Part of the Geology Commons

Anisotropy of magnetic susceptibility (AMS) results from sediments spanning the basal décollement of the Barbados accre-tionary prism show a striking progression across this structure that strongly supports the hypothesis that it is strongly overpres-sured. In the accretionary prism above the décollement, the minimum AMS axes are subhorizontal and nearly east–west trending, whereas the maximum AMS axes are nearly north–south trending, and shallowly inclined. At the top of the décolle-ment, the AMS minimum axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the décollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the décollement are consistent with lateral shortening caused by regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American Plate, and the maximum axes are trench-parallel. This abrupt change in AMS orienta-tions at the top of the décollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sedi-ments from the underthrust sediments. The decoupling horizon occurs at the top of the décollement zone, coinciding with the location of flowing, high-pressure fluids. Comparison with magnetic fabrics and susceptibilities of the seaward reference site (Site 672) indicates that the AMS fabrics at Sites 948 and 949 record the orientations of neocrystallized (Ti)magnetite and o