Hydrothermal recharge and discharge guided by basement outcrops on 0.7-3.6 Ma seafloor east of the Juan de Fuca Ridge: Observations and numerical models

[ 1] The nature of ridge-flank hydrothermal circulation guided by basement outcrops protruding through thick sediments is constrained on the eastern flank of the Juan de Fuca Ridge using combined bathymetric, seismic, and thermal observations and analytical and numerical calculations of coupled fluid-heat flow. Observational data near the western edge of the survey area indicate that young, cool hydrothermal fluids circulate rapidly through upper basement, probably both across-strike and along-strike of dominant structural trends. Data from the eastern end of survey coverage ( Second Ridge (SR)) indicate that upper basement is regionally nearly isothermal. A small number of basement outcrops in this area host focused hydrothermal discharge, as do additional basement outcrops to the north and south of the SR area. Numerical models of individual recharging and discharging outcrops, patterned after the Baby Bare and Grizzly Bare outcrops, suggest that local convection alone cannot explain observed patterns of seafloor heat flux near these features. Forced-flow simulations show that reasonable rates of hydrothermal recharge and discharge, inferred from independent observations, can explain nearby seafloor heat flux, provided that upper basement permeability within and near the outcrops falls within a range of 10(-13) to 10(-11) m(2). Free-flow simulations of fluid circulation between paired outcrops separated by 50 km, as are Baby Bare and Grizzly Bare outcrops, are most consistent with observations when regional basement permeability is 10(-11) to 10(-10) m(2). These simulations illustrate how sensitive these systems are to selection of appropriate properties and boundary and initial conditions.