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The first in situ measurements of permeability and fluid pore pressure along the décollement zone (a low-angle detachment fault) between the North American and Caribbean Plates were completed during Ocean Drilling Program Leg 156. Measurements of properties within the fault zone were attempted at two sites using a drill string packer. Interpretation of the packer data was complicated by variations in the baseline formation fluid pressure during the tests, as well as temporal and spatial variations in formation properties. These measurements may be interpreted to reflect a consistent relationship between bulk permeability and vertical effective stress and between bulk permeability and the modified pore-pressure ratio. Measured bulk permeabilities from the most reliable tests varied from 6 × 10 -16 m 2 to 1 × 10 -13 m 2 , whereas vertical effective stress varied from 1.7 to 0.1 MPa and the modified pore-pressure ratio varied from 0.45 to 0.95. These bulk permeability values and trends are consistent with a variety of independent, less direct estimates. These trends may help to explain the transient nature of prism dewatering and fluid flow along faults in this setting.

In situ bulk permeability was measured in a borehole that intersected the decollement zone (a low-angle detachment fault) between the North American and Caribbean plates. Permeability measurements were made at a variety of fluid-pressure conditions, defining a quantitative relation between bulk permeability and effective stress for this plateboundary fault zone. The bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure. This relation may help to explain the dynamics of fluid-fault interactions and the transient nature of hydrologic processes during deformation at convergent margins.

In situ and laboratory studies of permeability, conducted by Ocean Drilling Program scientists from Leg 156, provide constraints on parameters controlling the hydrogeologic system in the Barbados accretionary prism. Results from these studies indicate that core-scale and formation-scale permeability values differ by at least several orders of magnitude and are dependent on pore-fluid pressure and effective stress conditions. Direct measurement from packer experiments and indirect evidence from consolidation tests suggest that pore-fluid pressures are commonly above hydrostatic values and approach lithostatic values within the décollement zone. Permeability and fluid pressure conditions in the Barbados accretionary prism reflect the complexity of the hydrogeologic system of such an active tectonic environment.