Late-orogenic extension and strike-slip deformation in the Neogene of southeastern Spain

Abstract

The Betic Cordillera of southern Spain form the western end of the Alpine orogen, which during the Neogene was affected by large-scale late-orogenic extension. Extension, however, occurred within a setting of continuous slow convergence of the African and European plates. During the Neogene, shortening occurred in the External Zone of the Betic chain as a result of collision between the southern margins of Iberia and the Betic crust. Simultaneously, late-orogenic extension affected the Internal Zone of the Betics, resulting in the development of intramontane basins. Parts of the Internal Zone have subsided since the Miocene to form a large marine basin, the Alboran Sea. Seismic studies of the Alboran seafloor have revealed ubiquitous graben and half-graben structures of Miocene age. The Alboran basin and basins in the eastern Betics are currently underlain by an anomalously hot thinned continental crust. A prominent set of NE-SW trending strike-slip faults in the south-eastern part of the Betics form bounding faults along a number of late Miocene intramontane basins. This first-order characteristic invited earlier workers to interpret the intramontane basins as wrench basins, developed in response to wrench faulting along these faults. But recent fission-track studies in the Betic crust as well as the Miocene geology of the basins onland and in the Alboran Sea clearly indicate that extension of the Betic-Alboran domain continued well into the late Miocene. And studies of the Betic and Alboran crust led other workers relate the geological and geometrical developments of the Betic-Alboran region to orogenic collapse in response to convective removal of a thickened lithospheric root or to subduction roll-back followed by slab detachment. They interpret the late Miocene basins largely as extensional or back-arc basins. Our field studies of basins in the Betic External and Internal Zone clearly provide evidence that foreland basin development and thrusting in the External Zone continued well into the late Miocene and occurred simultaneously with extension and development of basins in the Internal Zone. The intramontane basins in the Internal Zone initiated at the beginning of the late Miocene and developed as genuinely extensional and subsiding basins in response to ~NE-SW and later ~N-S directed extension. In the late Miocene, progressively from the easternmost part of the Betics towards the southwest, extension in the basins was terminated followed by uplift. In the latest Miocene – early Pliocene NE-SW trending thrust and reverse faults developed, basins were inversed and basin sediments were folded. A number of the faults that belong to the prominent NE-SW trending fault zone initiated as normal faults during late Miocene basin development but became reactivated as oblique contraction faults in the latest Miocene – early Pliocene. In conclusion, the extending middle-upper parts of the Betic-Alboran crust were transported northward onto the Iberian foreland as inferred from late Miocene ages of thrusting and amount of shortening in the Betic External Zone. The structure of the extensional basins became modified by latest Miocene – early Pliocene large-scale reverse faulting accommodating ongoing convergence between Africa and Iberia. The development of the Betic might be related with orogenic collapse, but seem more consistent with slab-detachment.