Hydrogeology of Ridge-flank Hydrothermal Systems
Most of the hydrothermal circulation through the ocean crust, in terms of mass, heat, and many solute fluxes, occurs on ridge-flanks. Far from the magmatic influence of mid-ocean ridges, fluid flow is driven by lithospheric heating from below and channeled through volcanic rock outcrops that serve as high-permeability conduits between the ocean and the underlying volcanic crust. Field data in this setting is sparse due to difficulties associated with accessing these remote locations, making geologically accurate modeling particularly valuable to assessing the nature of ridge-flank hydrothermal circulation. Each study in this thesis applies a combination of modeling and field observations to constrain the hydrogeologic properties and behaviors of ridge-flank hydrothermal systems, including: (1) deriving permeability estimates from flowing subsea boreholes, (2) investigating the sustainability of outcrop-to-outcrop hydrothermal flow, and (3) constraining the properties and behaviors on a well-studied outcrop-to-outcrop system. In the first study, thermal records from flowing boreholes in young oceanic crust are used to assess borehole and formation properties, including permeability, using analytic equations and a Markov chain Monte Carlo analysis to quantify uncertainty. We find the median bulk permeability at all sites to be between 0.4 to 1.5 x 10^-11 m2, with a standard deviation of 0.2 to 0.3 log-cycles at each borehole. These results are remarkably homogenous, given the much larger variability in permeability measurements in the oceanic crust. Results from the second study illuminate the controls on hydrogeologic sustainability, flow rate, and preferred flow direction in outcrop-to-outcrop hydrothermal systems. We find that sustained flow between outcrops over tens of kilometers depends on a contrast in transmittance (the product of outcrop permeability and the area of outcrop exposure) between recharging and discharging sites, and that discharge is favored through less transmissive outcrops. These systems require aquifer permeability values ranging from 10^-12 to 10^-11 m2, consistent with field measurements and values inferred from the first chapter. In the third study, a suite of three-dimensional numerical simulations are used to characterize and constrain the permeability and thickness of the upper crustal aquifer, the permeability of outcrops, and the potential for multiple discharging outcrops and azimuthal permeability anisotropy to influence hydrothermal processes at a field site on the eastern flank of the Juan de Fuca Ridge.