DSDP Leg 78A and ODP Legs 110, 156, and 171A focused on the northern flank of the Tiburon Rise. A principal target has been penetration of the décollement zone or plate boundary fault. Here the décollement zone is relatively shallow, and the dominantly hemipelagic-pelagic sedimentary section offers good drilling conditions and good biostratigraphic resolution. This décollement zone (or basal detachment or sole thrust) separates the deformed thrust wedge above from the undeformed sedimentary rocks or basement below (e.g., Twiss and Moores, 1992). Thus, the décollement zone is the surface of maximum structural disharmony beneath the accretionary wedge or prism. We call it the décollement zone because it is of a finite thickness. Structural studies show that the décollement zone becomes better defined in a landward direction and is a shear zone up to 33 m thick at Sites 671 and 948 (Maltman et al., 1997; Shipboard Scientific Party, 1988a; 1995). Anomalies in pore-water chemistry (Gieskes et al., 1990; Kastner et al., 1997) and temperature (Fisher and Hounslow, 1990b) indicate focused fluid along a décollement zone and in subjacent sand layers. Models simulating this fluid expulsion from the prism suggest that the flow is transient (Bekins et al., 1995). The décollement zone is characterized by suprahydrostatic fluid pressures (Labaume et al., 1997; Screaton et al., 1997; Zwart et al., 1997).
A 3-D seismic reflection survey (Moore et al., 1995a; Shipley et al., 1994) has greatly improved the interpretation of drilling results from the northern Barbados accretionary prism (Figs. F2, F3). The décollement zone commonly coincides with a distinct and locally high-amplitude seismic reflection. Using the 3-D seismic image, the stratigraphic horizon that the décollement zone occupies can be extended seaward, east of the deformation front, as the proto-décollement zone (Fig. F3). The seismic survey also shows patches of positive and negative seismic polarity in the décollement zone (Fig. F2) (DiLeonardo et al., in press; Shipley et al., 1994). These polarity signatures may signify differing fluid regimes and stress states along the décollement zone (Bangs et al., 1996; Bangs et al., 1999; Shipley et al., 1994; Shipley et al., 1997). Determining the physical properties that define these polarity signatures was a major goal of Leg 171A.