Lateral slab tear tectonics of Calabria (S. Italy): investigating the STEP fault offshore eastern Sicily (the CIRCEE and DIONYSUS seismic surveys)

Abstract:

Subduction and roll-back of narrow slabs is common in the Mediterranean region and is necessarily accompanied by a lithospheric scale slab edge tear fault or “STEP” (Subduction Transform Edge Propagator). We report on two recent marine geophysical surveys conducted in the Ionian Sea, offshore Eastern Sicily to investigate this type of structure: CIRCEE October 2013 R/V Le Suroit and DIONYSUS October 2014 R/V Meteor. The aim of the CIRCEE survey was to map potentially seismogenic faults offshore eastern Sicily and to seek the surface expression of the STEP fault (through high-resolution 72-channel seismic reflection profiles and swath mapping bathymetry). Strong historical earthquakes have struck this region repeatedly, whose origin in some cases remains unknown (1169, 1542, 1693).

Two major crustal scale structures have been proposed as being related to the STEP: the Malta escarpment, and a combined normal-fault and strike-slip-fault system 20-50 km further east, striking roughly N50°W and well imaged by the CIRCEE data. The main objectives of the DIONYSUS deep seismic survey in autumn 2014 are to image the deep structure (crustal thickness, nature of the crust) of this ancient Tethyan age margin (likely a transform margin) and to seek deeper expressions of reactivation (lithospheric scale faulting) related to the slab tear. The internal geometry of the Calabrian subduction zone - the crystalline basement backstop, the slab dip, the accretionary wedge composition (detritic vs. evaporitic) and its thickness, is also a target of the deep seismic survey. To achieve these goals a German-French-Italian wide-angle seismic survey was performed in October 2014 using 60 OBS (30 from Kiel-Geomar and 30 from Ifremer/Univ. Brest) deployed along 4 long profiles, 3 of which are collocated along existing multi-channel seismic lines (Italian CROP profiles) depth processed at Geomar.