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Fission-track thermochronology of the Oman Mountains continental windows, and current problems of tectonic interpretation

¹ Laboratoire de Géodynamique et Thermochronologie, Faculté des Sciences Aïn Chok, BP 5366 Maârif, Casablanca, Maroc, e-mail: o.saddiqi{at}fsac.ac.ma
² Laboratoire de Géologie, UMR 8145 CNRS, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris cedex 05, France, e-mail: michard{at}geologie.ens.fr
³ CRPAA, UMR 5060 CNRS, Maison de l’Archéologie, Université Michel-de-Montaigne, Domaine Universitaire, 33607 Pessac cedex, France
⁴ Institut für Geowissenschaften, Postfach 601553, D-14415 Potsdam, Germany

The Oman Mountain belt offers a unique case study of an Alpine orogen where blueschist- and eclogite-facies metamorphism developed in continental rocks beneath an ophiolite in the absence of continental collision. We present hereafter the first systematic, fission-track (FT) study of the subophiolitic Jebel Akhdar and Saih Hatat windows in the central-southern Oman Mountains. Nineteen samples out of 28 yielded significant zircon and/or apatite FT results. All of the 9 samples from NE (internal) Saih Hatat, where metamorphism occurred at ca. 300°C < T < 550°C (low-grade blueschist to low-grade eclogite facies), yielded FT zircon ages at 66-70 Ma. Four samples from SW Saih Hatat (front of the Fe-Mg carpholite zone, ca. 280°C < T < 300°C) yielded 69-93 Ma dates, and 4 samples from Jebel Akhdar (ca. 250°C < T < 280°C) yielded 80-96 Ma dates, which reflects partial resetting of the zircon FT chronometre at T ca. 280°C. As eclogite metamorphism of the deepest Saih Hatat units is dated at ca. 80 Ma [Warren et al., 2003], we deduce an early cooling rate of 30°C/Ma for these units. Late thermal evolution is constrained by apatite FT length optimization analysis and includes, i) cooling under 60°C until 60-58 Ma (rate close to 10°C/Ma); and ii) heating over 80°C at ca. 20-8 Ma.

The Oman Mountains tectonic building is currently accounted for by contrasting models involving one or two subductions. Taking into account the structural and geophysical data on the Saih Hatat metamorphic pile, we interpret the reported thermal evolution in the framework of the classical, one subduction model: i) Permian rifting and Mesozoic spreading; ii) late Cretaceous intra-oceanic subduction followed by continental margin subduction/ophiolite obduction; iii) late Late Cretaceous-Paleocene synorogenic exhumation of the subducted units through extrusion within the subduction channel (relative extension), then isostatic rebound and extensional inversion of the obduction thrust; and iv) Neogene compression resulting in rejuvenation of the basal thrust beneath the continental windows, which are considered to be parautochthonous.

Key Words: Oman • Obduction • HP-LT metamorphism • Fission-tracks • Exhumation