Relationships between recent tectonic activity, mud diapirism and slope instability in the Gulf of Squillace (Calabrian Arc, Ionian Sea).

The Calabrian arc, is a 200 to 400 km wide accretionary wedge which connects the Southern Apennine chain, to the North, with the Sicilian-Maghrebian chain, to the Southwest. This wedge is still forming, as testified by seismic activity and by recent sea-bottom morphologies including tectonic lineaments, mud volcanoes and deep-incised canyons. In particular, mud volcanoes and mud diapirs show a strict relationship with the Late Pliocene to Recent tectono-sedimentary evolution of the accretionary wedge (Artoni et al, 2009). In order to better understand their origin and evolution, we focused on the Gulf of Squillace. This latter is located in the northern part of the Crotone-Spartivento forearc basin. The studied area was selected because of the consistent available data base which consists of 10 well logs, 330 km of public seismic reflection lines and about 350 km of higher resolution seismic profiles (Sparker lines “J”, ISMAR CNR, 1971-75). The latter profiles have been extensively analyzed because they clearly show diapiric structures, also having a sea-bottom expression. In the Gulf of Squillace the reconstructed complex structural setting shows the occurrence of a system consisting of transpressive and transtensive lines oriented WNW-ESE, which has been interpreted as the off-shore prosecution of the Catanzaro-Squillace transcurrent system (Tansi et al., 2007). All the available WSW-ENE seismic profiles show a wide basin which is confined to the South by WNW-ESE system of deep rooted normal faults, dipping to the North, associated with antithetic faults developing in the northern side of the basin. These latter faults flatten on shallower layers, interpreted as Late Miocene clays with intervening clastic gypsum intervals. The displacement along the southern master fault is associated to an increase in sediment accumulation, of Late Pliocene to Recent age, arranged in a growth geometry on the hanging-wall and documenting the fault activity during this period. Mud diapirs are mainly grouped in this sector of the basin, characterized by the increase in sediment thickness. These diapirs, in turn, are bounded by the major faults. The WNW-ESE seismic profiles also evidence another normal fault system oriented SW-NE, dipping toward the basin, which cuts the upper and middle slope of the gulf. These faults account for hundred of meters of displacement and are associated with significant gravitative processes. In this frame, the occurrence of mud diapirism is related at least to two favorable conditions: 1) overpressure generated by the asymmetric sediment accumulation on the underlying unconsolidated muds and 2) the propagation of the active normal faults system. This activity, in fact, represents the main trigger for fluid and mud rising up to the seafloor. Mud diapirs could be originated from Late Miocene succession, but also from deeper sources. The progressive extensional deformation associated with mud diapirs significantly influences the morphologic evolution of the slope and of the basin. Deep erosion occurs in canyons, that are aligned along the main faults and in some cases are also laterally bounded by the mud diapirs relief. The retrogressive and landward migration of the canyon heads is also strictly associated to the evolution of normal faults, oriented near parallel to the coastline, and associated to gravitative slope failures.