The low velocity zone

Abstract

The low velocity zone in tectonic and oceanic regions is too pronounced to be the effect of high temperature gradients alone. Partial melting is consistent with the low velocity, low Q and abrupt boundaries of this region of the upper mantle and is also consistent with measured heat flow values. The inferred low melting temperatures seem to indicate that the water pressure is sufficiently high to lower the solidus about 200° C to 400° C below laboratory determinations of the melting point of anhydrous silicates. The mechanical instability of a partially molten layer in the upper mantle is probably an important source of tectonic energy. The top of the low-velocity zone can be considered a self-lubricated surface upon which the top of the mantle and the crust can slide with very little friction. Lateral motion of the crust and upper mantle away from oceanic rises is counterbalanced by the flow of molten material in the low-velocity layer toward the rise where it eventually emerges as newcrust. If this lateral flow of molten material is not as efficient as the upward removal of magma, the regions of extrusion, such as oceanic rises, will migrate.