Anisotropy of magnetic susceptibility (AMS) results from sediments spanning the basal décollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is strongly overpressured. 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 décollement, 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 orientations at the top of the décollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments 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 or (Ti)maghemite, and so reflect decoupling of differential stresses (and perhaps also strains) at the top of the décollement. Further comparisons of susceptibility stratigraphy between sediments just above the lithostratigraphic Unit III/Unit II boundary at Sites 672 and 948 suggest that the lower portion of the structurally defined décollement at Site 948 may in fact be largely intact. This suggests that (1) there may be little displacement accommodated by sediments below about 498 mbsf; (2) the deformation structures observed in most of the décollement may have formed via low total strains (but perhaps under high strain rates?); and (3) the basal décollement of the Barbados prism is a narrow plane (490-492 mbsf), along which stresses are very effectively decoupled, rather than a thick zone of distributed deformation.
Proceedings of the Ocean Drilling Program, Scientific Results
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Housen, Bernard A., "Magnetic Anisotropy of Barbados Prism Sediments" (1997). Geology Faculty and Staff Publications. 33.