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Numerical simulations of hydrothermal convection in a sedimented ridge setting are presented to examine relationships between the large-scale permeability of basement, the vigor of convective circulation, the aspect ratio of convection cells, and the resulting magnitude of lateral differences in temperature (ΔT) and effective pressure (Δ PE) along a sediment/basalt interface. These modeling results provide insight to hydrologic conditions within Middle Valley and allow us to explore possible relations between the single-borehole permeability measurements made during Leg 139 and the effective permeability of the basement rock at the basin scale. With an assumed basal heat flux of 750 mW/m, there is a permeability range (5 × 10~ to 10~ m) beyond that corresponding to the onset of convection, where ΔT and ΔPE increase towards their peak values. Higher aspect ratio convection leads to greater variations in temperature and effective pressure along the interface. Over a permeability range of 2 orders of magnitude (1CT to 10~ m), ΔT and ΔPE decrease as the pressure gradients driving the steady convective flow decrease, as convection becomes more vigorous, and as the entire system is cooled. At yet higher permeability values, unsteady convection is the norm and ΔT and ΔPE decline to values less than 20°C and 0.10 MPa, respectively. If arguments in support of a near-isothermal sediment/basement interface in Middle Valley are valid, then the implication follows that the effective permeability of the hydrothermal system is considerably greater than the values measured in deeper sections of Hole 857D and in Hole 858G. In this case it is possible that discrete zones of much higher permeability, such as those encountered at several depths in Hole 857D, are sufficiently well connected to substantially increase the large-scale permeability of the sill/sediment sequence and upper igneous basement.

Hydrologic and Thermal Conditions at a Sediment/Basalt Interface: Implications for Interpretation of Field Measurements at Middle Valley