Déformation active permanente induite par le méga-chevauchement dans l’arc antillais: Apport des complexes récifaux quaternaires

The Lesser Antilles arc is located along the Northeastern edge of the Caribbean plate, submitted to the slow and oblique convergence (2 cm/yr) of the North and South American plates. This subduction is thought to be seismically little active, only two mega-thrust earthquakes (Mw>8) having occurred since the 19th century. Consequently, this subduction is still poorly known. Here, we propose to get information on this plate interface behavior and to assess its seismic potential by studying long-term over-riding plate deformations, witness of inter-plate processes. For this purpose, we acquired new very high-resolution marine geophysical data (bathymetry, imagery, seismic reflection profile, etc) in 2009 along the active volcanic arc (GWADASEIS campaign) and in 2010 (BATHYSAINTES campaign) in Les Saintes earthquake area (21/11/2004). This new dataset allows to observe and quantify two types of deformations along the Lesser Antilles arc: * Faulting within the over-riding plate and especially normal faulting along the active volcanic arc from Saba to St Lucia. We especially studied the segments responsible for the Mw 6.3 earthquakes of Montserrat-Redonda (16/03/1985) and Les Saintes (21/11/2004). From Redonda segment to Les Saintes one, active 30-40 km long normal faults are arranged in right-stepping echelons, indicating sinistral motion. Les Saintes segment geometry indicates a southward propagation of the fault system. Faults and volcanic centers geometries imply a tectonic control on volcanic complexes emplacement, faulting initiation being certainly no older than 1 Ma. Offsetting volcanoes and reef complexes, faults are transtensional to the North and slip at few mm/yr whereas they are essentially normal to the South and slip at few tenths of mm/yr. At the scale of the Lesser Antilles arc, in this peculiar slip-partitioning context, this change in strain accommodation indicates that Marie-Galante graben, an arc-perpendicular structure extending from the forearc to Basse-Terre volcanic island, plays a key role. * Vertical deformations of the over-riding plate on the basis of Quaternary reef terraces morphology. Reefs being markers of sea level, their morphology and structure over the long-term result from the interplay between eustatism and tectonics. The Lesser Antilles outer arc islands present on land uplifted reef terraces that are tilted westward, this is confirmed here by the morphology of Marie-Galante submarine reef platform and its modeling. Along the inner active arc, no terraces outcrop. Thanks to the high-resolution data, we investigated Les Saintes and Martinique drowned reef plateaus in order to describe their morphology and structure. Both platforms are overlain by Holocene reefs, drowned at 45 m bsl in Les Saintes and influenced by Mount Pelée activity in Martinique. The Holocene terraces cap piled-up reef terraces, separated by surfaces of erosion. This “layer-cake” morphology is indicative of subsidence, estimated at few tenths of mm/yr since 125 ka. These scenarios are tested and constrained by reef growth modeling of Les Saintes plateau on the basis of its morphology. The Lesser Antilles arc is thus tilted westward and subsides along the volcanic arc. As it is generally assumed that over-riding plate does not accumulate deformation due to seismic loading over the long-term, we discuss local to regional processes responsible for this long-term deformation pattern (subducting ridges, local faulting, underplating, etc). Finally, these observations could suggest that the Lesser Antilles subduction zone is mechanically coupled over long-term, being the site of deep processes along the plate interface. In this case, its seismic potential should not be under-estimated.