Leg 156

Barbados Ridge Accretionary Prism

Fluids critically affect the structural development and architecture of accretionary prisms and their potential evolution into mountain belts. ODP Leg 156 examined the rates, effects, and episodicity of the structural and fluid processes at one such regime, the Barbados Ridge accretionary prism. Drilling was concentrated on the decollement zone (or plate-boundary fault) beneath the prism where striking variations occur in the polarity of the seismic reflections. Negative-polarity areas were thought to represent high-porosity zones of high fluid pressure, whereas positive-polarity zones were inferred to have normal porosity decreasing with depth. Site 948 targeted the positive seismic polarity areas and Sites 947 and 949 examined the negative-polarity reflections.

Techniques which, prior to Leg 156, had rarely been used in scientific ocean drilling enabled measurements of in situ physical, thermal, and chemical properties at the time of drilling (logging-while-drilling or LWD) and through the emplacement of long-term borehole observatories (CORKs). The LWD program provided continuous resistivity, density, neutron, and gamma-ray logs from the seafloor to total depth, returning detailed records of porosity and compositional variation. Packer experiments were run within the decollement zone and vertical seismic profiles were collected inside the cased holes. An offset VSP run at Site 949 will enable shear-wave polarization studies of crack alignment and thus stress orientation and structural fabrics.

Porosities of approximately 70% were calculated for sediments just above the decollement. To maintain such high porosities at depth, compaction of the sediments must either be arrested during burial or fractures must be created and held open by the presence of high-pressure pore fluids, possibly a low-clorinity fluid influx. The temperature and chemical measurements have shown that dilute, relatively warm fluids under anomalously high pressures flow laterally along the decollement. Thin zones of high porosity and fluid pressures also were observed in regions of positive high-amplitude reflections; these zones do not affect the reflection characteristics because their thicknesses are below the vertical resolution of the 3D seismic survey. Thus, negative-amplitude regions probably represent high-fluid-pressure intervals of significantly larger extent which, to date, have not been successfully cored or logged. High-fluid pressure may exist all along the dÚcollement zone, but the pressure must be of differing magnitudes and potentially vary through time. The decollement zone may also have a differing fluid content.

The CORK deployed at Site 949 included a prototype geochemical sampler to continuously collect fluids from the hole, thus providing a time-series of water specimens with a temporal resolution of approximately two weeks and which, combined with the pressure and temperature records, will enable ongoing study of the temporal variations in fluid flow and composition.