On stochastic near-critical oscillations in the upper mantle

The problem of the determination of thermomechanical conditions in the upper mantle under a moving lithospheric plate at a given shear stress does not have a unique solution. Given a fixed heat flux from the lower mantle, two types of motion are possible in the mantle: subcritical (slower and colder) and supercritical (faster and hotter). In this work, it is shown that, if these modes are rather close to each other (in the near-critical state of the mantle), transitions from one mode to another and backward are possible. The calculated period of a change in the mantle state amounts to ～0.3 Myr for the Pacific plate. The oscillatory regime of the mantle state can manifest itself in the activity pulsations of hotspots located near fast separating mid-ocean ridges.