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