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

Abstract

[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 À13 to 10 À11 m 2 . Freeflow 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 À11 to 10 À10 m 2 . These simulations illustrate how sensitive these systems are to selection of appropriate properties and boundary and initial conditions.

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

“…Rapid turbidite deposition during Pleistocene sea level low-stands has buried much of the oceanic basement on the eastern flank of the JFR, except close to the active ridge and where a few seamounts and other basement outcrops penetrate regionally thick sediments [Davis et al, 1992;Underwood et al, 2005;Zühlsdorff et al, 2005]. Nearly complete burial of this ridge at an unusually young age makes this an ideal area to study ridge flank hydrothermal processes, where the rates and patterns of circulation are intense and particularly wellconstrained [e.g., Davis et al, 1997b;Fisher et al, 2003a;Hutnak et al, 2006].…”

Site U1301

“…A. Regional bathymetry (Davis, Fisher, Firth, et al, 1997) Hutnak et al, 2006). Part of Line GeoB00-466 (thick dashed line) is shown in Figure F2.…”

“…Sediment structures and the sediment/basalt interface are clearly visible, as are steeply dipping normal faults to the west of the holes (indicated with vertical lines that show total depth penetration). Both holes are located over the peak of a buried basement high, near the western edge of a major distributary channel for turbidites that flowed off the North American continental shelf (Davis et al, 1992;Zühlsdorff et al, 2005;Underwood et al, 2005;Hutnak et al, 2006). Proc.…”

Site 1027

“…A. Regional bathymetry (Zühls-dorff et al, 2005;Hutnak et al, 2006). Part of seismic Line GeoB00-203 (thick dashed line) is shown in Figure F2.…”

“…Sediment structures and the sediment/basalt interface are clearly visible, as are steeply dipping normal faults to the west of Hole 1026B and a small basement high about halfway between the holes. Hole 1027C is located below the center of a major distributary channel for turbidites that flowed off the North American continental shelf (Davis et al, 1992;Zühls-dorff et al, 2005;Underwood et al, 2005;Hutnak et al, 2006). …”