Tomographic constraints on the geodynamic evolution of the Italian region

In this paper we present P wave tomographic images of the mantle beneath Italy obtained by inverting similar to 6000 teleseismic P and PKP wave arrival times,accurately repicked, recorded in the time period 1988-1994 by the stations of the National Seismic Network of the Istituto Nazionale di Geofisica. 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 inverted with the well-established Aki-Christofferson-Husebye tomographic technique; different reference models and residuals computation have been tried to verify the stability of the results. The high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the Alps and the Apennines, looking for their lateral and vertical continuity down to 800 km depth. The main finding of this study is a continuous high-velocity body located between 250 and 670 km depth beneath the entire Apenninic system dipping toward the Tyrrhenian area, which continues upward segmented in two main anomalies in the northern Apenninic and the Calabrian Arcs. We interpret this high-velocity feature as the subducted oceanic lithosphere between the Eurasian and African plates, dipping down to the upper-lower mantle boundary beneath the Tyrrhenian Sea. The retrieved images 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 the normal mantle). Furthermore, our tomographic images, which focus on the deep geometry and continuity of the velocity structures, provide new keys to understanding the geodynamic evolution of the Italian region. The segmentation of the high-velocity slab upward suggests a complex evolution of the arc-trench system and the initially continuous subduction of the Ionian-Adriatic plate progressively developed in subordinate arcs, probably due to lateral heterogeneity of the subducting lithosphere.