Apennines tectonics as caused by lithosphere delamination
The deep structure of the crust and uppermost mantle, along with the intermediate-depth earthquakes, are consistent with a progressive delamination and foundering of the Adria lithosphere beneath the Apennines. Such a process was proposed in mid-1990 on the basis of broad low velocity and high attenuation in the Apennines mantle revealed by the propagation of Pn waves (Mele et al., 1996; 1998). It was competing with a model of Adria subduction, which was anchored on the positive velocity bodies diving in the mantle recognized by teleseismic tomography (Lucente et al., 1999, Piromallo and Morelli, 2003) and the presence of "sub-crustal" earthquakes (Selvaggi and Amato, 1992). The recent improvement of the seismological observational networks and the availability of a huge set of earthquake data (Castello et al., 2005) yielded high-resolution velocity and attenuation models of the uppermost mantle (Di Stefano et al., 2009; Chiarabba et al., 2009a; Piccinini et al., 2010). These studies localize the negative velocity and high attenuation zones in the mantle above a narrow stripe of deflected and dehydrating Adria crust. Furthermore, new teleseismic model (Giacomuzzi et al., 2010) shows pronounced low velocities in the Adria uppermost mantle. Thus, can such weak continental lithosphere subduct? On the other hand, intermediate-depth earthquakes only testify for down-dragging, and dehydration, of crustal material, compatible with both delamination and subduction processes. All these evidences may be explained by delamination and foundering of the Adria continental edge, with complexity and lateral irregularities that follows those of the original Tethys margin (figure 9).
The existence of a negative buoyancy between a continental lithosphere and a warm, hydrated and mobilized asthenosphere is required for delamination (Meissner and Mooney, 1998; Leech, 2001). In the Apennines, this condition is generated by a weakening of the Adria mantle, as a consequence of the Paleogene subduction. The negative velocity anomalies recently found by teleseismic tomography beneath Adria is evidence of such weakening (see Giacomuzzi et al., 2010).
The difference in the deep structure between the northern and central portion of the Apennines (figures 7 and 8) implies that the length of delaminated lithosphere is variable along the belt, also at short distances (hundred of kilometers). This is mirrored by the irregular front of compression and change in dip of the regional monocline (see Mariotti and Doglioni, 2000). We hypothesize that pre-existing variation of rheological properties along the Adria continental margin and a different timing, or velocity, of delamination causes these lateral irregularities. The delamination seems to be at a progressively younger stage moving from south to north, suggesting that the recent fast subduction and retreat of the Ionian slab in the southern Tyrrhenian region have favoured and accelerated the process. The Apennines delamination can be seen as a terminal process of evolution in the continental lithosphere at the margin of the subduction of the Ionian ocean.
In the northern Apennines (figures 7 and 9a) the delamination is ongoing in a restricted portion encompassing the Mt. Conero, promoting compression in the external area, deep rooted in the basement, and extension in the belt. The active foundering is testified by the intermediate depth seismicity well defining the narrow strip of dehydrating crust.
In central Apennines (Abruzzi, figures 8 and 9b), the foundering of the Adria lithosphere seems to be absent or ceased since the lack of intermediate-depth earthquakes and the marked negative velocity anomaly in the mantle (see also Chiarabba et al., 2009a). This evidence, along with the external position of the thrust front recently proposed by Scrocca (2006), might be consistent with a southward increase of delamination in the continental edge, i.e., greater toward the still subducting Ionian oceanic slab. The asthenosphere mobilization on top of the delaminated crust may be the cause of strong uplift and sustained topography invoked by D’Agostino and Mckenzie (2000) based on free air gravimetric anomalies, and consistent with a mature delamination (England and Houseman, 1989).