Structure of the thermal lithosphere and asthenosphere beneath oceans and continents

The lithosphere and asthenosphere make up a common geodynamic system but are characterized by different physical parameters. The former has a temperature of 1200–1300°C, a density of 3.3 g/cm³, and a viscosity of 10²² poise, while the latter has a density of 3.23 g/cm³, a viscosity in the range 10²¹-10^18–19 poise, and a temperature from 1200–1300°C to 1600–1700°C. The asthenosphere is distinguished by a great variability of its physical state in the lateral and vertical directions. This circumstance necessitates the recognition of the different types of the asthenosphere: seismic (LVZ zone), electrical, thermal, and seismological. The structure and the physical state of the thermal asthenosphere is considered in this paper on the basis of P-T parameters. Its state normally fits viscous Newtonian liquid beneath the continents and provides partial (5–20%) melting in spreading zones and along continental margins. No partial melting is detected beneath the main portion of the continents. The interaction between the asthenosphere and lithosphere is characterized by spatiotemporal migration of partial melting zones and asthenosphere upwelling, and such interaction determines the entire range of geodynamic processes from spreading and rifting to collision and vertical motions of different senses.