Abstract
Detailed structural and stratigraphic analysis of a three-dimensional seismic
data set reveals two scales of structural fabric in the westward dipping oceanic
basement under the toe of the northern Barbados accretionary prism. The
large-scale fabric consists of three NNE trending structural highs with relief
of about 300 m and spacing of 5-6 km. The small-scale structure is
characterized by a series of NNE trending normal faults, resulting in horsts and
graben with relief of 50-200 m and spacing of 0.2-2 km. The structural fabric
at both scales is preexisting. Structures in the underthrust sediment are
mostly secondary, having formed by differential compaction. Three distinctive
seismic stratigraphic units are defined in the underthrust sedimentary section.
The upper Cretaceous to lower Eocene unit mostly fills in basement lows,
resulting in great thickness variations. The thickness of the Oligocene and
middle to upper Eocene units is less variable. We estimate that the upper two
sedimentary units have lost 25% of their total initial void space owing to the
load of the accretionary prism within the first 3.5 km landward of the thrust
front. This suggests a current fluid expulsion rate of 1008 m3/yr per kilometer
of strike length from the toe. The fluid discharge is expected to increase to
1092 m3/yr within 64,000 years as a thicker sediment section is subducted. Our
results also suggest that the basement indirectly controls fluid movement in the
underthrust Oligocene unit by creating secondary normal faults that act as major
fluid conduits between the overlying dcollement and the underlying, more
permeable middle to upper Eocene turbidite-bearing section. Ponded sediments
between basement horst blocks are not faulted and may compact more slowly.
Reprinted by permission of the American Geophysical Union.