Home – Home – Jornals – Russian Geology and Geophysics 2011 number 9

Deep seismic investigation carried out in Russia in long-range profiles with peaceful nuclear explosions allowed clarifying in detail the structure of the upper mantle and the transition zone down to a depth of 700 km within the vast area of old and young platforms in Northern Eurasia. The variability of horizontal heterogeneity of the upper mantle with depth serves for the qualitative estimation of its rheologic properties. The upper part of the mantle (down to a depth of 80-100 km) is characterized by a block structure with significant velocity steps of seismic waves at the blocks often divided by deep faults. This is the most rigid part of lithosphere. Below 100 km, horizontal heterogeneity is insignificant, i.e., at these depths the substance is more plastic and not capable to retain block structure. On the lithosphere bottom at a depth of 200-250 km, a plasticity increase is observed as well but the zone of the lower velocities that might have been related to the area of partial melting (asthenosphere) has not been found. These three layers with different rheologic properties are divided by seismic boundaries presented by thin layering zones with alternating higher and lower velocities. At the specified depths, no phase boundaries have been distinguished.  These thin layering zones are assumed to form owing to high concentrations of mantle fluids at some depths where the mechanical properties and permeability of substance change. A minor quantity of fluids might result in streaks with partial or film melting at relatively low temperature, i.e., the appearance of weakened zones where subhorizontal shifts are possible. According to seismic data, seismic boundaries are observed at a depth of about 100 and 200 km in many world regions; they can be globally spread. There are signs that areas of xenolith formation and earthquake concentration, i.e., zones of high deformations, are confined to these depths.