Petrophysical Features of the Upper Mantle Structure beneath Northern Eurasia and Their Nature

Abstract

Deep seismic studies of the upper mantle conducted in Russia with the use of nuclear explosions and laboratory studies of mantle materials at high pressure and temperature have revealed new structural and petrophysical features of the upper mantle beneath Northern Eurasia. These features are hard to explain within the framework of current understanding of the structure of the continental lithosphere. For example, in the region of the asthenosphere distinguished according to the thermal field, no corresponding decrease in seismic velocities has been detected, but instead low-velocity layers have been identified at a depth of 100–150 km within the lithosphere. However, the asthenosphere may be distinguished by structural features of the seismic boundaries. This means that it is represented by a layer of elevated plasticity without partial melting. The laboratory studies also indicate that seismic velocities do not depend on the composition of mantle rocks and are controlled primarily by their temperature. This made it possible to infer the temperature regime of the upper mantle from seismic data. Calculations carried out on this basis have shown that the thickness of the lithosphere beneath the Siberian Craton does not vary and is 300–350 km. These data are inconsistent with evaluations of the lithosphere bottom depth based on the heat flow at the surface. This discrepancy may be explained by a stronger effect of deep fluids on the heat flows and on petrophysical parameters of mantle rocks. At depths where mechanical properties of rocks drastically change and their porosity increases, the density of the deep fluids decreases, and the fluids release much heat. As a result, low-velocity domains (plumes), which may contain partial melts, are formed and the surface heat flow increases. This may explain how low-velocity layers can be formed at a depth of 100–150 km and why the temperatures determined from seismic data differ from those derived from the heat flow data. Deep fluids also initiate physical and chemical transformations in mantle rocks, for instance, produce depleted material that has a reduced density but is characterized by the unvarying seismic velocity. Deviations from linear relationship between seismic velocity in a material and its density is also detected at integrated interpretations of seismic and gravimetric data. Physicochemical transformations of rocks in areas where deep fluid flows are focused can also account for the origin of complex reflective boundaries identified in the lithosphere.