Multiple scales of hydrothermal circulation in Middle Valley, northern Juan de Fuca Ridge: Physical constraints and geologic models

A synthesis of data including seafloor heat flow, water column thermal anom alies, shallow pore pressures, pore water chemical profiles, paired borehol e observations, and 3.5-kHz records help to characterize distinct hydrother mal circulation systems within and surrounding two active vent areas in Mid dle Valley, a sedimented rift on northern Juan de Fuca Ridge. We estimate t he total heat and fluid budgets for a 260 km(2) area encompassing 17 km of ridge to be 274 MW (16 MW km(-1) of ridge) and 0.157 m(3) s(-1), respective ly. About half of the seafloor heat loss is conductive, requiring efficient lateral fluid and heat transport in basement. Lateral head gradients (flui d driving forces) in basement between the primary fluid discharge site and the surrounding formation are very low. Simple mass balance and flow calcul ations indicate that the majority of the recharge to the primary hydrotherm al reservoir cannot occur through sediments immediately around the vent are a; recharge is probably focused along valley-bounding normal faults. Crusta l-scale lateral permeability in basement must be 10(-10) to 10(-12) m(2) in order to allow rapid flow and maintain nearly isothermal temperatures over many kilometers. On a smaller scale, heat flow patterns within the main ve nt area are controlled by the depth to the primary hydrothermal reservoir, proximity to active vents, and local recharge of a secondary circulation sy stem. Shallow underpressures and isolated zones of very low heat flow resul t from vigorous secondary circulation driven by pressure gradients associat ed with venting. Models of secondary circulation suggest that most of the o verpressure observed in hydrothermal basement is lost when the fluid enters the vents and that cooler sediment pore fluids flow toward and into the ve nts as a result of shallow underpressuring. Secondary circulation results i n significant heat flow reduction at the seafloor but adds minor sediment p ore fluid to vent discharge.