Upper-mantle structure and nature of deep processes in island arcs and trench systems

Upper-mantle structure and heat flow and gravity data are considered. It is shown that crustal isostasy disturbance is compensated for by inhomogeneities in the density of the upper mantle; compaction beneath trenches and discompaction under island arcs. On the basis of the available data an upper-mantle density model is constructed which is represented by a seismoactive layer with a surplus density of from 0.05 to 0.1 g/cm³ and a discompacted zone overlying that layer with a density deficiency of 0.05 g/cm³ beneath the island arc and 0.01 g/cm³ in the remaining area. The available data are in poor agreement with the concept of “new global tectonics”, therefore the formation of density inhomogeneities is associated with gravity differentiation of the mantle material. A scheme of this process is proposed according to which heated differentiates of the mantle material rise along deep faults or weakened zones from depths of 700 km and more. Upper-mantle material differentiation occurs mainly in several levels, but mostly at depths down to 150–200 km within the interval of 350–450 km. Separation of heavy components as a result of differentiation and the “zone melting process” causes a high velocity and correspondingly a denser inclined layer. Discompaction of the upper mantle overlying the focal plane is associated with its being heated by high-temperature melts circulating over the inclined fault.