249 Bollettino di Geofisica Teorica ed Applicata Vol. 55, n. 2, pp. 249-279; June 2014 DOI 10.4430/bgta0125 The backstop between the Mediterranean Ridge and western Peloponnese, Greece: its crust and tectonization. An active seismic experiment with ocean bottom seismographs J. MAKRIS 1 AND J. PAPOULIA 2 1 University of Hamburg, Germany 2 Hellenic Centre for Marine Research, Institute of Oceanography, Athens, Greece (Received: March 1, 2011; accepted: March 11, 2014) ABSTRACT Five wide aperture seismic lines were recorded with ocean bottom seismographs offshore western Peloponnese, between the Island of Zakynthos and the Gulf of Messinia. Four lines were ENE-WSW oriented, and one N-S crossing three of them. Two of the ENE-WSW lines extended from western Peloponnese to the Mediterranean Ridge, from the continental domain of western Greece to the oceanic of the Ionian Sea. Seismic energy was generated by a 48 l (~ 2976 in 3 ) air gun array tuned at low frequencies. Data were compiled as Common Station Gathers (CStGs) with two-point ray tracing forward modelling. The Vp-velocity models obtained by this procedure were further used to depth migrate the CStGs. Line length of the to the Mediterranean Ridge is continental of variable thickness, while west of the Moho depth of nearly 28 km, thinning in the Kyparissiakos basin to 22 km, and and the Ionian oceanic crust at the continent/ocean transition of the Mediterranean at the collision front of the Mediterranean Ridge. Vp velocities of soft sediments 6.2 km/s. The two limestones are separated by Triassic evaporites with Vp ~ 4.2 continental crust. Faults mapped in the Kyparissiakos basin are the continuation of onshore structures of western Peloponnese. We could identify the westernmost limit collision front of the Mediterranean Ridge, between the Ionian oceanic crust and the © 2014 – OGS