Can local tomography settle the matter about subduction in the central and northern Apennines?

Abstract

The many studies conducted on the Italian area led to several models to explain the present-day structural setting. Some of the most debated questions are the presence or not of continuous subduction and the presence or not of a slab detachment in the northern or in the central part of the Apenninic chain. The absence of a continuous, high velocity body beneath the Apennines has been interpreted by some researchers as an evidence of the detachment of the Apenninic slab. According to this view the Apenninic slab is expected to be inactive whether the Ionian lithosphere subducting underneath Calabria is considered to be on the verge of detaching or just detached. Other researchers however, suggest that a fairly continuous and fast slab exists beneath the Apennines and the Calabrian arc. Different geodynamical models have also been proposed for the Tyrrhenian area considering it as an active or as a passive margin. Our working group has conducted several seismic tomographies in the search of the geometry, size and extension with depth of the subduction under the Italian peninsula. While the images resulting from teleseismic data were clearly showing a subducting slab under the Calabrian arc, they were not conclusive for the rest of the Apennines since they were showing, only in the Northern sector, a likely subduction in the shallower part apparently detached from other high velocities body in the deeper zone. At that stage it was not possible to distinguish between thrust and subduction due to the poor horizontal resolution of the applied methodology. In order to analyze in more details this apparent discrepancy, a new seismic local tomography has been conducted with a very dense grid, the selection of a smaller area to be investigated (limited to the Apennines only) and the addition of new data: all these features contributed to partly improve the results, which cannot anyway extend beyond the maximum depth of seismic events. The main limitation in this kind of experiment is the lack of seismic events deeper than 60-70 km under the northern and central Apennines although, as many authors assume, is not itself an evidence against subduction. Analyzing different cross sections of the enhanced resolution tomography results, we do not see any slab in the northern-central Apennines in the first 80-100 km depth. The downgoing material (Adriatic plate) of this area has a rather low dip angle, as also partly shown by the distribution of the (few) deep seismic events. Along the central and also the northern part of the Apennines there are more overlapping than subducting geometries.