GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.1 511 The BSR identified along the seismic profile IT-154 (Fig. 1b) that crosses the sedimentary mound resting on the transitional crust of the Ona High (Civile et al. , 2012), shows similar characteristics. The BSR is almost parallel respect to the seabed topography and cross-cut the seismic reflectors of the sedimentary succession of the mound. The BSR amplitude is generated by a high contrast of acoustic impedance that produced a high energy absorption. The interval velocity (red line through the BSR) increases to more than 3000 m/s below the BSR, suggesting an increment of density. The BSR recognized in the NE sector of the mound could be an unconformity. This hypothesis is supported by the presence of onlap terminations of the seismic reflectors against the BSR. The depth migrated seismic profile (Fig. 1d) shows that the BSR is deeper than 500 m below the seabed. It also evidences the real dip of the reflectors, no more affected by pull-up effects (note the different dip of reflector A). Study area 2 - South Shetland Trench. In this area, a methane hydrate related BSR has been already recognized by Lodolo et al. , (2002). The BSR is identified along the seismic profile IT90-A44 (Fig. 2), where it develops for about 20 km, from the South Shetland Platform (SE) to the trench (NW). The cosine of phase attribute (Fig. 2c) highlights its continuity and the overlying sedimentary sequence, which is an accretionary wedge. The velocity analysis showed a decrease of the velocity values below the BSR (red line superimposed). We also observed a decrease of the frequency component below it, mainly below BSR1. Amound shaped structure, about 100 m high above seabed, has been observed through the trench (Fig. 2b). A reverse polarity reflector is located at its base (-100 m from the top) and the central part of the mound is Fig. 2 – a) portion of seismic profile IT90-A44 (location in map d) from the South Shetland Platform (SE) toward the South Shetland Trench (NW) where three BSR have been highlighted. b) zoom on the mound volcano in the depth domain: the polarity inversion could be produced by the top of methane gas accumulation, migrated through the fault system of the South Shetland accretionary complex; c) the cosine of phase seismic attribute better discriminates the three segments of BSR; the example of the interval velocity function (red line) shows a drop of velocity below the BSR2.