Near‐axis heat flow measurements on the northern Juan De Fuca Ridge: Implications for fluid circulation in oceanic crust

Present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process. In axial regions, abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid. In older crust, where the insulating sediment cover is thick, previous studies have shown that basement topography is the dominant forcing factor for within‐crust fluid circulation. In the intermediate region, where young crust is lightly sedimented, heat flow data are difficult to obtain with traditional techniques. To determine whether topography or permeability is the dominant process controlling fluid circulation in the near‐axis region, we conducted a profile of heat flow measurements using the submersible ALVIN, on the Endeavour Segment of the Juan de Fuca Ridge. Our data indicate that topographic forcing is responsible for the long wavelength variations, with high heat flow at the ridge summits, and low values in the inter‐ridge valleys. The locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge‐valley topographic pair comprises a single circulation cell. This model predicts that the source area for the high temperature axial vents may be in the flanking inter‐ridge valleys.