Seismic structure and the active Hellenic subduction in the Ionian islands
Résumé
In the region of the Ionian Islands of western Greece, the active margin of the Hellenic domain passes from oceanic subduction in the south to continental collision in the north, linked by the right-lateral Cephalonia transform fault. A slightly landward dipping interface revealed at 13 km depth by a single previous line in the channel between Cephalonia and Zante has been suggested as the interplate subduction boundary. New marine multichannel reflection profiles and OBS refraction and wide-angle reflection data confirm the reflector as a regional feature. These data evidence its extension to the south, where large, low-angle thrust earthquakes occur offshore to Zante. The new profiles establish a coincidence between the focal depths of these large subduction events and the imaged bright reflective level, confirming its tentative interpretation as the interplate boundary, which generally appears with a positive reflection polarity. In this context, the Ionian Islands outcrop corresponds to a shallowing of the interplate boudary from south to north. In the south, offshore Zante, the interplate boundary comprises a stratified zone that may be considered as the sedimentary cover of the Ionian Basin oceanic-like crust, which forms the lower plate here. The shallower position and single-cycle reflection character of the interplate further north suggest that the lower plate could there be the Apulian paleomargin to that basin.