Moho depth and Vp/Vs ratio in peninsular Italy from teleseismic receiver functions

We present a new data set of Moho depth estimates for peninsular Italy, obtained from receiver functions analysis and a grid search approach. Teleseismic records from 174 broadband seismic stations were used to compute the receiver function data set. In general, our results agree well with previous knowledge, adding original information in many regions where previous data were lacking or uncertain. We find broad regions with consistent Moho depth, namely, (1) the peri-Tyrrhenian areas bordering the homonymous sea have a 20 to 25 km deep Moho; (2) the foreland regions (Adriatic, Apulia, Iblean) with 30-35 km deep Moho; (3) the Apennines belt, where we find westward deepening of the Adriatic Moho down to 50 km depth, that well delineate the westward Adria subduction, overlayed by the shallow Tyrrhenian Moho; (4) Calabria shows a flat, 40 km deep Moho suddenly dipping at high angle from the Tyrrhenian coast westward, defining the Ionian subduction; and (5) a clear separation between northern and southern Apennines is marked by a region of almost flat Moho, possible evidence of a detached slab in between. We also find that in the northern Apennines the shallow Tyrrhenian Moho is continuous for more than 200 km from the sea to under the northern Apennines, with a generally flat geometry and a few undulations suggesting either the "boudinage'' or episodic pulses of extension of the Tyrrhenian crust during its Pliocene-Quaternary stretching. The eastward extent of the Tyrrhenian Moho below the Apenninic belt approximately corresponds to the eastward extent of the observed extension in the upper crust as mapped by the normal faulting earthquakes. In this portion of the Apennines, we interpret an intracrustal discontinuity, SW dipping below the belt parallel to the Adria Moho, as the top of the lower crust being subducted beneath the Apennines. The "subcrustal'' earthquakes of this region (down to 80 km depth) seem to be located within this subducted crustal layer. The southern Apennines show the same general geometry of subduction as the northern arc, although the former have a less defined on-land extended region and no earthquakes in the lower crust-uppermost mantle. In between, the central Apennines present a different Moho geometry: no clear doubling of the two plates but rather a constant bilateral deepening possibly associated to a crustal root, with no evidence of a subduction setting. This reinforces the hypothesis of a detached slab in this area. We also present the first large-scale mapping of average crustal V-p/V-s ratios for peninsular Italy. The results, although representing averages over the whole crust, enhance some interesting features. The most evident are belts of high V-p/V-s in the thinned crust of the Apennines where extension has long been active in the Quaternary, probably as an effect of highly fractured rocks with abundant fluid circulation. Strong correlation exists between areas of high V-p/V-s and Quaternary volcanoes, suggesting the presence of fluids at depth.