GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 601 MEDOC-CHIANTI data revealed a substantially different geological structure. The area thought to be floored by a basaltic crust formed by seafloor spreading (Kasten and Mascle, 1990) in Magnaghi and Vavilov basins appears made of exhumed mantle covered by few hundred meters to ~2 km of sediment and basalt (Prada et al. , 2014, 2015). The exhumation of mantle rocks in the area appears to precede the emplacement of large intrusions of MOR-type fissural volcanism and later intraplate basaltic volcanism (Prada et al. , 2016). Along transect E-F (Fig. 1) and GH, velocity changes laterally abruptly in sediments and basement, indicating crustal thinning toward the center of the basin. Based on velocity values and vertical velocity gradients three domains can be identified: continental crust, magmatic crust and serpentinized mantle. The serpentinized mantle extends in W-E direction for more than 100 km. The absence of a Moho discontinuity under Vavilov and Magnaghi basins is also consistent with the lack of a basal reflection in coincident MCS lines MEDOC 6 and 4 (Fig. 1). In these profiles, Moho reflections terminate abruptly at the location where WAS PmP reflections also stop. Additionally, recent analysis of converted S-waves from MEDOC ocean bottom seismometers (OBS) deployed in the area of mantle exhumation, reported Vp/Vs and Poisson’s ratios within the range of partially serpentinized peridotite, rather than oceanic gabbros/diabase (Prada et al. , 2016), further supporting the presence of exhumed mantle. In the Vavilov basin, a 30-m thick section of serpentinized peridotite was drilled at ODP site 651 under a section of altered peridotite clasts sandwiched between two layers of basalt and Fig. 1 - Shaded relief of bathymetry and topography of the Tyrrhenian region. Black thick and red lines depict the WAS and MCS trancsects respectively, acquired during the MEDOC and CHIANTI experiment. White lines are the location of the other available MCS lines: MS, CROP, SITHERE, CS89. The MS are under reprocessing (Diviacco et al. , 2015). The location of OBS, and OBHs used to acquire WAS data are marked by yellow circles in marine area and by red squares on land. Green poligons show the location of ODP and DSDP sites sampled in the seventies and eighties (Dietrich et al. , 1977; Kastens and Mascle, 1990).