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.

We present 206 new heat flow measurements in the Indian Ocean. These and approximately 300 previously published heat flow values are individually evaluated for sedimentary environment and instrumental performance. The relationship between average heat flow and age is found to be little affected by selection of the most reliable experiments, although the scatter about the mean is significantly lowered. The variation of mean heat flow with age is found to be very similar to that in the eastern Pacific and Atlantic oceans: There is a crestal low heat flow zone with large variability, a transition zone within which the heat flow increases from values considerably below to values in agreement with predictions from thermal models of the oceanic lithosphere, and a region where heat flow values are in accord with theoretical predictions. However, the transition zone occurs over different crustal ages from ocean to ocean: 40-60 m.y. in the Indian Ocean, 4-6 m.y. in the Galfipagos spreading center, 10-15 m.y. on the East Pacific Rise, and 50-70 m.y. on the Mid-Atlantic Ridge. The transition zone generally corresponds to a sea floor age where (1) sedimentary thickness increases to >300 m, (2) sea floor roughness is significantly smoothed by sediment blanketing, and (3) the carbonate content of surface sediments decreases to <40%. The transition zone occurs where water circulation in the oceanic crust stops affecting the surface heat flow strongly. There are two possible explanations for the transition. First, a change in composition from carbonate to siliceous sediments results in a decrease in bulk permeability. This combined with general thickening of the sedimentary blanket with aging results in the deposition of an impermeable layer which prevents the convective exchange of heat from the oceanic crust to the ocean. Second, hydrothermal flow within the oceanic crust is plugged by filling of circulation cracks in the oceanic crust. The fact that in several basins of the Indian Ocean the heat flow transition corresponds with the carbonatesiliceous boundary is support for the former mechanism. However, the fact that locations of increases in velocity of seismic layer 2A generally correspond to the transition regions in the Atlantic and Pacific oceans provides support for the latter mechanism. Heat flow measurements in the world's oceans allow us to calculate the variations of bulk permeability and basal temperature in the oceanic crust as a function of age and to evaluate the geochemical implications of the variation in these parameters between oceans. The combination of conductive heat flow and elevation versus age observations in old lithosphere demonstrates the deviation from t •/2 cooling in the Indian Ocean and indicates that the Mozambique and western Somali basins are considerably older than preliminary deep-sea drilling results suggest.

We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2‐m.y.‐old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190±10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350–370 m, indicating that ocean bottom water was flowing down the hole into the upper ∼100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 6000–7000 1/h in late 1979 to about 1500 1/h 2 years later; altogether over 50×106 kg of seawater has been drawn into the basement. We estimate a permeability of ≳6×10−14 m2 for the reservoir in the upper ∼100 m of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a).

A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5‐km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over‐all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free‐air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid‐Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.

Over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the Nicoya Peninsula is characterized by remarkably low heat flow; values over the accretionary prism average 28 mW/m², and values in the trench and the ocean crust seaward of the trench average 14 mW/m². We attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation. Thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow. Disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits.