The Malaguide-Alpujarride boundary: a major extensional contact in the Internal Zone of the eastern Betic Cordillera, SE Spain

Abstract

The Internal Zone of the Betic Cordillera in southern Spain was formed by thrusting and horizontal ductile shortening in the Late Cretaceous (?)-Palaeogene, but was subjected to a major phase of vertical thinning during the Neogene. The contact between the Malaguide (the structurally highest thrust complex) and the underlying Alpujarride Complex, is an example of several major tectonic contacts across which the stratigraphy has been repeated by thrusting, but the metamorphic grade increases abruptly downwards. Detailed work in the eastern Betic Cordillera documents the relationships across this contact. In the Sierra Espuña, petrographic and illite crystallinity studies show that the highest imbricate thrusts of Malaguide rocks have only been diagenetically altered, whereas the deeper parts of the thrust stack exhibit anchizone facies metamorphism. In the northern Sierra de las Estancias, apatite fission track studies confirm that there, too, Malaguide rocks have never been above 200 °C. In both areas, the underlying Alpujarride rocks exhibit epizone or greenschist facies metamorphism and a major fault zone with both brittle and ductile structures separates the two complexes. The observed rapid increase in metamorphic grade over very small structural thicknesses suggests that part of the original metamorphic zonation has been excised across the contact. Thus, in its present form, we interpret the Malaguide/ Alpujarride boundary as an extensional contact. The extensional motion took place in Early-Middle Miocene times, as constrained by fission-track cooling ages. The fault zone at the contact is sub-horizontal in the Sierra Espuña, but in the Sierra de las Estancias it is mainly sub-vertical. Kinematic data from a 5–20 m thick calcmylonite layer within the fault zone in the Sierra de las Estancias, corrected for later tilting, indicates that the extensional movement was originally directed with the top to the east-northeast. The interpretation of the Malaguide/Alpujarride boundary as an extensional contact is similar to recent interpretations of the Nevado-Filabride/Alpujarride contact at deeper structural levels in the central and eastern Betics. The identification of another major extension fault system confirms that Miocene extension was widespread at all structural levels. During the early-middle Miocene, the Sierra Espuña and the northern margin of the Sierra de las Estancias lay on the boundary between the region undergoing horizontal extension in the Internal Zones and the Alboran Sea to the south, and the region undergoing horizontal shortening in the External Zones to the north. As a result, the detailed history of the Alpujarride/Malaguide contact is complicated: extensional structures cross-cut structures related to earlier Alpine shortening and were then affected by further shortening as convergence with the External Zones continued.