Evidence for serpentinization of the Ionian upper mantle from simultaneous inversion of P- and S-wave arrival times

Simultaneous inversion of P- and S-wave arrival times, collected during a 3-years Ocean Bottom Seismometer with Hydrophone (OBS/H) monitoring campaign, yields 1D P- and S-wave velocity models for the Ionian lithosphere (Central Mediterranean).The 1D model highlights the presence, in the Ionian upper mantle, of two layers characterized by high seismic P-wave velocity (S1 and S2, 6.3-6.7 and 7.5 km/s, respectively). These two layers, with thicknesses of about 3.3 km and 5 km, respectively, and ranging from similar to 8 to similar to 16 km in depth, are characterized by low S-wave velocity (S1 =3.05-3.2 km/s, S2 = 3.85 km/s) and high values of V-P/V-S (S1 = 2.06-2.09, S2 = 1.95). This is a characteristic feature, often encountered in passive margins and is generally interpreted as partly serpentinized peridotite. The V-P, V-S and V-P/V-S values of S1 are consistent with 55-65% of serpentinization of the upper mantle, while the S2 ones are consistent with 15-25% of serpentinization. This research provides a crucial hint about the debated nature of the Ionian crust, suggesting its oceanic structure. (C) 2016 Elsevier Ltd. All rights reserved.