| Abstract: | Theoretical considerations and exploratory experiments indicate that most of the extreme change of large-strain “viscosities” between the solidus and the liquidus of rocks is concentrated about a non-zero rheological critical melt percentage (RCMP). Usually the RCMP is within the range of 20 ± 10 vol%. Below it, both bulk shear and segregative-compactive filter pressing are drastically slowed down, requiring strains in the solid fraction and squeezing of thin melt films. Many geological consequences of these critical phenomena may be far reaching. In particular, most of the upper-mantle low-velocity zone (LVZ), if partially melted, is likely to be stabilized somewhat below the RCMP, with runaway excesses of heat, melt and shear strain creating local excursions over the RCMP. Flow may be thus concentrated in narrow high-melt channels, within a relatively unsheared LVZ. |
