THE RELATIVE IMPORTANCE OF PLATE-DRIVEN AND BUOYANCY-DRIVEN FLOW AT MIDOCEAN RIDGES

The dynamical interaction between three-dimensional (3-D) buoyant flow and plate-driven mantle flow beneath a mid-ocean ridge is examined us ing a combination of laboratory and numerical experiments. In a unique laboratory setup a layer of strongly temperature-dependent viscous fl uid is heated from below and cooled from above to drive thermal convec tion. Forced, plate-driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface. Uniform viscosity 3-D numerical models have been designed to simulate the laboratory runs, to provide additional information on the flow, and to attempt to isola te the effects of variable viscosity. In one type of experiment we mod eled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis. Tn a second set of experiment s the entire base of the tank is heated in order to investigate the in teraction between plate-driven flow and larger-scale (upper mantle) co nvection. The pattern of segment-scale (similar to 100-150 km) convect ion beneath a spreading center is found to be a strong function of the spreading rate and the Rayleigh number (Ra) of the buoyant flow. Pure ly two-dimensional (2-D) flow exists only in the case of low Ra (simil ar to 10(5)) and faster spreading rates (>4-6 cm/yr half rate). Three dimensionality is strongly enhanced during transient pulses of upwelli ng. Convection on the scale of the upper mantle can contribute signifi cant long-wavelength spatial (similar to 600-1000 km) and temporal (20 -40 m.y.) variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume-ridge interact ion. For a given Ra the upwelling near the spreading axis can essentia lly be described by three regimes: weakly 3-D at the slow spreading ra tes, strongly 3-D at slow to intermediate rates, and 2-D at fast sprea ding rates. The regime boundaries shift toward higher plate velocities with increasing Ra. Comparison of the laboratory and numerical experi ments indicates that temperature-dependent viscosity may strengthen th e position of focused upwelling centers and cause a sharper transition between 2-D and 3-D upwelling patterns. Taken together, these results suggest that buoyancy-driven, dynamic flow is an important element in the geodynamics of mid-ocean ridges.