Recently acquired single-and multichannel seismic reflection profiles and over 500 heat flow measurements augment SeaBeam bathymetric and SeaMARC II and I side-scan acoustic data to provide new constraints on the tectonic setting and thermal structure of the sediment-filled rift, Middle Valley, of the northern Juan de Fuca Ridge. Over most of the length of the Juan de Fuca Ridge, spreading takes place along high-standing volcanic ridges, which are broken only by relatively small axial rift grabens. Near the northern end of the ridge south of the Sovanco fracture zone intersection, the volcanic supply is diminished, probably because of the lateral heat loss at the end of the ridge and the northern migration of the ridge in the wake of the lithospheric Explorer plate. This has resulted in the formation of deep rift valleys at the spreading axis. The deepest of these, Middle Valley, has been buried syntectonically by Pleistocene turbidite sediment. Heat flow in this valley varies inversely with sediment thickness, suggesting that the sediment forms a hydrologic seal over permeable igneous crust, where efficient hydrothermal circulation maintains relatively uniform temperatures. This simple model is investigated by comparing directly the thermal regime at depth and the seismic structure of the valley. Temperatures at depth are estimated from seafloor heat flow measurements using sediment physical properties derived from multichannel seismic (MCS) velocities. Although it is transitional and poorly defined in places, the acoustically defined sediment-basement contact is estimated to be typically about 300° C. Numerous local heat flow anomalies are observed in the valley. Two are associated with known hydrothermal vents also of roughly 300° C. Another lies near the normal-fault scarp that forms the eastern boundary of the valley, where basement is exposed at the seafloor, although in general, heat flow in the vicinity of the fault is relatively low, as are estimated basement temperatures. This suggests that the fault and the exposure of basement may serve primarily as a conduit for diffuse recharge of crustal fluids. Other heat flow anomalies and associated vents or vent fields overlie buried basement edifices where the sediment cover is locally attenuated. There is no significant heat flow anomaly and apparently no hydrothermal discharge associated with the most recent (Holocene) intrusive activity in the valley. The location of hydrothermal upflow zones appears to be influenced more by permeability structure, which is inferred to be controlled primarily by basement topography and variations in sediment thickness, rather than by the location of heat sources. The generally continuous, low-permeability sediment cover allows local vents to tap large areas of high-temperature permeable crust. The 300° C hydrothermal fluid temperatures currently present in Middle Valley are low compared to those required to produce solutions of high metal concentration. The presence of base-metal sulfide deposits in the valley suggests eith...

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Ocean Drilling Program Leg 168 investigated three hydrothermal regimes located on the sediment-covered eastern flank of the Juan de Fuca Ridge. The three regimes are (1) a transition from very young (<0.8 Ma) unsedimented igneous crust to sediment-buried crust (0.8-1.2 Ma), (2) an older (~3.5 Ma) area with large topographic relief (>300 m) of igneous crust under a thick (250-600 m) sediment cover, and (3) an intermediate age (1.4-2.6 Ma) area of low relief igneous crust buried continuously over tens of kilometers by sediments averaging about 300 m thick. Ten sites were cored and roughly 1500 m of sediment and basalt was recovered. A total of four CORKs were emplaced, with thermistor strings and osmotic water samplers, in Holes 1024C, 1025C, 1026B, and 1027C. Temperature, pressure, and water chemistry data will be recorded from these holes over the next 3 to 5 years. The first direct BACKGROUND

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