Topographically driven subcritical hydrothermal convection in the oceanic crust

Abstract

A simple analytical model for investigating topographically induced subcritical hydrothermal convection in either barren or thickly sedimented oceanic crust is developed. The results show that for unsedimented, low-amplitude, wave-like topography, the maximum topographically induced vertical convective flow may be of the order of 10 −10 m/s for a bulk crustal permeability of 10 −14 m 2 and topographic wavelength of 10 3 m. The fluid ascends under topographic highs and descends at topographic lows. The convective heat flux is about 15% of the conductive background. The magnitude of the convective flow is independent of the topographic wave-length/layer depth ratio, L/H, if L ≤ H/2, but for LåH, the convective flow depends on ( H/L) 2. If the crustal topography is completely covered with sediment of lower permeability, the convective flow is substantially reduced, perhaps by several orders of magnitude in some cases. Because the thermal conductivity of the sediment is less than that of the crust, the fluid descends at topographic highs and ascends at topographic lows. Since the actual oceanic crustal topography is quite irregular, with a number of different amplitude and wave-length scales, the actual topographic convection pattern may be much more complicated than the model results given here. This may be especially true in regions where large-amplitude outcrops occasionally protrude through a moderately thick sediment blanket and if supercritical Rayleigh convection is also present.