Partial melting processes at mid-ocean ridges generate oceanic lithosphere which is chemically and mineralogically zoned. Basaltic oceanic crust overlies a 20–30 km thick zone of depleted upper mantle. This zone has undergone partial melting and is largely free of the high density phase, garnet, has a higher MgO/FeO ratio, and in consequence has a lower density (Δ p = 0 06 gm/cm3), than the undepleted mantle of the lower lithosphere. The lithosphere does not become gravitationally unstable upon the asthenosphere for 40–50 Ma, when increased density resulting from thermal contraction has offset the compositional buoyancy of the depleted zone and crust. During subduction the basaltic crust inverts to eclogite and the net compositional buoyancy of the lithosphere is eliminated. However, as the subducted lithosphere is heated it becomes less rigid and density differences both between different parts of the descending lithosphere and the surrounding mantle become important. The dense eclogite layer sinks through the underlying depleted zone at a rate determined by the temperature-dependent rheology. With further heating the depleted zone becomes less dense than the overlying undepleted mantle and will diapirically rise some 300-400 km behind the trench depending upon the angle and rate of subduction and the age of the subducted plate. Such diapirs are able to intitiate behind-arc spreading. In a continental setting the diapirs could both heat the lithosphere and produce exceptional elevation.

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