On the rheologically critical melt fraction

Our previous work from regional geoid inversion over oceans has shown that there exists a low viscosity zone (LVZ) under the 660 km boundary. Nonlinear inversion has been applied for fitting to the intermediate-wavelength oceanic geoid without invoking a priori any impermeable boundary. In this study we have conducted a systematic search of the dependence of the depth of this regional boundary from 660 to 1000 km. We found that the use of this boundary at the depth of the LVZ exerts little influence on the inversion. The LVZ can act regionally as an impermeable boundary, and enhances very fast lateral flow at the uppermost lower mantle. On the other hand, impermeable boundary alone is not caused by a LVZ, as shown also by analysis of the 2D flow patterns, driven by internal density anomalies inferred from tomography. From the intermediate-wavelength geoid, we can not resolve the existence of layering, when the LVZ exists beneath the 660 km boundary. However, a LVZ is needed to fit the geoid. This LVZ can have profound influence on the geodynamical behavior of mantle circulation and the dispersal of geochemical anomalies and the generation of seismic anisotropy.