GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 Structural features appear to be the primary controller of the NE-SW and NNW-SSE high-absorption trends across the bay (De Rosa et al., 1995; Maestro et al., 2007) (Figure 1a). Lopes et al. (2015) generated a tectonic map highlighting a Riedel deformation model. At 1 km node spacing, the widest high-absorption anomaly within the bay has an orientation between NS and NNW-SSE, coinciding with the Hero Fracture Zone, almost orthogonal to the extensive dynamics of the Strait of Bransfield. The NE-SW anomaly coincides with submarine faults of similar strike, whose downthrow focuses on the central part of the anomaly (Figure 2c). The anomalies at 2-km node spacing show a sigmoidal shape at both frequencies, but particularly at 15 Hz, stretching in a direction roughly parallel to the regional distension (Figure 2c, orange line). This sigmoidal shape is interpreted as the signature of how the caldera subsided (Lopes et al., 2015). At 6 Hz (thus at greater depths ( ∼ 1 km)) the NNE-SSW high-absorption axis is parallel to Riedel's R fractures in the system proposed by Lopes et al. (2015). The spatial correlation between high absorption and tectonic features thus suggests that the high-absorption anomalies detect the shallower product of deeper tectonically-controlled reservoirs within the bay. The preferential NW-SE and NNE-SSW high-absorption orientations also coincide with the spatial variations of specific hydrothermal outputs, whose location is controlled by tectonics (Alvarez-Valero et al., 2020; De Rosa et al., 1995) (Figure 2d). There is extensive evidence of the dependence of seismic attenuation and absorption on thermal variations at the mantle, crustal, and volcano scales (De Lorenzo et al., 2001, Abers et al., 2006; Cammarano & Romanowicz, 2008;), with high thermal potential the most likely explanation for absorption increases at locations Aa1-Aa3. The NE-SW and NNE-SSW high-absorption trends correspond to the measured variations in Mn and As, respectively (Rey et al., 2002; Somoza et al., 2004) (Figure 2d). At 15 Hz (shallower depths) the orientation of the high-absorption anomaly coincides with the axis of maximum concentration of Mn. This signature is related to the interaction between the sediments and fluids emitted from the submarine volcanic vents, which also develop NE-SW (Marti et al., 1996; Somoza et al., 2004). At 6 Hz, the NNW-SSE high-absorption signature aligns to the maximum distribution of As (Figure 2d). High As values are associated with water-rock interactions more than magma (Somoza et al., 2004), providing further evidence that magma storage is likely deeper than our investigation volume, and absorption anomalies are primarily controlled by tectonics. Fumarolic centres are the most superficial expression of a deeper hydrothermal system. Gaseous components such as helium and carbon dioxide in fumaroles come from the underlying magma and are released to the surface through faults, and fractures (Kusakabe et al., 2009). The fumaroles distribute consistently in regions of high absorption (Rey et al., 2002) (Figure 2d, stars). The source of these fluids is the complex network of individual shallow magma storage zones with variable volumes, compositions and sizes, and placed at depths approximately between 10 and 2 km b.s.l. (Geyer et al., 2019). The detected high-absorption anomalies represent areas where fluids at high temperatures uprise, migrating preferentially through faults and fractures located at the border of the primary high-absorption volumes (hot-fluids reservoirs).