No abstract

Transects of the submersible Alvin across rock outcrops in the Oregon subduction zone have furnished information on the structural and stratigraphic framework of this accretionary complex. Communities of clams and tube worms, and authigenic carbonate mineral precipitates, are associated with venting sites of cool fluids located on a fault-bend anticline at a water depth of 2036 meters. The distribution of animals and carbonates suggests up-dip migration of fluids from both shallow and deep sources along permeable strata or fault zones within these clastic deposits. Methane is enriched in the water column over one vent site, and carbonate minerals and animal tissues are highly enriched in carbon-12. The animals use methane as an energy and food source in symbiosis with microorganisms. Oxidized methane is also the carbon source for the authigenic carbonates that cement the sediments of the accretionary complex. The animal communities and carbonates observed in the Oregon subduction zone occur in strata as old as 2.0 million years and provide criteria for identifying other localities where modern and ancient accreted deposits have vented methane, hydrocarbons, and other nutrient-bearing fluids.

A new tool has been developed and successfully deployed that provides a means by which cased reentry holes drilled by the Ocean Drilling Program can be hydrologically sealed and instrumented. The seal prevents flow of water into or out of a hole that would otherwise cause severe thermal and chemical disturbance to the formation drilled. The seal is capable of withstanding both positive and negative differential pressures that may be present in hydrothermally or tectonically active environments, and can be removed for later drilling operations. The instrumentation, developed for initial deployments during Ocean Drilling Program Leg 139, was designed to monitor the formation temperatures and pressures for up to 2 years during and following the recovery from drilling disturbances. A recording gauge measures absolute pressure in the borehole below the seal. A sensor string containing 10 thermistors and a fluid-sampling tube hang in the hole below the recording package. The sampling tube extends through the seal to a port, allowing the differential pressure to be measured and fluids to be tapped from deep within the hole. Data recovery and fluid sampling can be done via submersible or remotely operated vehicle.Two units were deployed in the sediment-filled Middle Valley rift of the northern Juan de Fuca Ridge. The first was installed in Hole 857D, which was drilled to a total depth of 936 meters below seafloor, through 470 m of turbidite sediment and into a highly permeable sediment-sill complex that is inferred to be a sediment-sealed hydrothermal "reservoir." The second was deployed in Hole 858G, which was drilled to a total depth of 433 meters below seafloor into a buried volcanic edifice that underlies a hydrothermal vent field. Data were recovered successfully from both holes about three weeks after deployment. Temperatures and pressures recorded in Hole 858G show that formation conditions are severely affected by drilling and by downward fluid flow into the formation through a nearby uncased exploratory hole that was not adequately sealed with cement. The differential pressure across the seal in Hole 858G was initially -0.24 MPa, and at the time of the data recovery was becoming increasingly negative. Temperatures and pressures recorded in Hole 857D appear to be recovering slowly from drilling disturbances. Initially, the differential pressure across the seal in this hole was -1.10 MPa; at the time of the data recovery, the differential had reduced to -0.55 MPa. The instruments will continue to monitor formation pressures and temperatures once per hour for 2 years.

Abstract. Accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence. Compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration. These latter processes, in conjunction with fermentation or thermal maturation of entrained organic matter, yield hydrocarbon-bearing pore fluids that are expelled from the prism.

Diatomaceous mudstones at depth under the deep sea terrace and the trench inner slope off Japan have been variably affected by tectonic stress. Veins, healed fractures, and microfaults occur at all sites except the shallow Site 435 on the upper trench inner slope and Site 436 on the Pacific Plate. Veins, fractures, and faults occur in cores from below 620 meters (lower to middle Miocene) in the landward sites (438 and 439) on the deep sea terrace, and are probably related to normal faulting seen in seismic records. The depth to "consolidated" sediment and to the first occurrence of veins and healed fractures shallows progressively toward the trench. The intensity of deformation also appears to increase seaward. However, no sediments younger than upper Pliocene are deformed. Open fractures may exist in situ at Sites 434 and 441 at levels between about 150 and 500 meters sub-bottom. The Japan Transect sediments-in contrast to deposits in the zone of initial deformation at other convergent margins-though highly deformed, are not highly overconsolidated. However, sediment at depth in the trench inner slope sites is overconsolidated relative to that at the same depth in the landward reference site. None of the deformed Japan margin sediments recovered at Legs 56 and 57 sites originated by accretion of oceanic plate material-also in contrast to sediments at some of the margins previously studied. We suggest that tectonic stress related to convergence has been communicated to the slope sediments on the trench inner slope, either continuously or periodically, causing rapid tectonic dewatering and inducing fracturing and faulting. If episodic, the latest of these deformational periods may have occurred during the late Pliocene. The faults and fractures are either rehealed by continued overburden pressure (sediment loading) or may remain open at shallower levels. Fracturing and dewatering of semiconsolidated sediment beneath an unconsolidated but impermeable mud veneer may cause overpressured zones at depths of 200 to 500 meters. These overpressured zones possibly locally reduce shear strength and cause downslope mass movement of sediment, even on low-angle slopes on the trench inner slope.