Contrasting two-dimensional and three-dimensional models of outcrop-to-outcrop hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge

Abstract:

We present results from two-dimensional and three-dimensional coupled (fluid and heat flow) simulations of ridge-flank hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge. Field studies in this region demonstrate the existence of an active hydrothermal siphon operating between two seamounts separated by ~50 km, and provide quantitative constraints that help to determine which simulations are successful in replicating known properties and processes. Constraints from field observations include (a) the flow rate between the outcrops, (b) the presence of secondary convection within the basement aquifer, leading to simultaneous recharge and discharge through a single outcrop (in additional to siphon flow between outcrops), (c) direct measurements of crustal permeability in basement boreholes, and (d) the lack of a regional seafloor heat flux anomaly as a consequence of outcrop-to-outcrop circulation. New simulations include an assessment of crustal permeability and thickness, outcrop permeability, and a comparison of simulation results using different geometries. Three-dimensional simulations are more consistent with field observations than their two-dimensional counterparts and indicate a crustal aquifer of ≤300 m thick having a bulk permeability between 3×10^-13 and 2×10^-12 m², values consistent with borehole measurements. In addition, we find fluid flow rates and crustal cooling efficiencies that are an order of magnitude greater in three-dimensional simulations than inferred from two-dimensional simulations using equivalent properties. These results show that three-dimensional simulations of outcrop-to-outcrop hydrothermal circulation on a ridge flank improves the geological and geometric accuracy of results, in comparison to models run in two dimensions.