In this study we construct a thermal and mechanical model for the genesis of oceanic crust. Magma is halted in its ascent within the oceanic crust when it reaches a freezing horizon, where the dilational volume change associated with magma freezing leads to viscous stresses that favor magma ponding near the freezing horizon. To model the steady state thermal impact of crustal accretion via dike injection and pillow flows, we treat all crustal accretion in rocks cooler than a magma ''solidus'' to occur in a narrow 250-m-wide dike-like region centered about the ridge axis. The rest of the oceanic crust is modeled to be emplaced as a steady state magma lens directly beneath the ''solidus'' freezing horizon where the steady state emplacement rate is determined by the constraint that this lens supply all crust that is not emplaced through diking/extrusion above the magma lens. If hydrothermal heat transport within crustal rocks cooler than 600 ¿C removes heat 8 times as efficiently as heat conduction, then we find that a steady state magma lens will only exist within the crust for ridges spreading faster than a 25 mm/yr half rate.

The depth dependence of the magma lens with spreading rate is in good agreement with seismic observations. These results suggest that a fairly delicate balance between magmatic heat injection during crustal accretion and hydrothermal heat removal leads to a strongly different crustal thermal structure at fast and slow spreading ridge axes. Our results support the hypothesis that median valley topography is due to extension of strong ridge axis lithosphere; it is the difference in thermal regime that is directly responsible for the striking difference between the typical median valley seen at slow spreading ridges (e.g., Mid-Atlantic Ridge) and the axial high seen at fast spreading ridges (e.g., East Pacific Rise). This paradigm for the origin of a median valley at a slow spreading ridge predicts that along axis variations in median valley topography of a slow spreading center reflect variations in recent magmatic heat input along a segment, that is, that the axial topography is a good time-averaged indicator of the relative importance of hydrothermal cooling and magmatic injection along a given section of a ridge segment. We determine the accumulated crustal strain associated with lower crustal flow which supports the hypothesis that the Oman Ophiolite crust created at a paleo-analogue to a fast spreading ridge and also suggests that crustal strain, and not cumulate layering, may be the dominant physical process that generates ''layered gabbros'' within the Oman Ophiolite. ¿ American Geophysical Union 1993