DYNAMIC-MODELS OF SUBDUCTION - GEOPHYSICAL AND GEOLOGICAL EVIDENCE INTHE TYRRHENIAN SEA

Predictions based on a 2-D finite-element model for subduction underne ath the Calabrian Are in southern Italy are compared with a variety of geophysical and geological data, such as the present-day stress patte rn within the slab, uplift from the elevation of marine terraces in Ca labria and subsidence in the Tyrrhenian Marsili Basin from ODP Leg 107 . We model the behaviour of the slab driven by slab pull, in agreement with the present tectonic style in this part of the Mediterranean as suggested by several investigators. The model accounts for the crustal , lithospheric and mantle structures in a vertical cross-section perpe ndicular to the Calabrian subduction zone. The shape of the slab is co nstrained on the basis of new tomographic images in the southern Tyrrh enian Sea, which were obtained from the regional seismic stations of t he Istituto Nazionale di Geofisica, while the rheological properties o f the mantle are taken from global dynamic models. Density contrasts b etween the subducted slab and the surrounding mantle, based on petrolo gical models, drive the flow in our viscoelastic model; stress values, displacements and vertical velocities at the surface are sampled at d ifferent times after loading until dynamic equilibrium is reached. Our estimates are appropriate for a time window of 100 kyr; the validity of our comparison with the geological record is based on the assumptio n that the tectonic configuration in the past was not substantially di fferent from that of the present day. Two families of models, with unl ocked and locked subduction faults, are considered. The unlocked model s allow for roll-back of the trench of about 20 mm yr(-1), in agreemen t with some geological estimates; the same family of models predicts u plift of the Calabrian Are of about 1 mm yr(-1) and subsidence in the Marsili Basin of 1-2 mm yr(-1), in agreement with geological surveys. The deviatoric stress obtained from the unlocked model is consistent w ith the continuous distribution of deep seismicity in the southern Tyr rhenian Sea, with minor concentration within the lithospheric wedge. L ocked models fail to reproduce these geophysical and geological observ ations. Predictions derived from a detached slab model are not consist ent with the continuous hypocentral distribution of deep seismicity in the southern Tyrrhenian Sea. Deformation at the surface and the stres s patterns at depth for a detached slab differ substantially from thos e of a continuous plate: dynamic topography and horizontal motions are reduced, when compared with the continuous plate, with deviatoric str esses concentrated within the relict slab. Our results indicate that s ubduction is a major tectonic process in the southern Tyrrhenian Sea.