The backstop between the Mediterranean Ridge and western Peloponnese, Greece: its crust and tectonization. An active seismic experiment with ocean bottom seismographs

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 in3) air gun array tuned at low frequencies. Data were compiled as Common Station Gathers (CStGs) and were evaluated by first break tomography and layer tomography combined 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 profiles varied between 60 and 180 km, with seismic station spaced between 2.5 and 5.5 km inline, and shoots fired at 125 m. The crust from western Peloponnese to the Mediterranean Ridge is continental of variable thickness, while west of the backstop the crust is of oceanic origin, 5.5 to 6 km thick. South Zakynthos has Moho depth of nearly 28 km, thinning in the Kyparissiakos basin to 22 km, and south of Messinia Peninsula to 21 km. At the collision front between the backstop and the Ionian oceanic crust at the continent/ocean transition of the Mediterranean Ridge, the subducted oceanic crust was also mapped, adding 5.5 to 6 km to the crustal thickness. Sediments in most parts of the profiles range from 6 to 8 km in the Alpine domain and in the backstop between 4 and 6 km, having maximum thickness at the collision front of the Mediterranean Ridge. Vp velocities of soft sediments above the high velocity Alpine limestone have values of 1.8, 2.9, 3.8 and 4.8 km/s, while the Alpine limestones range from 5.6 to 5.8 km/s, and the pre-Apulia (Paxos) limestones and sediments, below the Alpine napes, Vp wave speeds of 6.0 to 6.2 km/s. The two limestones are separated by Triassic evaporites with Vp ~ 4.2 km/s. Alpine limestones do not exist in the backstop domain, which is exclusively built by pre-Apulia limestones and sediments, overlying a stretched and thinned continental crust. Faults mapped in the Kyparissiakos basin are the continuation of onshore structures of western Peloponnese. We could identify the westernmost limit of the Ionian zone and the extend of the pre-Apulia backstop that widens from north to south, from 20 to 40 km west of Zakynthos and Strophades Island to more than 80 km SW of Messinia Peninsula. Deformation of the sedimentary sequence at the collision front of the Mediterranean Ridge, between the Ionian oceanic crust and the thin continental crust at the backstop, is very intense, and the backstop, particularly at its western side, is fractured by several major faults.