The thermal regime of mid-ocean ridges: geological perspectives and numerical modelling

The thermal regime of mid-ocean ridges determines their spreading modes (i.e., the combination of mid-ocean ridge tectonic, magmatic and hydrothermal processes that control the composition and structure of the oceanic lithosphere). It is determined by the balance of heat supply and heat loss in the axial region. Most heat is supplied through magma, while hydrothermal energy fluxes depend on the permeability and depth extent of the hydrothermal cooling domain, and on the thickness of the conductive boundary layer at its base. At fast spreading ridges, the flux of melt is high, the thermal regime is hot, and melt resides at depths of only a few kms in a steady state fashion, well within the reach of vigourous axial hydrothermal convection. At slow ridges, the melt flux is lower, the thermal regime is colder, so that melt can only reside durably at depths that are commonly > 10 km, out of the reach of vigourous (high permeability) hydrothermal systems. Melt there, however, is commonly injected higher up in the axial lithosphere, forming transient melt bodies in colder host rocks and triggering high temperature, black smoker, hydrothermal systems.

Here we report on numerical models that explore the thermal effects of varying both the melt flux and the depth of magma emplacement, a parameter that previously published mid-ocean ridge thermal models did not take into account. Our models do predict the large variability in thermal regime that is documented at slow ridges, from cold detachment-dominated settings, to hotter melt-rich segment centers. We discuss these results and the strengths and limitations of the modelling approach. We also explore the potential effects of varying the melt flux and melt emplacement depth with time at a given slow spreading ridge location, on crustal construction processes and on the respective roles of faults and melt intrusions to accommodate plate divergence.