The nature of oceanic lower crust and shallow mantle emplaced at low spreading rates

Abstract

Thermal calculations indicate that at sea floor spreading rates too low to sustain a steady-state magma chamber, most of the upwelling melt still freezes within ∼10 km of the sea floor beneath a mid-ocean ridge. A thickened lower crustal section containing crust-mantle mix is thus predicted. Reduced melting in the source region is an expected effect of low spreading rates. However, cooling from the surface limits the degree to which mafic crustal material is separated from mantle at spreading rates higher than those at which reduced melting becomes important. Evidence for ultramafic contributions to the crust at low spreading rates includes a distinct seismic velocity profile for crust from very slow ridges, evidence of aging of the lower crust, spreading-rate-dependent differences in reflection seismic structure and character, and seismic crustal thicknesses apparently greater than adiabatic crustal thicknesses. Crustal structure inferred from the thermal results agrees with the model of Cannat [Cannat, M., 1993. Emplacement of mantle rocks in the seafloor at mid-ocean ridges. J. Geophys. Res. 98, 4163–4172], which includes a lower crust of mixed composition achieved without the requirement of gross tectonic rearrangement of an originally stratified crustal section.