HR: 15:10h
AN: T23F-07    [Abstracts]
TI: Hydrothermal Circulation Within and Between Basement Outcrops on a Young Ridge Flank: Numerical Models and Thermal Constraints
AU: * Hutnak, M
EM: mhutnak@pmc.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Department 1156 High St., Santa Cruz, CA 95064 United States
AU: Fisher, A T
EM: afisher@pmc.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Department 1156 High St., Santa Cruz, CA 95064 United States
AU: Stauffer, P
EM: stauffer@lanl.gov
AF: Los Alamos National Lab, Earth and Environmental Sciences MS T003, Los Alamos, NM 87545 United States
AU: Gable, C W
EM: gable@lanl.gov
AF: Los Alamos National Lab, Earth and Environmental Sciences MS T003, Los Alamos, NM 87545 United States
AB: We use two-dimensional, finite-element models of coupled heat and fluid flow to investigate local and large-scale heat and fluid transport around and between basement outcrops on a young ridge flank. System geometries and properties are based on observations and measurements on the 3.4-3.6 Ma eastern flank of the Juan de Fuca Ridge. A small area of basement exposure (Baby Bare outcrop) experiences focused hydrothermal discharge, whereas a much larger feature (Grizzly Bare outcrop) 50 km to the south is a site of hydrothermal recharge. Observations of seafloor heat flow, subseafloor pressures, and basement fluid geochemistry at and near these outcrops constrain acceptable model results. Single-outcrop simulations suggest that local convection alone (represented by a high Nusselt number proxy) cannot explain the near-outcrop heat flow patterns; rapid through-flow is required. Venting of at least 5 L/s through the smaller outcrop, a volumetric flow rate consistent with earlier estimates based on plume and outcrop measurements, is needed to match seafloor heat flow patterns. Heat flow patterns are more variable and complex near the larger, recharging outcrop. Simulations that include 5-20 L/s of recharge through this feature can replicate first-order trends in the data, but small-scale variations are likely to result from heterogeneous flow paths and vigorous, local convection. Two-outcrop simulations started with a warm hydrostatic initial condition, based on a conductive model, result in rapid fluid flow from the smaller outcrop to the larger outcrop, inconsistent with observations. Flow can be sustained in the opposite (correct) direction if it is initially forced, which generates a hydrothermal siphon between the two features. Free flow simulations maintain rapid circulation at rates consistent with observations (specific discharge of m/yr to tens of m/yr), provided basement permeability is on the order of 10^-10 m² or greater. Lateral flow rates scale inversely with the thickness of the permeable basement layer. The differential pressure needed to drive this circulation, created by the siphon, is on the order of tens to hundreds of kPa, with greater differential pressure needed when basement permeability is lower.
DE: 3015 Heat flow (benthic)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3021 Marine hydrogeology
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: Fall Meeting 2005