Análisis estructural y evolución tectonometamórfica del basamento de las cuencas neógenas de Vera y Huércal-Overa, Béticas orientales

Abstract

The basement of the Vera and Huercal Overa basins (southeastern Betics) is formed by rocks of the Alboran Crustal Domain; a terrain which collided with the South-Iberian and Maghrebian continental margins in the Lower Miocen, resulting in the formation of the Gibraltar Arca mountain chain. The Alpujarride complex which occupies an intermediate structural position within the Alboran Domain, above the Nevado-Filabride and below the Malaguide complexes, include at least three tectonic units in the southeastern Betics. From bottom to top in the structural sequence, these units are Almagro, Almanzora and Variegato. The metapelitic rocks of these units show significative differences in their tectonic fabrics and in their P-T metamorphic paths. The lower Almagro unit underwent low-P/low-T metamorphism (300 ºC and 3-4 kbar) and its metapelites are slates with no differentiated metamorphic fabric, which show two sets of spaced axial-plane cleavages. The intermediate Almanzora unit hast two differentiated metamorphic fabrics. The oldest foliation (Ss) preserved in quartz-rich domains of a crenulation cleavage (Scc) grew during high-P/low-T metamorphism (between 300 ºC/12 kbar and 350 ºC/6kbar). The Scc cleavage developed after an isobaric heating to 475 ºC at 5 kbar and registers an initial isothermal decompression to 475 ºC at 3 kbar followed by cooling to 300 ºC at 2 kbar. A brittle spaced cleavage axial plane to N-vergent asymmetric folds cuts the Scc fabric. At the top of the Alpujarride tectonic pile the Variegato unit, includes up to three imbrications formed from top to bottom by garnet schists, fine-grained schists and Triassic carbonates. The main Scc foliation in the garnet schists grew during a nearly isothermal decompression betweeen 500 ºC at 8 kbar and525 ºC at 2 kbar. In the Variegato dark schists, the spaced crenulation cleavage associated to N-vergent folds is defined by muscovite and chlorite lepidoblasts and is overprinted by the growth of andalousite porphyroblasts. This assemblage equilibrated at 450-460 ºC at 2 kbar. The Variegato fine-grained schists include a high-P/low-T Mg-carpholite-bearing assemblage, within pre-Scc quartz veins equilibrated at 8-10 kbar and approximately 400 ºC. Furthermore, a chlorite + phengite + quartz assemblage defining the Sc relic foliation in lenticular domains of the Scc cleavage shows local equilibria at 11 to 9 kbar at 400 ºC. The superposition of these units was related with N-directed compressional brittle-ductile shear zones and associated N-vergent asymmetric folds, which were active at a late stage of the metamorphic evolution after coaxial ductile flattening of the Variegato and Almanzora units. The thrust pile that resulted from this late compressional event was later thinned by two consecutive brittle extensional systems with northward and southwestward tectonic transport. Finally, these metamorphic rocks were exhumed to the surface in the core of E-W oriented anticlinal ridges, which developed during the Upper Neogene and the Quaternary. Folding and strike-slip faulting of the Miocene Alboran basin produced the present basin and range morphology of the southeastern Betics, which is characterised by the formation of isolated sedimentary basins in the synclines.