General features of the uppermost mantle stratification from long-range seismic profiles

Abstract

Long-range seismic studies made by Russian institutions during the last two decades reached depths of 700–800 km on the continent (Peace Nuclear Explosions were used as a source) and 100 km in the ocean (ocean bottom stations recording 500-kg explosions in water). Comparison of the wave-fields of these and other seismic profiles in Russia, Western Europe and the South Atlantic shows that the classical lithosphere-asthenosphere model is too simple to explain the observed data. A stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle. One of the layers is globally observed at a depth of 80 to 100 km. It is located in the thermal lithosphere beneath old platforms, at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid-oceanic ridges. Relatively high velocities (8.4 to 8.5 km/s) are typical for this boundary both in the old platforms and in the high heat flow oceanic areas. A change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level. Such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere.