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

Abstract

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.

QP and QS in the upper mantle beneath the Iberian peninsula from recordings of the very deep Granada earthquake of April 11, 2010

“…Early tomographic images [Blanco and Spakman, 1993] show a high velocity body between 250 and 650 km depth, suggesting a scenario of slab detachment. More recently, Spakman and Wortel [2004] describe the 3-D geometry of a continuous slab extending to 660 km depth, supporting models of westward slab roll-back in the BeticRif-Alboran region [Lonergan and White, 1997;Faccenna et al, 2004;Jolivet et al, 2008]. Ongoing eastward subduction has been proposed by Gutscher et al [2002], however no signature of such a process shows up in the regional seismotectonic patterns and the GPS velocity field [Stich et al, 2005[Stich et al, , 2006.…”

Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece)

“…During extension, the middle and the lower crust of these basins, ductilely deformed, has been exhumed in several metamorphic core complexes separated from the upper crust by detachments. In the MCCs of the Tyrrhenian Sea (Figure 1a), the ductile sense of shear associated to detachments is mainly top-to-the-east [Rossetti et al, 1999;Jolivet et al, 2008]. In the MCCs of the Alboran Sea (Figure 1a), back-arc extension induced the formation of detachments mainly associated to a top-to-thewest kinematics [Augier et al, 2005;Jabaloy et al, 1993;Jolivet et al, 2008;Platt et al, 2006].…”

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

“…However, very rapidly slab rupture occurred below the African margin and it facilitated its tearing in two different directions: towards the present-day Calabrian Arc, creating the Tyrrhenian Basin, and towards the present-day Gibraltar Arc, creating the Algerian Basin (Jolivet et al, 2008;Lonergan and White, 1997;Mauffret et al, 2004;Spakman and Wortel, 2004;Van Hinsbergen et al, 2014).…”

“…The West Mediterranean back-arc basins developed since the Oligocene in relation with the roll-back of the subduction of the Ligurian-Maghrebian Tethys 1 (LMT) (Bouillin, 1986;Doglioni et al, 1999;Frizon de Lamotte et al, 2000;Gueguen et al, 1998;Jolivet and Faccenna, 2000;Jolivet et al, 2008;Lonergan and White, 1997;Malinverno and Ryan, 1986;Réhault et al, 1984;Vergés and Sàbat, 1999;Fig. 1).…”