Slope Instability on the French Guiana Transform Margin from Swath-Bathymetry and 3.5 kHz Echograms

Abstract : Although transform margins represent ~30% of rifted margins around the world, few studies have investigated mass-movement processes in such areas and their links with this specific structural context. The French Guiana transform margin and adjacent Demerara abyssal plain have been surveyed during the GUYAPLAC cruise, collecting multibeam bathymetric data, backscatter imagery, 3.5 kHz echograms and 6-channel seismic profiles. The study area is divided into three domains: the shallow Demerara plateau, the Guiana slope and rise, and the Demerara abyssal plain. The Demerara plateau displays multi-scale slope instabilities from huge deep-seated collapses of the whole margin to surficial creeping folds and recent slumps. Giant elongated pockmarks have been also observed for the first time in this area. Fluid escape is common everywhere on the plateau and probably enhances slope instability. On the Guiana slope and rise, large stacked lobate masses have been identified testifying to repetitive failure events. Fluid escape is also ubiquitous there, suggesting a dewatering of debris flows due to sediment loading. Two main types of sedimentary structures are observed on the Demerara Abyssal Plain: small meandering channels of the Amazon Fan at its eastern edge and sediment waves at its western edge, along the foot of Demerara continental slope.