STRAIN DECOUPLING ACROSS THE DECOLLEMENT OF THE BARBADOS ACCRETIONARYPRISM

The interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate, particularly the possibility tha t overpressuring within the basal decollement may enable mechanical de coupling of the prism from the subducting material, Anisotropy of magn etic susceptibility (AMS) data from sediments spanning the basal decol lement of the Barhados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured. In the accretionary prism, above the decollem ent, the minimum AMS axes are subhorizontal and oriented nearly east-w est, whereas the maximum AMS axes are oriented nearly north-south and shallowly inclined, At the top of the decollement, the minimum AMS axe s orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediment s below, The AMS orientations in the prism sediments above the decolle ment are consistent with lateral shortening due to regional tectonic s tress, as the minimum axes generally parallel the convergence vector o f the subducting South American plate and the maximum axes are trench- parallel. Because the orientations of the AMS axes in deformed sedimen ts usually parallel the orientations of the principal strains, the AMS results indicate that the incremental strain state in the Barbados pr ism is one dominated by subhorizontal shortening, In contrast, the AMS axes within and below the decollement are consistent,vith a strain st ate dominated by vertical shortening (compaction), This abrupt change in AMS orientations at the top of the decollement at Site 948 is a dir ect manifestation of mechanical decoupling of the off-scraped prism se diments from the underthrust sediments, The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of fl owing, high-pressure fluids.