Heat and fluid flux at a crustal scale: Observations and models of coupled transport in young oceanic lithosphere

Hydrothermal circulation of seawater through the oceanic crust profoundly influences lithospheric and oceanic evolution. The upper oceanic crust comprises an aquifer of global proportions, and sedimentation acts to hydrologically isolate the crustal aquifer from the overlying ocean. Thermally-significant exchange of fluids between the crustal aquifer and overlying ocean can only occur where seamounts or other basaltic outcrops penetrate the sediment blanket. Although much progress has been made in the last several decades in elucidating the nature of hydrothermal circulation in oceanic basement, many of the physical properties and processes remain poorly understood or inadequately quantified.; My dissertation research addresses several fundamental gaps in our understanding of ridge-flank hydrological processes, namely the roles of seamounts and basement outcrops is facilitating the exchange of fluid between the crustal aquifer and overlying ocean, the extents and rates of hydrothermal circulation in uppermost basement, the bulk crustal permeability distributions required to support the fluid fluxes, the thermal effects of sedimentation on measurements of seafloor heat flux, and the timescales required for conductive thermal rebound to occur once basement outcrops become buried.; Interpretation of measurements of seafloor heat flux on relatively young (0.7-24 Ma) on the eastern flanks of the Juan de Fuca Ridge (JFR) and East Pacific Rise (EPR) is facilitated by collocation of measurements on bathymetric maps and along seismic reflection profiles, and these data are used as constraints for numerical models of coupled heat-fluid flow. Observational data from the eastern flank of the JFR indicate that fluids circulate rapidly through upper basement close to the ridge axis, both along- and across-strike of dominant structural trends. Numerical models of both individual and paired recharging and discharging outcrops are most consistent with regional upper basement permeabilities of 10-11 to 10-8 m2, and crustal aquifer thicknesses of 100-600 m. Numerical thermal models of sedimentation indicate that incomplete conductive thermal rebound may be responsible for the observed variability in seafloor heat flux measurements on the JFR, and may also bias measurements made on moderate to old seafloor areas even where there is little evidence for ridge-flank hydrothermal circulation at present.