Geodynamic models of Late Oligocene subduction initiation in the Western Mediterannean

The present day tectonic setting of the Western Mediterranean, located in the convergence zone between Africa and Europe, is the result of slab roll back and formation of extensional basins since Late Oligocene- Early Miocene. Despite the important role of subduction in the evolution of the region, little attention has been paid to the mechanisms involved in the initiation of subduction in the Late Oligocene-Early Miocene. Two different scenarios have been proposed: (1) Initiation of subduction at a pre-existing fault along the former Iberian passive margin including Corsica and Sardinia (PM). In this scenario, the pre-existing fault resulted from a previous period of the rollback of the Alpine subduction zone along a STEP (Subduction- Transform Edge Propagator) fault. (2) Polarity reversal of a southeast-dipping subduction zone (PR). According to this scenario, the northwest-dipping subduction zone was initiated in the back-thrust belt of an earlier southeast-dipping subduction zone, which was the extension of Alpine subduction extending southwestward up to the Betics. In this study, we investigate these scenarios using 2-D thermo-mechanical modeling. Mechanical properties of the models are found to be particularly relevant in our study; we use elastic, (power law) viscous, and brittle rheologies to represent temperature and pressure dependent rock behavior. Constraints on our models come from published geological data and paleogeographic reconstructions for the Late Oligocene- Early Miocene. The results of PM model indicate that the main resistive force during subduction initiation process is the elastic bending, which is overcome initially by tectonic forces such as ridge push and eventually by slab pull when subduction reaches the self-sustaining stage. Results show that subduction can be initiated along STEP faults with a shallow dip angle (< 70 degrees). Back-arc extension and formation of extensional basins start soon after initiation ( ~ 4 Myr), and subduction becomes self-sustaining after less than 7 Myr. We find similar results for the importance of the elastic bending resistive force in the PR models. In PR models, we investigate how various back-thrust belt dip angles, the back-thrust/trench distances, and suction forces from the detached slab affect the model results.