GEOMETRY OF SUPRA-PRISM BASINS OF SOUTHER N BARBADOS WEDGE

A stratigraphic pattern study of supra-prism basins located in the sou thern part of the Barbados wedge was performed using original oceanogr aphic data. These consisted of seismic reflection profiles using both air gun and high resolution (3.5 kHz) techniques and side scan sonar i mages. Three sites with different structural location within the accre tionary complex were surveyed. The migration of the depocentres of pig gy-back basins and the geometrical relationships between the thrust fa ults and the syntectonic deposits show that three different deformatio ns, with distinct scales, control evolution of frontal basins. (1) A r apid tilting (about 1-2 degrees/40 000 years), strictly localised at t he back limb of anticlinal ridges. This deformation has been attribute d to migration of active axial surfaces during fault-bend fold propaga tion. (2) A complex activation of the major thrust system, at a kilome tric scale: blind thrusts corresponding to frontal propagation develop seaward under abyssal plain whereas reactivating of thrusts occurs ba ckward. Thrust reactivating results either in a whole displacement fro m decollement zone to the surface or in a passive re-adjustment linked to the imbrication of new frontal sheets. (3) A long scale (greater t han 10 km) arcward tilting that leads to backward migration of piggy-b ack basins depocentres. This deformation could be linked to the bendin g of the oceanic crust driven to the subduction zone or could be induc ed by staircase thrust trajectory. In the inner areas of the prism, th e influence of mud diapirism increases, partly in response to very thi ck sediments and tectonic disturbances induced by fault reactivating. The superimposition of clay diapiric structures to deformations linked to tectonic accretion and oceanic basement ridges leads to complex mo rphologies characterised by steep topographic features. Finally, this study outlines the influence of oceanic floor topography on the evolut ion of the wedge. Oceanic basement ridges act as barrages to sediment transport and consequently have a strong control on depth of the decol lement, spacing of the anticlinal ridges and size of piggy-back basins and lead to the development of transfer faults and offsets of the def ormation front. Finally, this study shows that stratal pattern of supr a-prism basins varies with : (1) lateral depth variations of tectonic decollement, (2) inherited structures of oceanic basement, (3) inner o r frontal location within the accretionary complex. The displacement a nd evolution of oceanic wedges follow the predictions of both numerica l and analogue modelling : (1) new imbricates are accreted at the fron t, (2) out-off sequence reactivations occur along the nearest thrusts located backward.