Problems with imaging cellular hydrothermal convection in oceanic crust

Heat-flow variations are commonly used to infer the existence of hydrothermal convection in young oceanic crust and, in special cases, the geometry and vigor of convection. In a small area on the eastern flank of the Juan de Fuca Ridge where the seafloor and seismic basement relief are only about 10 m, a coherent pattern of heat-now variation having 40 mW m(-2) peak-to-peak amplitude and half-wavelength of about 600 m has been previously observed and interpreted to be caused by cellular convection within the extrusive layer 2a of the oceanic crust. In August, 1995, new seismic reflection and heat-flow data were collected to define better the structure of basement in this area, and to determine the planform of the heat-flow variations revealed along the single transect of the earlier observational study. The new, higher resolution seismic results show considerable intra-crustal structure; in particular, the uppermost crust is reverberant beneath areas of lower heat flow, and a diffractive character extends close to the basement surface beneath areas of higher heat flow. This relationship suggests that the heat-flow variations may be caused by variations in depth to the top of a high-permeability intracrustal zone that is not coincident with acoustic basement, where relatively uniform temperatures are maintained by vigorous hydrothermal circulation of unresolved planform. If this is the case, the search that began in the 1970's for heat-flow variations that might reveal the scale of cellular convection, and hence by inference the penetration depth of hydrothermal circulation in the oceanic crust, may have been frustrated once again.