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Subduction, convergence and the mode of backarc extension in the Mediterranean region

¹ Laboratoire de Tectonique, UMR 7072, Université Pierre et Marie Curie, T 46-00 E2, case 129, 4 Place Jussieu, 75252 Paris cedex, France. laurent.jolivet{at}lgs.jussieu.fr
² Institut des Sciences de la Terre d’Orléans, Bâtiment Géosciences, BP 6759, 45067 Orléans cedex 2, France
³ Dipartimento di Scienze Geologiche, University of Roma Tre, Largo San Murialdo 1, 00146 Rome, Italy.
⁴ Institut de Géologie et d’Hydrogéologie, Université de Neuchâtel, 11 rue Emile Argand, CP 158, 2009 Neuchâtel, Suisse
⁵ Schlumberger SRPC, Well Integrity Technologies, 1 rue Becquerel, BP 202, 92142 Clamart cedex, France.
⁶ Geosciences, UMR 4661, Université de Rennes I, 263 Avenue du Général Leclerc, 35042 Rennes cedex, France
⁷ Departamento de Geodinámica, Instituto Andaluz de Ciencias de la Tierra, Universidad – CSIC, Fuentenueva s/n, 18071 Granada, Spain

30-35 Ma ago a major change occurred in the Mediterranean region, from a regionally compressional subduction coeval with the formation of Alpine mountain belts, to extensional subduction and backarc rifting. Backarc extension was accompanied by gravitational spreading of the mountain belts formed before this Oligocene revolution. Syn-rift basins formed during this process above detachments and low-angle normal faults. Parameters that control the formation and the kinematics of such flat-lying detachments are still poorly understood. From the Aegean Sea to the Tyrrhenian Sea and the Alboran Sea, we have analysed onshore the deformation and P-T-t evolution of the ductile crust exhumed by extension, and the transition from ductile to brittle conditions as well as the relations between deep deformation and basin formation. We show that the sense of shear along crustal-scale detachments is toward the trench when subduction proceeds with little or no convergence (northern Tyrrhenian and Alboran after 20 Ma) and away from the trench in the case of true convergence (Aegean). We tentatively propose a scheme explaining how interactions between the subducting slab and the mantle control the basal shear below the upper plate and the geometry and distribution of detachments and associated sedimentary basins. We propose that ablative subduction below the Aegean is responsible for the observed kinematics on detachments (i.e. away from the trench). The example of the Betic Cordillera and the Rif orogen, where the directions of stretching were different in the lower and the upper crust and changed through time, is also discussed following this hypothesis.

Key Words: Mediterranean tectonics • Backarc basins • Post-orogenic extension • Exhumation • Slab retreat • Ablative subduction • Basal shear