HR: 10:35h
AN: T22E-02 INVITED    [Abstracts]
TI: Do Fault-Controlled Hydrothermal Systems Control the Thermal Evolution of the Lower Crust at Fast-Spreading Ridges?
AU: * Coogan, L A
EM: lacoogan@uvic.ca
AF: University of Victoria, School of Earth and Ocean Sciences, Victoria, BC V8W 3P6, Canada
AB: Some geophysical data (e.g. seismic, compliance, bathymetry) suggest that the lower oceanic crust loses heat far more rapidly in the near-axis region that is predicted by conductive cooling models. Hydrothermal circulation through the lower oceanic crust provides one potential mechanism to cool the lower crust in the near axis region. However, there is scant evidence in samples of the lower oceanic crust for extensive pervasive fluid flow occurring over a wide temperature range. This suggests that any large-volume fluid flow must be channelised, most likely in fault-zones. Observations in the Oman ophiolite demonstrate that faults in the plutonic complex can form at high-temperatures. The Sr-isotopic composition of rocks within these fault zones are as elevated as those of the sheeted dike complex (~0.7045) requiring large fluid fluxes (Coogan et al., 2006). Thermal modeling suggests that fluid flow in faults in the lower crust could lead to much more heterogeneous cooling than if the permeability is constant. If fluid flow in the lower crust is focused in faults, with heat conducted through the wall-rock to the fluid in a fault, then the separation of faults and how far from the axis they form are critical parameters in controlling the impact of fluid flow on the axial thermal structure. The best estimate of the separation of lower crustal faults in the Oman ophiolite is approximately 1km but in the Troodos ophiolite, where this is better constrained by seismic data, it is approximately 7 km (Mackenzie et al., 2006). We currently have little data to support the existence of deeply penetrating faults in modern lower oceanic crust close to the ridge axis. Future off-axis micro-seismic surveys, and geological observations of lower crust exposed at tectonic windows, provide perhaps our best opportunities to better understand if such faults exist and, if so, what role they play in controlling the hydrology of the lower oceanic crust. References: Coogan et al, 2006, Am. J. Sci. v306, p389; Mackenzie et al, 2006, Geophys. J. Int. v167, p1385.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3021 Marine hydrogeology
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting