2. Tectonic Setting

The western Mediterranean region has undergone significant extension despite continuous convergence of Africa and Eurasia since the Cretaceous . The most striking evidence for this extension is the Neogene age of the oceanic crust beneath most of the western Mediterranean (Figure 1a). The original Mesozoic oceanic crust of the Neo-Tethys that lay between Africa and Europe was almost completely recycled into the mantle during the Late Oligocene and Miocene as a subduction zone rolled back to the south toward Africa and southeast toward Calabria (see Figures 1b and 1c) . A narrow NE-SW trending mountain belt generated by Cretaceous-Paleogene convergence that is believed to have existed along the present-day coasts of SE France and eastern Spain collapsed behind the retreating subduction zone and was dispersed across the western Mediterranean. Fragments of this mountain belt now form the Internal zones of the Betics, Rif, Tell, and Calabria .

Situated at the western edge of the extended region, the Alboran region suffered the least extension. It has also experienced the least amount of convergence relative to the rest of the Alpine belt (~200 km since the Cretaceous ). Convergence was initially NNE-SSW to N-S before shifting to NW-SE in the Tortonian (~10-7 Ma) . The plate boundary between Africa and Iberia is diffuse with Neogene continental deformation distributed over a broad zone more than 500 km wide stretching from the High Atlas in Morocco to the Betic Cordillera in Spain (Figure 2). Marine studies in the Gulf of Cadiz and off the Atlantic coast of Morocco demonstrate that the diffuse character of this deformation extends offshore with no clear plate boundary visible to the west until the Gloria Fault (not on map, ~20°W) .

The Alboran Sea is bounded by the arcuate Betic and Rif mountain belts (see Figures 1a and 2). These mountains are typically subdivided into Internal and External zones (Figure 2) owing to a pronounced difference in timing, style of deformation, and lithology. A region of highly deformed flysch separates these zones in the western Betics and Rif. The Internal zones are fragments of a collisional mountain belt composed of Mesozoic marine sediments shortened during Cretaceous and Paleogene convergence. The crustal thickness beneath this belt may locally have been in excess of 50 km . The flysch was deposited on the southern and western margins of the belt on thinned continental or possibly oceanic crust . During formation of the belt, the External zones were relatively stable continental margin areas subject to marine deposition.

During the Late Oligocene-Early Miocene (~25 Ma) the belt collapsed along with limited injection of basaltic dikes beneath the Betic (northern) portion of the belt (Figure 2) . The extension seems to have occurred initially along NNW dipping faults followed by more substantial extension along WSW dipping low angle faults . Deep crustal rocks were rapidly exhumed from depths of 40 km at a minimum of 4 km/Ma . Peridotites emplaced at the base of the extending crust in the Early Miocene were rapidly exhumed beneath extensional faults to form the Spanish Ronda and Moroccan Beni Boussera peridotite massifs (Figure 2). Extension in this convergent setting was accommodated by thrusting in the bounding flysch and External zones with a primarily WNW, W, and WSW directed transport direction in the Betics, Gibraltar, and Rif regions, respectively . This coeval extension of the Internal zones and shortening in the External zones was accompanied by distributed calc-alkaline volcanism predominantly in the Internal zones but also on the perimeter of the External zones (Figure 2). By Burdigalian times (~21-16 Ma) the western portions of the belt (now the West Alboran Basin (WAB) (Figure 2)) had subsided below sea level and were subject to marine deposition . Active extension of the Alboran Sea ceased in the Middle Miocene (Serravallian/Tortonian, ~10 Ma) but subsidence continued in the WAB and to a limited extent in the east Alboran basin (EAB) . Extension ceased in the Betics in the Tortonian (perhaps as early as the Serravallian ). Predominantly west directed transport continued in the Rif and Betics until the end of the Miocene . The total amount of westerly directed shortening is significant but not well determined with estimates ranging between 200 and 500 km .

In the Pliocene and Quaternary gross plate motions reasserted control of the deformation in the region with primarily NNW-SSE shortening and strike-slip faulting . Uplift of the margins of the extended region differentiated the Rif and Betic Internal zones from the Alboran Sea. Limited potassic volcanism (shoshonites and lamprophyres) occurred in the eastern External zones during the Pliocene and was superseded by alkaline basaltic magmatism in the Quaternary.

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