Morphotectonics of the Gorringe Bank summit, eastern Atlantic Ocean, based on high-resolution multibeam bathymetry

Abstract

High-resolution swath bathymetry of the Gorringe Bank, located along the Europe–Africa obliquely convergent plate boundary offshore SW Portugal, allowed investigation of its shallow-depth morphotectonic structure as a contribution 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 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 (except 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, point to displacement toward the Horseshoe Abyssal Plain to the south at least at upper crustal levels.