1.12 - Crust and Lithospheric Structure – Seismic Structure of Mid-Ocean Ridges

Abstract

New seafloor is created at mid-ocean ridges where plates separate. Pressure-release melting of upwelling mantle generates basaltic magma that migrates to the surface to form the oceanic crust. Most seismological studies of the mantle beneath spreading ridges find that anomalous structure extends no deeper than 200–300km, indicating that the upwelling is passively driven by the separation of the plates and not by buoyant convection originating deep in the mantle. Recent studies have observed an anisotropic pattern suggesting that upwelling extends deeper than 300km, but there is no indication of partial melting or anomalous temperatures extending to that depth; the observation needs to be confirmed. In the upper 100–150km of the mantle, there is a broad region of very low shear velocities beneath the ridge, consistent with passive upwelling and a wide region of melt production. At shallower levels, there is evidence of three-dimensional variations in mantle structure, suggesting that the variations in density induced by melt production and extraction influence mantle flow. These variations are most clearly manifested in along-axis changes in crustal structure at slow-, intermediate-, and fast-spreading ridges. Seismic velocities in the crust are strongly influenced by porosity, temperature, and the presence of melt. At fast-spreading ridges, a crustal magma chamber is commonly observed at depths of ∼1.5 km, underlain by a low-velocity region extending to the base of the crust that may contain a small melt fraction. The depth of the magma chamber increases with decreasing spreading rate. At slow spreading rates, the magma chamber is intermittent and low velocities tend to be concentrated in the centers of ridge segments, indicating focused delivery of melt. At full spreading rates less than ∼15mmyr−1, basaltic crust is sometimes missing entirely as the rate of melt production is greatly reduced.