GNGTS – Atti del 20° Convegno Nazionale / 01.13 G. Lavecchia, P. Boncio and N. Creati GeoSis-Lab, Dipartimento di Scienze della Terra, Università, Chieti IS THERE REALLY NEED OF A SUBDUCTION MODEL TO EXPLAIN DEEP SEISMICITY AND ACTIVE TECTONICS IN CENTRAL ITALY? During the last 30 years, the development of Apennine fold-and-thrust belt has been almost unanimous interpreted within a subduction-related geodynamic context. The deep (up to about 80 km) seismicity recorded in the northern Apennines has been considered one of the evidences for a west-dipping subducting slab (Selvaggi, 2000). In contrast, the occurrence of intra-Apennine Pleistocene melilitite and carbonatite igneous centers (Stoppa and Woolley, 1997), which are globally associated with continental rift-environment, has been considered irreconcilable with a subduction mode (Lavecchia and Stoppa, 1996). In this paper, we analyze the geometric and rheologic relationships between the deep seismicity and the lithospheric tectonic structure in order to verify if a model different from the subduction one might be proposed. We plotted available seismological databases (Selvaggi and Amato, 1992; Collettini et al.,1997; Boncio et al.,1999) onto two regional geologic sections coinciding with the trace of the Piombino-Ancona Deep Seismic Soundings profile (DSS 78, Ponziani et al., 1995) and of the Punta Ala-Gabicce Near Vertical Reflection seismic profile from (CROP 03, Barchi et al., 1998). The hypocentral distribution of analyzed earthquakes projected along the traces of the CROP 03 and DSS 78 profiles defines a seismic strip approximately 30 km wide and 100 km long, deepening westward at about 30° and reaching a maximum depth of about 70 km. The seismogenic volume is located along and beneath the outermost SW-dipping reverse shear zone imaged by the CROP 03 profile, which dislocates the base of the crust and merges out along the Adriatic border. An along-dip non-homogeneous distribution of the seismicity is evident: hypocenters are concentrated within the upper crust (5-10 km), at the top of the lower crust (20-25 km) and around the Moho discontinuity (35-40 km); two major seismic gaps are observed from 10 to 20 km and from 25 to 35 km. The along dip alternation of seismic and aseismic zones well fit with here calculated strength profiles. In conclusion, the northern Apennine deep seismicity may reflect an on-going reverse shearing of the Adriatic lithosphere and, in our opinion, does not show any link with a subduction plane. Furthermore, the cutoff of the seismicity at a depth of about 70 km is well compatible with the “step-like” geometry of the lithosphere-asthenosphere boundary beneath central Italy proposed by Lavecchia et al. (2001). One of the problem arising from an interpretation which does not admit a subduction plane beneath central Italy may regard the genesis and the orogenic engine of the Apennine fold-and-thrust belt. An answer to the question may be found in Lavecchia et al. (1995) and Decandia et al. (1998), who showed that the Tyrrhenian rifting may create horizontal forces capable to generate the Apennine fold and thrust belt and to justify the Adriatic lithosphere flexure.