Details

• Lecturer: Agustín Martín-Algarra, Departamento de Estratigrafía y Paleontología de la Universidad de Granada (UGR)
• Date: 19th July 2017 12:00 am
• Place: Sala d’Actes de l’Institut de Ciències de la Terra Jaume Almera (ICTJA)
• Further information: Dr. Jaume Vergés

Abstract

Geochronological data obtained by the zircon U-Pb SHRIMP and (U-Th)/He methods from several Malaguide-Alpujarride units of the central sector of the Betic Cordillera constrain the tectono-metamorphic history and Alpine exhumation of these units. (U-Th)/He dating of two Malaguide Paleozoic conglomerates provided cooling ages of 37 and 18 Ma. The former age records unroofing of the source area associated with the earliest Alpine tectonic event affecting the Malaguide unit, while the latter age records the final exhumation related with the youngest Alpine tectonics in this domain. Upper Alpujarride metapelites and gneisses cooled at 18-20 Ma according to zircon (U-Th)/He dating, which followed medium-grade Early Miocene metamorphism and a pre-Alpine (late Variscan) magmatic and metamorphic history dated by U-Pb SHRIMP in the same rocks. Intermediate Alpujarride schists of Permian-Lower Triassic stratigraphic age provided (U-Th)/He Langhian ages (15.4±0.2 Ma), which indicate rapid cooling and high exhumation rates related to fast tectonic unroofing. Tectonic models explaining exhumation must take into account that extensional collapse and crustal thinning affected the Alpujarride-Malaguide crustal segment at the end of the Variscan orogeny, and that the subsequent Alpine orogeny produced substantial tectonic burial followed by exhumation of deep crustal Alpujarride sections (including mantle rocks at their base). Persistent Alpine shortening explains the tectonic evolution of the Alpujarride-Malaguide tectonic stack, including: (i) subduction-related, Early Miocene thrusting and metamorphism (at ca. 22-20 Ma); (ii) syncollisional tectonic exhumation (with rates around 5 km/Ma) of the upper part of the thrust stack at ca. 20-18 Ma; and (iii) syncollisional exhumation of the Iower part of the thrust stack at ca. 17-16 Ma. Continental collision of the Internal Domain (overriding plate and accretionary complex) with the External Domain (down-going plate) involved a relatively small upper plate (Internal Domain stack, including still hot crustal pieces in its lower part, due to Early Miocene Alpine metamorphism) and a larger buoyant lower plate (cold Iberian crust of the Betic External Domain). This forced persistent thrusting at depth and extrusion of the frontal part of the orogenic wedge, whereas coeval extension affected the upper part of the wedge at its rear. Rapid exhumation and cooling of the whole Alpujarride-Malaguide nappe stack at ca. 15.5 Ma predates the final exhumation of the deepest units of the belt (Nevado-Filabride Complex), which started at ca. 12 Ma.