Ocean Drilling Program (ODP) Leg 156 revisited the northern Barbados Ridge, where the previous Deep Sea Drilling Program Leg 78A and ODP Leg 110 studied the frontal part of this accretionary prism. Drilling and logging-while-drilling at Sites 947, 948, and 949 successfully identified major thrust faults and the décollement, which was the target of several downhole experiments. Two of the eight holes drilled were equipped with borehole observatories that will monitor temperature, pressure, and fluid flow over the next years. Coring at Hole 948C recovered 180 m of sediment, centered around the décollement, which was positively identified based on structural information. The aim of this study is the evaluation of the possible correlation of preferred orientation of acoustic properties and the direction of maximum compressive strain in the frontal part of the accretionary prism. For this purpose, shipboard P-wave velocities from Holes 948C and 949B were reoriented. This information was then used to compare the directional properties of accreted and subducted sediments. In Hole 948C, lowest transverse velocities (Tmin ) were observed to be consistently oriented perpendicular to the maximum horizontal compressive stress, believed to be parallel to the convergence vector. In the underthrust domain of Hole 948C, several preferred orientations for Tmin were detected, but no correlation with the geotectonic reference frame could be identified. Acoustic anisotropy does not show a comparable pattern in Hole 948C. It is concluded that the observed directional dependence of P-wave velocity in the accreted sediment domain in Hole 948B is the result of moderate to steeply inclined bedding, although this conclusion can not adequately be tested due to the lack of corrected structural data.
Proceedings of the Ocean Drilling Program, Scientific Results
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The Proceedings of the Ocean Drilling Program are published in College Station, TX.
Housen, Bernard A.; Bruckmann, W.; and Moran, K., "Directional Properties of P-wave Velocities and Acoustic Anisotropy in Different Structural Domains of the Northern Barbados Ridge Accretionary Prism Complex" (1998). Geology. 32.