Tomographic constraints on the geodynamic evolution of the Italian region

In this paper we present P wave tomographic images of the mantle beneath It aly obtained by inverting similar to 6000 teleseismic P and PKP wave arriva l times,accurately repicked, recorded in the time period 1988-1994 by the s tations of the National Seismic Network of the Istituto Nazionale di Geofis ica. We pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set. The data were invert ed with the well-established Aki-Christofferson-Husebye tomographic techniq ue; different reference models and residuals computation have been tried to verify the stability of the results. The high quality of the repicked arri val times allows us to enhance the definition of the deep structures beneat h both the Alps and the Apennines, looking for their lateral and vertical c ontinuity down to 800 km depth. The main finding of this study is a continu ous high-velocity body located between 250 and 670 km depth beneath the ent ire Apenninic system dipping toward the Tyrrhenian area, which continues up ward segmented in two main anomalies in the northern Apenninic and the Cala brian Arcs. We interpret this high-velocity feature as the subducted oceani c lithosphere between the Eurasian and African plates, dipping down to the upper-lower mantle boundary beneath the Tyrrhenian Sea. The retrieved image s of the lithosphere subducting beneath Apennines are reliable in terms of thickness (about 80-90 km) and P wave velocity contrast (2-4% higher than t he normal mantle). Furthermore, our tomographic images, which focus on the deep geometry and continuity of the velocity structures, provide new keys t o understanding the geodynamic evolution of the Italian region. The segment ation of the high-velocity slab upward suggests a complex evolution of the arc-trench system and the initially continuous subduction of the Ionian-Adr iatic plate progressively developed in subordinate arcs, probably due to la teral heterogeneity of the subducting lithosphere.