GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 589 et al. , 1994; Hayward et al. , 1999; Roque, 2007; Neves et al. , 2009; Martinez-Loriente et al. , 2013). Discussion and conclusions. The integrated analysis presented in this study allowed to recognize two horizons that are present in all the seismic profiles, correlated to anAptian condensed sequence, and to a Middle Miocene unconformity in DSDP site 135, south of the Horseshoe Abyssal Plain. The Middle Miocene geometrical unconformity corresponds to the age when Africa started moving towards NW, relatively to Eurasia, and the development of the NE – SW trending thrusts that accommodate convergence. In the seismic sections crossing the Gorringe’s northern flank in the Tagus Abyssal Plain, the presence of a maximum of three wedge – shaped chaotic bodies underlying the abyssal plain sediments at increasing depths was observed (Fig. 2). These units lie above the Middle Miocene unconformity, indicating that at least three large mass failures have occurred since the uplift of the seamount. Moreover, a large mass transport deposit is visible on the Gorringe’s northern flank: the North Gorringe Avalanche displays a scarp almost 20 km long (along strike), and a height drop of ca. 2500 m, for a calculated total volume of 80 km 3 (Lo Iacono et al. , 2012). The presence of the North Gorringe Avalanche supports the hypothesis of the gravitic origin of all the buried chaotic bodies, rather than a tectonic one. Profile BS14 crosses the North Gorringe Avalanche’s deposit; the data were processed with a new flow that includes pre-stack time migration. Although the final seismic image did not improve significantly, iterative pre- stack time migration helped constraining the seismic velocities in different parts of the section, allowing to identify abrupt lateral velocity changes in structures such as faults and chaotic bodies (Fig. 3). These sharp and punctual negative velocity anomalies are probably caused by overpressured fluids circulating along the faults and within the chaotic bodies. Among other factors, such as steep slopes and pervasive fracturing, the presence of overpressured fluids along the faults is a preconditioning factor for the occurrence of large mass failures. Thus it is important to consider the role of large submarine mass failures in generating tsunamis for the assessment of the real hazard for the NE Atlantic coasts of Iberian Peninsula, Morrocco and the Atlantic Archipelagos. Fig. 2 - TGS – 5 seismic section (upper panel) and interpretation (lower panel). The Gorringe main thrust is inferred on the basis of the regional tectonics and stuctural analysis. Two regional markers are visible in red and green: a Lower Aptian condensed sequence and a Middle Miocene unconformity, respectively. The wedge – shaped seismically chaotic bodies are shown in red, orange and yellow, and represent three main submarine mass transport deposits emplaced since the Middle Miocene uplift of the Gorringe Bank. Several backthrusts affect the entire seamount’s interior. See box for location in Fig. 1.