Abstract
We have developed a refined model of coupled heat and fluid flow to
numerically simulate off-axis hydrothermal circulation through the
upper oceanic crust. The new representation includes mesh elements
with curved sides and noncentered nodes. These curvilinear elements
allow improved simulation of topography at the seafloor and within
underlying sediment and basaltic layers. Curvilinear simulations
of circulation at Deep Sea Drilling Project/Ocean Drilling Program
site 504 confirm the broad conclusions derived from the rectilinear
simulations of Fisher et al. (1990) but also suggest that permeability
within the upper few hundred meters of crust around hole 504B must
be concentrated within several narrow zones. This interpretation
is consistent with core observations and wireline logs from hole
504B. Within these highly permeable zones, absolute permeability
may be several orders of magnitude higher than the bulk permeability
measured for the upper crust as a whole. A series of detailed parametric
simulations was conducted to determine the quantitative importance
of basal heat flow, seafloor bathymetry, basement relief, and differential
sediment thickness in modifying the geometry and intensity of off-axis
hydrothermal circulation. These studies reveal that basement relief
and differential sediment thickness are more important than seafloor
bathymetry in enhancing off-axis convection within the seafloor.
Observed natural correlations between seafloor bathymetry and heat
flow may exist because bathymetry is a proxy for the other two important
parameters, basement relief and differential sediment thickness.
Off-axis convection at geochemically significant velocities is possible
even with heat input appropriate for some of the oldest oceanic crust.
A thick sediment layer over older crust may cause enough conductive
refraction at the seafloor to mask the variations in seafloor heat
flow often associated with off-axis hydrothermal convection.
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