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 anomalies, shallow pore pressures, pore water chemical profiles, paired borehole observations, and 3.5-kHz records help to characterize distinct hydrothermal circulation systems within and surrounding two active vent areas in Middle Valley, a sedimented rift on northern Juan de Fuca Ridge. We estimate the 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), respectively. About half of the seafloor heat loss is conductive, requiring efficient lateral fluid and heat transport in basement. Lateral head gradients (fluid driving forces) in basement between the primary fluid discharge site and the surrounding formation are very low. Simple mass balance and flow calculations indicate that the majority of the recharge to the primary hydrothermal reservoir cannot occur through sediments immediately around the vent area; recharge is probably focused along valley-bounding normal faults. Crustal-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 vent area are controlled by the depth to the primary hydrothermal reservoir, proximity to active vents, and local recharge of a secondary circulation system. Shallow underpressures and isolated zones of very low heat flow result from vigorous secondary circulation driven by pressure gradients associated with venting. Models of secondary circulation suggest that most of the overpressure observed in hydrothermal basement is lost when the fluid enters the vents and that cooler sediment pore fluids flow toward and into the vents as a result of shallow underpressuring. Secondary circulation results in significant heat flow reduction at the seafloor but adds minor sediment pore fluid to vent discharge.