Variations in ultrasonic velocity and density with pore pressure in the décollement zone, northern Barbados Ridge accretionary prism

Abstract

In a recent three-dimensional multichannel seismic reflection survey, the decollement zone of the northern Barbados Ridge accretionary prism exhibits both negative and positive-polarity, high-amplitude fault plane reflections. The negative-polarity regions of this reflector have been interpreted as a response to elevated fluid pressure with attendant low velocity and densit y in the fault zone. Here, we use core samples from above, within, and below the decollement at Site 948 on Ocean Drilling Program Leg 156 to evaluate the effect of varying fluid pressure (or effective pressure) conditions on the P-wave velocity and density of the samples. At Site 948, the decollement zone exhibits a positive-polarity fault plane reflection probably caused by a shift to higher density across a lithologic boundary coincident with the decollement zone. The density distribution observed in the borehole and velocities consistent with inferred fluid pressures suggest a positive impedance contrast. The negative-polarity waveform of th e reflector below Site 947 requires a low-impedance zone superimposed on this lithologic boundary. Overpressures above estimates for hydrostatic or ambient conditions at Site 948 cause a velocity decrease of ~100 -200 m/s, and a percent porosity increase of up to ~4%. The minimum acoustic impedance measured for the upper part of the decollement is ~2.83 × 10 6 Mg/m 2 s at near-zero effective pressure. Published synthetic seismic models for examples of the negative-polarity reflections require reflection coefficients that imply much lower impedance in this interval. We conclude that significant hydraulic dilation of th e fault at lithostatic fluid pressure is necessary to generate the observed reflectivity.