Alex Maltman,2 Pierre Labaume,3 and Bernard Housen 4


The base of the Barbados accretionary prism is defined by a décollement, which separates material accreting to the Caribbean Plate from underthrusting Atlantic Ocean sediment. A three-dimensional seismic survey has shown the structure to contain intervals of negative polarity, interpreted as representing pockets of overpressured fluid. Consequently, Ocean Drilling Program Leg 156 was designed specifically to investigate the hydrogeological and deformational behavior of the décollement.

Analysis of recovered cores shows the structure to comprise a zone of intensified but heterogeneous deformation, 31 m thick, but 39 m thick if suprajacent breccia and various physico-chemical anomalies are included. The top of the décollement shows a pronounced change in the orientation of magnetic anisotropy, indicating efficient decoupling between the prism and the lower material. Core-scale deformation features consist principally of fracture networks, stratal disruption and, most especially, zones of scaly fabric. On the basis of thin-section and scanning electron microscope/transmission electron microscopy observations, the scaly fabric is fractal-like, with zones of sheared clay wrapping around relatively undeformed lenses, at all scales down to that of the individual particles. The scaly zones, consisting of a combination of a pervasive flattening fabric and distributed slip surfaces, are viewed as S-C structures. These develop as shear strains supplant initial flattening microstructures, which is termed spaced foliation. Continuing shear strain progressively dissects the undeformed lenses to produce an overall zone that becomes more intensely sheared, better defined, and narrower—the difference between drilling Site 949 and the more landward Site 948. Fabric-parallel color changes and mineralized veins testify to the structural influence on fluid migration along the décollement. This takes place in periodic pulses, judging by compound nature of the veins and the inferred intermittent collapse of pores during shear-zone formation. That is, the deformation and drainage of the décollement are highly heterogeneous, both spatially and through time.

1 Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, 156: College Station, TX (Ocean Drilling Program).
2 Institute of Earth Studies, University of Wales, Aberystwyth, Wales, SY23 3DB, United Kingdom. ajm@aber.ac.uk
3 Laboratoire de Géophysique Interne et Tectonophysique, CNRS-Université Joseph Fourier, BP 53X, 38041 Grenoble Cedex 9, France.
4 Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455,U.S.A.