Benoît Loubrieu

Evidence for Quaternary convergence across the North America- South America plate boundary zone, east of the Lesser Antilles

New seismic and bathymetric data reveal that the plate boundary zone between the North American and South American plates, east of the Caribbean, has occupied a >200-km-wide zone of localized basement uplift and folding, and of faulting of the sedimentary cover, during the early Pleistocene to Holocene. This zone, which includes the Barracuda Ridge and Tiburon Rise, exhibits north-south compressional structures and continues eastward up to 500 km from the boundary of the Caribbean plate. A sequence of young turbiditedominated sediments fi lls the basin between these two ridges and shows synsedimentary deformation; the base of the sequence is of early Pleistocene age, dated by biostratigraphic correlation with Deep Sea Drilling Project Site 27. Recent deformation in the turbidite-fi lled trough north of the Barracuda Ridge is shown by uplift of the seafl oor at the crests of anticlines, across which the shallowest sediment layers thin or pinch out.

Mass Movements in a Transform Margin Setting: The Example of the Eastern Demerara Rise

The eastern Demerara Rise located offshore French Guiana was surveyed in 2003 (GUYAPLAC cruise, part of the French EXTRAPLAC program) using multibeam bathymetry and imagery, 6-channel seismic data and 3–5 kHz echosounding. Analysis of seismic data shows that the flank of the Demerara Rise endures repetitive sliding of its Paleogene to Neogene sedimentary cover towards the ocean. Fluid escapes seem to be closely associated with the activity of those slides and deep faults seems to impact the location of the main headscarp. We suspect fluid overpressures and the specific architecture of transform boundaries (“free border”) to be key parameters in the development of wide MTD’s retrogressively eroding the eastern Demerara Rise.

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

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.