J. Casey Moore 2 and H. Tobin 3


Sediment void ratio of the Barbados accretionary prism and adjacent deposits varies with effective stress and is a basis for estimating long-term variations in fluid pressure. Consolidation tests show that void ratio decreases at differing linear rates with the log of increasing effective stress in carbonate-and clay-rich lithologies from the oceanic reference Site 672 of the northern Barbados Ridge. Using these relationships, we estimate vertical effective stress and fluid pressure (overburden stress minus vertical effective stress) from sites with continuous physical property profiles. The compressive state of stress in the accretionary prism requires correction of effective stress values inferred directly from consolidation tests. We utilize physical property data from both in situ logging-while-drilling (LWD) measurements and measurements on individual core samples. Good correlation between LWD and core sample physical properties indicates the latter are reliable and argues for minimal rebound from in situ conditions.

Estimated fluid pressures are near lithostatic in the décollement zone and proto-décollement zone. Zones of high fluid pressure generally correlate with zones of mud- and mineral-filled veins, suggesting the latter are hydrofractures. Thrust faults tend to occur in intervals of high fluid pressure. Fluid pressures tend to be highest in clay-rich zones. This last correlation could be a result of misrecognition of bound water as porosity, but use of the same techniques to calibrate the consolidation tests as well as to determine the porosity of the samples accounts for this ambiguity. Concentration of deformation in the clay-rich zones may be because of their low coefficient of friction in addition to their high fluid pressure.

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 Earth Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, U.S.A. casey@earthsci.ucsc.edu
3 Department of Geophysics, Stanford University, Stanford, CA 94305-2215, U.S.A.