low-velocity zone

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low-velocity zone (LVZ) The zone within the upper mantle beneath the oceans within which seismic P-waves are slowed and S-waves are slowed and partially absorbed. The top of the zone is some 40–60 km deep near the oceanic spreading ridges, and this depth increases to 120–160 km beneath the older oceanic crust. The bottom of the zone is poorly defined, but in the region of 250–300 km in depth. Beneath the continents, a restricted low-velocity zone occurs beneath crust areas subjected to orogenesis during the last 600 million years or so, but is not found beneath cratonic (see CRATON) areas. It is attributed to the presence of a 0.1% fluid phase and commonly ascribed to the partial melting of mantle rocks at these depths. It is often considered coincident with the asthenosphere, but probably this is valid only for oceanic areas.