Morphotectonics of the Gorringe Bank summit, Eastern Atlantic Ocean, based on high-resolution multibeam bathymetry
Ferranti Luigi 1 , Passaro Salvatore 2 , de Alteriis Giovanni 2
(1) DiSTAR - Dipartimento di Scienze della Terra, delle Risorse e dell’Ambiente, Università Federico II, Napoli, Italy.
(2) IAMC-CNR, Istituto per l’Ambiente Marino Costiero, Calata Porta di Massa, Napoli, Italy 7
Quaternary International 332 (2014) 99e114
High-resolution swath bathymetry of the Gorringe Bank, located along the Europe-Africa obliquely convergent plate boundary offshore SW Portugal, allowed to investigate its shallow-depth morphotectonic structure as a contribute to understanding the interplay of sea-level changes and active deformation of the region. Dominant ENE-WSW to NNE-WSW, and subordinate NW-SE trending morphologic alignments were mapped on the Digital Terrain Model (DTM) built from multibeam data for both the Gettysburg and Ormonde summits of the bank. Based on appraisal of regional structural and seismological data, the dominant and subordinate sets are interpreted to reflect dip-slip to left-oblique reverse faults, and normal faults or dyke-like joints, respectively. Several terrace remnants were mapped on the DTM as well and were grouped into five orders at depths ranging from ~40 to ~120 m. Morphological features suggest that the terraces were formed in a coastal environment likely during Middle-Late Pleistocene sea-level changes. The terraces are probably polycyclic implying minor vertical tectonic motion. However, the whole terrace set but the deeper and younger one, which was mainly shaped during the LGM lowstand, is consistently up to ~10 m lower at Ormonde in the east relative to Gettysburg in the west, suggesting displacement across a NNE-SSW striking fault mapped in the saddle between the two seamounts. The structure assemblage is argued to represent the shallow splay system of a crustal-scale fault ramp underlying the bank. The ENE-WSW reverse faults have a southward polarity and, together with the overall northwest tilt of the bank, points to displacement toward the Horseshoe Abyssal Plain to the south at least at upper crustal levels.
Key-words: Multibeam bathymetry; Morphotectonics; Submerged coastal terraces; Gorringe Bank; Atlantic Ocean
Legenda: Tectonic setting of the SW Iberian margin. Bathymetry data from GEBCO08 Digital Atlas (www.nbi.ac.uk). GB = Gorringe Bank; MPB = Marquês de Pombal Bank. Base of ocean-continent boundary (OCB, thick gray line) after Gonzalez-Fernandez et al. (2001) in the south and Rovere et al. (2004) in the north. Faults (dashed where inferred or where different interpretations are given) modified after Tortella et al. (1997); Hayward et al. (1999); Borges et al. (2001); Gracia et al. (2003); Zitellini et al. (2009); Iribarren et al. (2007); Rosas et al. (2009); Terrinha et al. (2009). Fault labels: GF = Gorringe Fault; HF = Horseshoe Fault; MPF = Marquês de Pombal Fault. Earthquake epicenters from the USGS NEIC Catalog (http://www.neic.cr.usgs.gov) for the period 1973–2004, hypocenter depth of 0–70 km. Focal mechanisms of crustal and lithospheric earthquakes after Grimison and Chen (1986), Buforn et al. (1988), Moreira (1991), Ribeiro et al. (1996), Mezcua and Rueda (1997), Pondrelli et al. (1999), Borges et al. (2001), Buforn et al. (2004), Stich et al. (2005), and Catalogues IGN-Madrid, MedNet Meredian, and Regional and Harvard CMT. Epicenter of the 1755 Lisbon earthquake (diamond) from the USGS NEIC. Inset shows the plate-kinematic setting of the central part of the eastern Atlantic ocean; convergence rates between Africa and Eurasia after Serpelloni et al. (2007)..