Introduction
The geodynamic evolution of Italy and the Mediterranean has been controlled by the Mesozoic Tethyan rifting and the later Tertiary subduction zones. The Permian-Mesozoic rifting developed separating Europe from Africa, and possibly some smaller intervening microplates. At the same time, toward the east, the Cinmerian NE-directed subduction zone was operating in the eastern Mediterranean-Asian area. The rifting in the Mediterranean evolved on a lithosphere previously thickened by the Paleozoic orogens. The stretching of the lithosphere locally evolved to oceanization in some areas of the Mediterranean (Ligure-Piemontese, Valais, Pennidic zone, eastern Mediterranean), and generated articulated continent-ocean transitions (i.e., passive continental margins) with undulated shapes. During the early Cretaceous, the Alpine subduction initiated to transfer some oceanic basins underneath the Adriatic-African plates. Collision possibly started during the Eocene. At a later stage, possibly during the Eocene-Oligocene, the opposite Apennines subduction initiated along the retrobelt of the Alps where oceanic or thinned continental lithosphere was present. The Alpine and Apennines orogens involved the passive margin sequences into their accretionary prisms. Most of the oceanic lithosphere was consumed into subduction, and only some small fragments of oceanic suites have been sandwiched into the belts. The oceanic crust formed during the expansion was almost totally assimilated in the subduction and dragged deep in the mantle, although it is possible to see its fragments involved in the collision now present in the Western Alps and North-western Apennines as ophiolites. The crust involved in the collisional process got thicker and thicker, uplifting for isostatic adjustment, thus starting the Alpine orogenic process. The deformation in the Alps started first in the Adriatic margin (African promontory). The European continental margin and the related sedimentary prism deformed last, thrusting over the European foreland to complete the generation of the Alpine orogen. At the same time, there were other active subductions, like the north-eastward directed subduction of the Adriatic plate below Eurasia (along the Dinarides) and westward subduction below the Apennines. In this belt, the slab retreat generated a different style of orogen, with a rather lateral growth, lower topography, shallower decollement and deeper foredeep. In the hanging wall of the Apennines slab, a backarc basin opened to form the present western Mediterranean basins. During the backarc rifting, since about 20 Ma, a fragment of the European margin detached from the continent and, rotating counterclockwise by about 60°, moved the Sardinia-Corsica block south-eastward to its present position. Behind this movement the crust thinned, forming the Algerian-Provencal basin. Then, since about 15 Ma, to the east of the Sardinia-Corsica block, a new tensional process caused the opening of the Tyrrhenian basin (Gueguen et al. 1997; Carminati et al., 2004). These rotations and expansions modified and completed the structure of the Apennines chain. Evidence of these processes are the Calabrian-Peloritan arc’s reliefs, whose “Alpine” rocks represent a relict of alpine chain, “boudinated” and dragged inside the accretionary prism of Apennines during the eastward migration of the subduction hinge. The rifting accompanied the eastward retreat of the Apenninic subduction zone, and the slab retreat kinematically requires a contemporaneous eastward mantle flow, regardless of whether this is the cause or a consequence of the retreat (Doglioni et al., 1999). Therefore, the Italian and Mediterranean mantle should have recorded this long history with relevant anisotropies. However, the debate about the geodynamic evolution of the Mediterranean area is still open.
The advantage of joining geophysical models with geological and petrological data in order to understand the complex evolution of Italic region is discussed in Panza et al. (2007a) which presents a reliable model of the Tyrrhenian Sea. The study described, for the first time, a very shallow crust–mantle transition and a very low S-wave velocity (VS) just below it, in correspondence of the submarine volcanic bodies (Magnaghi, Marsili and Vavilov) indicating the presence of high amounts of magma. Panza et al. (2007b) integrated crustal geological and geophysical constraints with the VS models along the TRANSMED III geotraverse. As a result a new model of the mantle flow in a backarc setting, which first reveals an easterly rising low-velocity zone (LVZ) in the active part of the Tyrrhenian basin, is obtained, thus an upper mantle circulation in the Western Mediterranean is suggested. This upper mantle circulation, mostly easterly directed, affects the boundary between upper asthenosphere (LVZ) and lower asthenosphere, which undulates between about 180 and 280 km. An explanation for the detected shallow, very low velocity mantle in the non-volcanic part of Tyrrhenian region is presented in Frezzotti et al. (2009). These anomalous layers were generated by the melting of sediments and/or continental crust of the subducted Adriatic-Ionian (African) lithosphere at temperatures above 1100°C and pressure greater than 4 GPa (130 km). The resulting low fractions of carbonate-rich melts have low density and viscosity and can migrate upward forming a carbonated partially molten CO2-rich mantle in the depth range from 130 to 60 km. Carbonate-rich melts upwelling to depth less than 60-70 km induce massive outgassing of CO2 that can migrate and be accumulated beneath the Moho and within the lower crust.
This article analyzes the central Mediterranean mantle through surface waves tomography and the comparison of crustal and mantle structure with seismicity. The systematic inversion of recent damaging earthquakes that have occurred in the Italic region is performed through a powerful non-linear technique named INPAR (Guidarelli and Panza, 2006, and references therein). The space distribution of such events is analyzed and related to the different rheologic-mechanic properties of the upper and lower crust and uppermost mantle. This led to a drastic relocation and change of mechanism of a major event of the Umbria-Marche 1997 seismic sequence with respect to the solution reported by Centroid Moment Tensor (CMT) and Regional Centroid Moment Tensor (RCMT) bulletins, providing new constraints to the geological setting and local stress field.
We contribute to the debate on the geodynamical evolution of the Italic region with a cellular model of the lithosphere-asthenosphere system of the area in terms of VS distribution with depth (VS-depth) structure. The model presented in this work is constrained with independent studies concerning Moho depth (Dezes and Ziegler, 2001; Grad and Tiira, 2008; Tesauro et al., 2008), seismicity-depth distribution (ISC, 2007), VP tomographic data (Piromallo and Morelli, 2003) and other independent information as heat flow (Hurting et al., 1991; Della Vedova et al., 2001) and gravimetric anomaly (ISPRA, ENI, OGS, 2009) data.