Seismically Inferred Fault Zone Properties and Fluid Migration along the Barbados Ridge Decollement

Nathan L. Bangs, Tom H. Shipley, and Gregory F. Moore

Three-dimensional seismic reflection data acquired in a 5 × 25 km area of the toe of the Barbados Ridge accretionary complex reveal the amplitude and waveform associated with the reflection from the decollement plate boundary fault. Reflections are analyzed using computer-automated modeling of amplitudes and waveforms developed to quantitatively measure reflection coefficients and examine fault zone physical properties related to the behavior of pore fluids migrating along the decollement.

Modeling shows that the decollement reflection is usually caused by a thin low-velocity, low-density (and presumably high porosity) layer. A map of layer thickness indicates smooth thickness variations from 8 to 20 m over 500 m distances and trends that parallel the strike of the Barbados Ridge. In a 2x6 km region of high amplitude, a 100-200 m/s low-velocity anomaly and a corresponding low-density anomaly is inferred in the fault zone from reflection coefficients at the top of the layer. The map patterns of reflection properties reveal km-wide areas of connected high-porosity, and presumably high-permeability zones. The magnitude of the reflection coefficients at the base of the layer reveal connected km-wide areas where the amplitude is equal to that at the top of the layer and othe areas where it is only half as large. This variation may indicate differences in the history of fluid movements through the fault zone. The fault probably develops near-lithostatic fluid pressure in the km-wide high-amplitude areas. The high fluid pressure maintains a weak fault and dilates the fault zone allowing focussed flow along a permeable fault plane.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California