2-DIMENSIONAL AND 3-DIMENSIONAL MODELING OF HYDROTHERMAL CONVECTION IN THE SEDIMENTED MIDDLE-VALLEY SEGMENT, JUAN-DE-FUCA-RIDGE

The heat flow map derived from 550 measurements collected in a the sou thern portion of the sedimented rift in Middle Valley, northern Juan d e Fuca Ridge, displays kilometer-sized quasi-circular regions of very high heat flow. Some of these domains, explored during Ocean Drilling Program (ODP) leg 139, are thought to be discharge zones of large-scal e hydrothermal plumes. To understand this unique data set, we modeled the kilometer-scale hydrothermal circulation within both the sedimenta ry and the igneous crust, using a set of two- and three-dimensional mo dels that use a numerical technique based on horizontal spectral decom position of the flow equations. These models include variations in the viscosity and density of the hydrothermal fluids with temperature. We examine the variations in flow patterns due to different permeability -versus-depth distribution within sediment and pillow layers. Models w ith the same permeability in both layers do not match the seafloor hea t flow field in Middle Valley. When the permeability decreases from th e bottom to the top of the simulation domain by a factor greater than 20, convection assumes a plume pattern to produce surface heat flow co mparable to that observed in Middle Valley. Within the models the rati o of the heat flux above the recharge and discharge domains is directl y related to the vertical harmonic mean of the permeability field. A v alue of 7 X 10(-16) m(2) provides a good match to the heat flow observ ations. The Darcy velocities of the hydrothermal fluids in the dischar ge areas approach 16 cm/yr while in the recharge areas they are lower than 3 cm/yr. These rates and the temperature inside the plumes are su fficiently high to produce the observed massive sulfide deposits and m ineral alterations in 1-2 X 10(5) years. The dynamic pressure produced by the hydrothermal flow matches the pressure measured in drill sites . This process may play a major role in compaction, fracturing, and up lift of the sediment cover. For example, the dynamic pressure in the a scending plume equals the lithostatic pressure at a depth of 50 m. Res ulting hydrofracturing could explain the genesis of the vent fields as sociated with the hydrothermal discharge.