Abstract

Hydrothermal circulation extracts a significant fraction of lithospheric heat from the ocean crust, with most of this advective heat loss occurring on ridge flanks, far from mid-ocean ridges. Faults in ocean crust are common in many settings, and may serve as high-transmissivity structures that facilitate advective transport and focus discharge of fluid, heat, and solutes below and at the seafloor. Coupled flow along fault zones has been invoked in a variety of settings, but circulation patterns are not well constrained by observational data or earlier models. We present results from three-dimensional, fully coupled numerical simulations of fluid and heat flow in sediment-covered ridge-flank ocean crust cut by a fault. We explore a range of fault and surrounding crustal characteristics, including crust and fault permeability, fault dip angle, thickness, and depth. We are particularly interested in resolving relations between fault and crustal characteristics and seafloor heat flux patterns.

Simulation results show variability in patterns of fluid circulation and seafloor heat flux as a function of fault geometry and crustal properties. The seafloor heat flux pattern above fault traces tends to show variability along strike (in response to underlying regions of rapid upward and downward flow along the fault trace), and asymmetry in seafloor heat flux anomalies, with higher values above the fault trace and lower values in the immediately surrounding seafloor, especially above the hanging wall. The negative anomaly is generally greater when the fault dip angle is lower.

Higher permeability in the crustal rocks adjacent to the fault zone tend result in small-scale convection and small-amplitude variations in seafloor heat flux, and more diffuse convection cells in the fault zone itself. Convection in the surrounding crust decreases the importance of the fault zone in extracting lithospheric heat. Simulations also show that faults that penetrate deeper into the crust produce a significantly larger seafloor heat flux anomaly than do shallower faults, indicating that deeper faults extract lithospheric heat more efficiently. Patterns of seafloor heat flux from these simulations indicate that fault-zone hydrothermal circulation should produce thermal anomalies that are detectable in field measurements. Linking field observations directly to numerical simulations can provide better understanding of the geometry and properties of faults and fluid flow patterns in the volcanic ocean crust.