OFF-AXIS HYDROTHERMAL CIRCULATION - PARAMETRIC TESTS OF A REFINED MODEL OF PROCESSES AT DEEP-SEA DRILLING PROJECT OCEAN DRILLING PROGRAM SITE 504

We have developed a refined model of coupled heat and fluid flow to nu merically simulate off-axis hydrothermal circulation through the upper oceanic crust. The new representation includes mesh elements with cur ved sides and noncentered nodes. These curvilinear elements allow impr oved simulation of topography at the seafloor and within underlying se diment 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 e t al. (1990) but also suggest that permeability within the upper few h undred meters of crust around hole 504B must be concentrated within se veral narrow zones. This interpretation is consistent with core observ ations and wireline logs from hole 504B. Within these highly permeable zones, absolute permeability may be several orders of magnitude highe r 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 stu dies reveal that basement relief and differential sediment thickness a re more important than seafloor bathymetry in enhancing off-axis conve ction within the seafloor. Observed natural correlations between seafl oor bathymetry and heat flow may exist because bathymetry is a proxy f or the other two important parameters, basement relief and differentia l sediment thickness. Off-axis convection at geochemically significant velocities is possible even with heat input appropriate for some of t he oldest oceanic crust. A thick sediment layer over older crust may c ause enough conductive refraction at the seafloor to mask the variatio ns in seafloor heat flow often associated with off-axis hydrothermal c onvection.