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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The identification and quantification of conductive and convective components in the heat transfer of a sedimentary basin is demonstrated for the Rheingraben. Three different methods of varying complexity as well as three independent data sets are employed: (1) energy budget considerations based on hydraulically perturbed thermal data from shallow boreholes (<500m), (2) 1-D vertical Peclet number analysis of thermal data from 22 deep boreholes (> lo00 m), and (3) 2-D finite difference modelling of the fully coupled fluid flow and heat transport equations on a vertical cross-section of the entire Rheingraben. Energy budget considerations yield a conductive basal heat flow density of 84 + 40/-10 mW mP2, and in good agreement with this Peclet number analysis, gives median values in the range 90 f 35 mW m-'.In the first case, the basement is formed by low permeable, tertiary sediments at about 500 m depth, and in the second by the transition from the sedimentary graben fill to the crystalline basement at depths of between 2000 and 4000m. It is shown how results from numerical modelling support the flow field assumptions made by methods (1) and (2), as well as the value of 80 f 10 mW m-' for average basal heat flow density entering the graben from below. Conversely, the Peclet number range Pe I 1.2 inferred from method (2) can be applied for a (at least partial) calibration of the fully coupled hydrothermal model calculatioris. This technique is suggested as a potentially interesting thermal method for constraining regional-scale permeability.An interpretation of heat transport is presented that satisfies the experimentally established patterns of both temperature and heat flow density in the Rheingraben. Moreover, it is demonstrated that the thermal anomalies along the western rim of the graben (such as Pechelbronn, France or Landau, Germany) can be convincingly explained by a basin-wide, deep rooted E-W groundwater circulation that locally enhances a background basal heat flow density of about 80 mW m-' on average by 50 per cent and at individual sites by as much as 120 per cent.

Multiple penetration heat flow surveys that employ “violin bow” multithermistor array instruments generate large quantities of high‐quality data. To cope with the problems associated with the reduction of these data, an algorithm has been developed which can be used to reduce the data automatically on a microcomputer. The algorithm is designed for use with a pulsed line source method of conductivity measurement, although it could be modified easily for use with the continuous line source method. The multiply iterative algorithm deals, on a sensor by sensor basis, with errors associated with nonideal probe construction and sediment entry, factors that affect the determination of both in situ temperatures and thermal conductivities. Numerical tests of the algorithm show that it is accurate and stable and fast enough for post‐real‐time reduction of data at sea. Practical tests on high‐quality data from the western Pacific show that accuracies are limited by the instrumental resolution; in these cases, the reduction algorithm provided determinations of undisturbed in situ temperatures to better than 1 mK and of in situ conductivities to within 1%.