INFLUENCE OF BASEMENT TOPOGRAPHY ON HYDROTHERMAL CIRCULATION IN SEDIMENT-BURIED IGNEOUS OCEANIC CRUST

Abstract

Hydrothermal convection in the upper oceanic crust has been inferred to be a globally common and important process. Under the simplest conditions of planar boundaries, lateral dimensions of convection cells provide a strong constraint on the vertical extent of significant permeability, and on the depth of penetration of convection. It has been suggested, however, that topography can exert a strong influence on the pattern of circulation, potentially making it impossible to use patterns of heat-flow variations to constrain the depth of circulation. We have investigated convection as it is influenced by sediment-buried basement topography for a range of Rayleigh numbers from well below critical conditions (Rac) up to the limit of steady-state convection (~ 10 Rac), and for a variety of topographic wavelengths and amplitudes, using an advanced numerical modelling scheme. We find that convection is always stimulated by topography at sub-critical conditions, although flow velocities are in general too small to be thermally significant. Sub-critical flow is controlled by the buried topography, with upwelling beneath warm basement troughs, lateral flow rising along the basement-sediment interface, and descending flow beneath basement ridges. The opposite direction of flow occurs only when the slope of isotherms in basement is reversed, as in the case where the sediment cover is conformal and thin. At Rayleigh numbers between critical and about 5 Rac, convection occurs as a result of Rayleigh instability, with a natural half-wavelength between 1 and 2 times the convective layer thickness, similar to that which is present in the absence of topography. Buried topography influences the pattern of flow only weakly. At Rayleigh numbers higher than about 5 Rac, whether the pattern of convection is controlled by topography depends on the slope and wavelength of the topography. Over most of the ranges of these parameters representative of the topography of typical oceanic crust, however, the convection pattern follows topography similar to the sub-critical topographically driven convection, but with much higher velocities. The results are not applicable to Rayleigh numbers above about 10 Rac, conditions that are probably common in buried young oceanic crust, which requires transient simulations. Fluid flow is also stimulated under the influence of topography both through the sediments that bury the permeable upper crust, and in deeper basement. Owing to low characteristic permeabilities, this flow is thermally insignificant, but it may be geochemically important because it must persist as long as the perturbations caused by the buried basement topography and the convection in the permeable part of the crust are present.