The median valley, a result of magma fracture beneath mid-ocean ridges

Abstract

The shape of the ocean floor has been successfully modelled using the framework of plate tectonics^1–4. Lithospheric plates approximately 100 km thick lie above the hotter, less viscous asthensophere. At the ridges, the lithosphere thins. Plate separation is accompanied by creation of new ocean floor as the asthenosphere flows upwards and cools on approaching the surface. As a result, heat is concentrated beneath the ridge axes. Densities are lower due to thermal expansion, so that for iso-static equilibrium the sea floor sits some 6 km shallower than older sea floor, well off axis, where the temperature anomaly is absent. At fast spreading ridges (velocity v > 5 cm yr⁻¹) such as the East Pacific Rise, this description fits well. A central ridge crest is flanked on either side by bathymetry which deepens in agreement with thermal models^3–6 at a rate propor tional to the square root of age of creation. However at slow spreading ridges, v < 5 cm yr⁻¹, a major central graben of order 1 km deep replaces the central ridge crest, although the square root of age dependence is observed well away from the axes. Isostatic equilibrium is not achieved⁷, since the sea floor lies 1–2 km below the equilibrium level. In this report I demonstrate that this graben or median valley can result from steady state dyke intrusion into, and brittle failure of, the axial lithosphere.