THE STRUCTURE AND RECENT DEFORMATION OF THE EXTERNAL CALABRIAN ARC (ACCRETIONARY WEDGE): IMPLICATIONS FOR REGIONAL HAZARD ASSESSMENT

Subduction of the Ionian Sea lithosphere beneath Calabria and E Sicily has shaped the evolution of Southern Italy since 5 Ma. We present reprocessed 96-channel seismic reflection data acquired during the PRISMED survey (1993), and ARCHIMEDE survey (1997), both onboard R/V Le Nadir crossing the Ionian Abyssal Plain and the deep offshore portions of the Calabrian accretionary wedge (“External Calabrian arc”).

Our seismic data provide clear images of the the toe of the accretionary wedge (from the Ionian abyssal plain across the deformation front). Here, the base of the Messinian evaporites serves as a weak detachment above which the entire Messinian salt and the overlying Plio-Quaternary (PQ) sediments are accreted. Repeated imbricated thrusting and some back-thrusting within the Calabrian wedge allows a doubling of the thickness of the salt layer within 30 km of the deformation front. Pre-stack depth migration of seismic line PM01 allows deeper imaging of the entire frontal wedge (post-Messinian) as well as the boundary to the older internal Calabrian wedge (pre-Messinian in age).

To the east of the Malta Escarpment (East Sicily), another major tectonic structure is imaged by the Archimede profiles. The structure offsets the top of the Pre-Messinian deposits and the underlying units by 0.5 - 1 sTWT increasing from S to N. This N150°E oriented lithospheric scale fault is interpreted as a tear fault (“STEP” fault) which has allowed the roll-back of the Ionian slab. Syn-tectonic sedimentation of Plio-Quaternary units above this fault suggest ongoing deformation related to this tear fault. Together with folding of Plio-Quaternary strata at the toe of the wedge, including Augias mega-turbidite dated at 3500 BP, this suggests ongoing activity of the subduction system. Available GPS data suggest slow eastward motion (3-4mm/yr) of Calabrian stations in a Nubia fixed reference frame, consistent with slab roll-back and capable of driving E vergent deformation in the accretionary wedge. Slip along the shallow NW dipping subduction fault plane offers a possible explanation for the 1693 Catania earthquake and tsunami. The seismogenic potential of the subduction zone and a better understanding of typical earthquake magnitudes and recurrence intervals bear strongly on hazard assessment for Southern Italy.