ELSEVIER Tectonophysics 285 (1998) 253-273 TECTONOPHYSICS Late exhumation stages of the Alpujarride Complex (western Betic Cordilleras, Spain): new thermochronological and structural data on Los Reales and Ojen nappes Marc Sosson a,*, Anne-Claire Morillon a, Jacques Bourgois b, Gilbert Frraud c, Grrard Poupeau d, Pierre Saint-Marc a " Giosciences Azur,, UMR 6526, CNRS-UNSA, 250 rue Albert Einstein, Sophia-Antopolis, 06560 Valbonne, France h Laboratoire de G(ocl~'namique, Tectonique et Environnement, CNRS-UPMC, Bo~te 119, 4 Place Jussieu, 75252 Paris cedex 05, France Gdosciences Azur, UMR 6526, CNRS-UNSA, Pan: Valrose, 96 Av. de Valrose, 06100 Nice, France ~lLaboratoire de G~odynamique des Cha~nes Alpines, UPRES-A 5025, CNRS, Universit( Joseph Fourier, 15 rue Maurice Gignoux, 38031 Grenoble, France Received 1 October 1995; accepted 15 June 1996 Abstract New thermochronological and structural studies were conducted to quantify the cooling and late exhumation histories of the Internal Zone of the western Betic Cordilleras. The study was carried out in the Ojen-Marbella region where the Ojen and Los Reales nappes of the Alpujarride Complex outcrop. 4°Ar/39Ar single-grain analyses of muscovite and biotite display plateau ages of 19 Ma, on the same rocks. Apatite fission-track datings give ages of 18-16 Ma for the last step of cooling below 110°C, and confined track measurements indicate a very rapid cooling between 110° and <60°C. The combination of both data suggests a fast cooling phase at 19-16 Ma with a gradient higher than 100°C/Ma between 500 ± 50°C and <60°(;. This fast cooling was associated with extensional tectonics accommodating thinning of the metamorphic alpine-type pile of nappes. The exhumation rate ranges from 1 to 3 km/Ma during the 19-18 Ma interval and is close to 1 km/Ma in the 18-16 Ma interval. Moreover an uplift of the Ojen and Los Reales units began after the Early Pliocene (5 Ma). The sediments sealing the strike-slip contact forming the boundary between the Alpujarride Complex and the Neomumidian Formation of Early Miocene age contain Early Pliocene planktonic foraminiferal assemblages (Zone N18). Benthic foraminiferal assemblages of these sediments, actually outcropping at 150 m above sea level, indicate a deposition in the upper bathyal zone ranging from 200 to 600 m water depth. Our conclusions confirm and precise the exhumation processes during the 19 to 16 Ma interval, and uplift of the Internal Zones since 5 Ma. © 1998 Elsevier Science B.V. All rights reserved. Keywords: exhumation; thermochronology; Betic Cordillera; Ar/Ar; fission track * Corresponding author. Tel.: +33-4-9395-4255; Fax: +33-4-9365-2717: E-mail: sosson@faille.unice.fr 0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PIl S0040-1951(97)00274-6