). Data compiled from Boccaletti et al. (1990), Oldow et al. (1990), van Dijk & Okkes (1991), Caby & Hammor (1992), Carmignani et al. (1994), Crespo-Blanc et al. (1994), Ferranti et al. (1996), Saadallah & Caby (1996), Azañon et al. (1997), Sowerbutts (2000) and Crespo-Blanc & Campos (2001). |
The mountains of the Betic in southern Spain and the Rif in northern Morocco surround the Alboran Sea to form an arc shape orogenic belt in the westernmost Mediterranean (Figure 5). This belt marks the western terminus of the Alpine orogen. The rocks of the Rif-Betic cordillera are usually divided to three main zones: the Internal Zone, the External Zone and the Flysch Zone. The Internal Zone consists of allochthonous Paleozoic to Early Miocene rocks, which were thrust onto the External Zone during the Miocene (Crespo-Blanc et al. 1994, Crespo-Blanc & Campos 2001). Alpine deformation and metamorphism affected the Internal Zone during the Cretaceous and the Tertiary, and crustal rocks were buried to great depths and underwent high-pressure metamorphism (de Jong 1990). The External Zone consists of Mesozoic to Tertiary rocks, which represent the passive margin of Africa and Iberia deforming during Alpine orogeny. The Flysch Zone mainly consists of Early Cretaceous to Early Miocene deep marine clastic deposits (Wildi 1983).
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| Figure 5 Geological map of the Alboran Sea and the Rif-Betic cordillera (after Platt & Vissers 1989). |
Structural and metamorphic relationships in the Rif-Betic cordillera, particularly from the Internal Zone, show that several contractional and extensional episodes took place. The earliest extension was probably associated with rapid isothermal exhumation of the high-pressure rocks in the Late Oligocene – Early Miocene, and probably commenced at ~30 Ma (Azañón et al. 1997, Platt et al. 1998). Few authors have demonstrated that coeval with crustal shortening in the External Zone, earlier thrust faults in the Internal Zone were rejuvenated as extensional detachments (Azañón et al. 1997, Martínez-Martínez & Azañón 1997). Extension led to vertical thinning and isothermal exhumation of high-pressure rocks now exposed beneath low-angle normal faults (Platt et al. 1983, Azañón et al. 1997, Balanya et al. 1997). Furthermore, during the Early Neogene, a slab of diamond-bearing mantle peridotites was exhumed and juxtaposed amidst crustal rocks of the Internal Zone (Van der Val & Vissers 1993). Radiometric dating of the high-pressure rocks suggests a rapid exhumation during the Late Oligocene - Early Miocene (27-18 Ma) (Monie et al. 1994, Platt et al. 1998).
The floor of the Alboran Sea consists of rocks similar to those found in the Rif-Betic cordillera, and covered by Early Miocene syn-rift deposits and post-rift marine sediments (Comas et al. 1992, Platt et al. 1998). It is therefore evident that the region was subjected to widespread extension during the Middle Miocene that led to the formation of the Alboran Sea. However, contemporaneously with extension in the Alboran Sea, thin-skinned thrusting and folding took place in the External Zones of the Rif-Betic cordillera (Platt & Vissers 1989, Platzman et al. 1993, Crespo-Blanc & Campos 2001). This deformation was accompanied by a considerable amount of block rotations around vertical axes, with clockwise rotations in the Rif and counterclockwise rotations in the Betic (Allerton et al. 1993, Platzman et al. 1993, Lonergan & White 1997).
The mountain chain of the Maghrebides in Northern Africa and in Sicily consists of a stack of south-verging thrust sheets that bridges the Apennines of Italy with the Rif Mountains of Morocco (Figure 1). Most rocks exposed in the Maghrebides are non-metamorphic sedimentary units of Mesozoic to Early Miocene age deposited on the southern margin of the Tethys Ocean (Wildi 1983). The southern boundary of the Maghrebides is a low-angle thrust fault (the Tellian Front) that delimits the Maghrebides nappes from the autochthonous terranes of the Atlas chain (Wildi 1983). More internal zones of the orogen are found in northern Algeria and Tunisia (Grande and Petite Kabylies), as well as in a submerged fold-and-thrust belt between Sicily and Tunisia (Compagnoni et al. 1989, Tricart et al. 1994). These internal terranes originated in the Alpine orogen and were overthrust onto the Maghrebides during the opening of the western Mediterranean basins (Cohen 1980).
The Kabylies consist of a Hercynian basement and Mesozoic sediments, metamorphosed and strongly deformed during Alpine orogenesis. Alpine metamorphism took place during the Cretaceous and the Tertiary (Peucat & Bossière 1981, Monié et al. 1984; 1988; 1992, Cheilletz et al. 1999), and involved metamorphism at high-pressure conditions. High-pressure rocks are presently exposed in metamorphic core complexes, and are directly overlain by rocks that did not suffer Alpine metamorphism (Caby & Hammor 1992, Saadallah & Caby 1996). Their exhumation seems to be associated with horizontal extension accommodated along low-angle-normal faults (Caby & Hammor 1992, Saadallah & Caby 1996). Deformation along these faults is usually associated with flat lying foliations and mylonitic shear zones with top-to-the-NW sense of shear (Caby & Hammor 1992, Saadallah & Caby 1996). Radiometric dating indicates that extensional deformation probably occurred at 25-16 Ma (Moniéet al. 1984, 1988, 1992).
Thrusting in the External Maghrebides commenced at the Early/Middle Miocene and was generally directed southward and southeastward (Frizon de Lamotte et al. 2000 and references therein). Since the Tortonian, the locus of major crustal shortening has migrated southward until a ~400 km wide area was structured as a fold-and thrust belt (Tricart et al. 1994).