Influence of fault zones on the self-organization of hydrothermal outflow and recharge: Constraints from analog experiments

Hydrothermal circulation at the axis of mid-ocean ridges profoundly affects chemical and biological processes in the deep ocean and influences the thermo-mechanical state of young oceanic lithosphere. Yet, the sub-seafloor geometry of fluid pathways and its relation to sharp spatial gradients in crustal permeability (e.g., fault zones) remain enigmatic. Structurally, fault zones commonly consist of high permeability damaged crust surrounding a relatively impermeable gouge layer. Here we present new laboratory models of hydrothermal circulation aimed at constraining the role of such fault zones on the self-organization of hydrothermal convection along slow-spreading ridges. Oceanic crust analogs of known permeability are constructed using one of three sizes of glass spheres (diameters: 0.250 mm, 6 mm, or 15 mm). Analog fault zones are constructed of 3-D printed plastic spheres 12 mm in diameter with an impermeable plastic “gouge” in the center of the fault. These materials are saturated with water and heated at their base by a resistive silicone strip heater to initiate thermal convection. A layer of water (analog ocean) overlies the porous medium and allows an "open-top" boundary condition. Areas of fluid discharge from the crust into the ocean are identified by illuminating microscopic glass particles carried by the fluid, using laser sheets. Using particle image velocimetry, we estimate fluid discharge rates as well as the location and extent of fluid recharge. Thermocouples distributed throughout the crust provide insights into the geometry of convection cells at depth, and enable estimates of convective heat flux, which can be compared to the heat supplied at the base of the system. By varying the permeability contrast between the crustal analogs and fault damage zone (5 - 5000), the dip of the high-permeability fault zones (30° - 90°), and the imposed basal temperature (20°C - 80°C) we investigate the role of fault damage zones in the localization of hydrothermal outflow and recharge. Preliminary results indicate alternating zones of outflow and recharge focus within the high permeability fault damage zone.