Thomas H. Shipley,2 Gregory F. Moore,3 Harold J. Tobin,4 and J. Casey Moore 5


A three-dimensional seismic reflection experiment has revealed a distinctive distribution of waveforms and amplitudes associated with the décollement reflection in the Barbados Trench. One objective of Ocean Drilling Program Leg 156 was to calibrate this seismic reflection response, which was thought to be related to variations in fault zone properties. Site 948 is characterized by a normal polarity reflection that is correlated to the décollement. Sampling and logging reveal a 40-m-thick fault zone, with density that increases across the zone as the clay mineralogy changes from smectite rich to illite rich downhole. A synthetic seismogram, derived from the logging data, shows that the lithology-driven density shift is responsible for generating the normal polarity reflection even though the log, packer, and long-term pressure measurements confirm the presence of abnormally high pressures. The objective of Site 947, to characterize the more common, high-amplitude negative polarity waveform, was not completed due to technical difficulties. However, a density log, run at the beginning of the cruise, did reveal a 14-m-thick 1.6 g/cm3 layer 540-554 m below seafloor (mbsf). The bit reached within 60 m of the décollement, and the density log went to 565.5 mbsf. The low-density layer correlates with a fault splay off the décollement, having similar seismic characteristics to that of the negative polarity décollement and can serve as an analogue. Although it is unsatisfying not to have directly calibrated the décollement at Site 947, the fault splay revealed characteristics that are consistent with modeling done before the cruise that required a 12- to 14-m-thick low impedance layer to account for the observed waveform. These very low densities are probably created by introduction of additional fluids and fluidized sediments during hydrofracturing, which are maintained by lithostatic pressure. The requirement for extremely low density indicates that a >12-m-thick lithostatic pressured hydrofractured zone occurs within the thicker décollement. The variations in the amplitude of the negative polarity waveform relate mainly to the amount of enhanced fluid content. The mapped pattern of amplitudes suggests a broad northeast-trending channel that possibly focused fluid transport along the décollement. However, numerous fault splays intersect the décollement, which may cause barriers that prevent rapid fluid flow along the décollement.

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 for Geophysics, University of Texas, Austin, TX 78759-8397, U.S.A. tom@utig.ig.utexas.edu
3 Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822-2285, U.S.A.
4 Department of Geophysics, Stanford University, Stanford, CA 94305-2215, U.S.A.
5 Earth Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064-0001,U.S.A.