THE EFFECTS OF SPREADING RATE, THE MAGMA BUDGET, AND THE GEOMETRY OF MAGMA EMPLACEMENT ON THE AXIAL HEAT-FLUX AT MIDOCEAN RIDGES

We explore two-dimensional, steady state, flow and thermal models of o ceanic spreading center structure. These models include the effects of hydrothermal heat transport and crustal accretion for two basic accre tion geometries: (1) a lens model where all crust below the sheeted di ke layer passes through a magma lens at the base of the dike layer bef ore subsiding and flowing to deeper levels; and (2) a dike model where crust is emplaced in a vertical slot from the surface to Moho and sub sequently moves horizontally (rigidly) away from the axis of accretion . The axial heat flux resulting from these two end-member accretion ge ometries is compared, and an assessment is made of the feasibility of using various observations to differentiate between these two accretio n geometries. We find that the steady state axial heat flux is predomi nantly influenced by three factors: the spreading rate, the magmatic b udget (crustal thickness) and the efficiency of hydrothermal cooling. The total steady state heat flux from the neovolcanic zone (2 km wide) increases almost linearly with the spreading rate for both the dike a nd lens accretion geometries. The major difference between the dike an d the lens models is nonthermal: they predict different accumulated st rain distributions within off-axis crust. Crustal flow due to crustal accretion within a crustal-height ''dike'' leads to little accumulated strain, while intense crustal strain results from crustal subsidence and flow below a steady stale magma lens. Ophiolite and marine seismic observations of crustal layering appear to be the strongest observati onal tests to discriminate between these two accretion geometries; the y currently favor a lens-like model of lower crustal accretion.