DYNAMICS OF THE MANTLE IN SUBDUCTION ZONES ( exemplified by the Cretan Arc )

A flow structure in the upper mantle has been obtained along the meridional profile crossing the Mediterranean Cretan arc. Numerical modeling of the flow is based on the assumption that velocity anomalies in a tomographic model of the region reflect density variations in the mantle. Agreement between calculated and observed parameters, first of all the recent vertical rate of the crust movement, is reached only for the no-slip upper boundary of the convecting mantle (lithosphere base), while in the majority of numerical models of the mantle flow a rigid slippery boundary is usually adopted. The maximum of calculated shear stress of the viscous moving mantle appears to coincide with the position of deep earthquake hypocenters of the region, they both being beyond the slab, part the oceanic lithosphere subducting into the mantle. Seismic events probably result there from momentary displacements of the mantle moving relative to the rigid slab. This conclusion is alternative to the usually accepted mechanism of the deep focus seismicity assuming a fragile momentary break of the slab itself as it moves in the mantle.