RHEOLOGY OF THE UPPER-MANTLE - SOME GEODYNAMIC AND MAGMATOGENIC IMPLICATIONS

Basic magmatism originates by partial melting in the upper mantle. In order to place this process in a geodynamic context, this paper focuse s on the long-term theological properties of upper mantle materials an d on their lateral variations. Geotherms vary between cases where the solidus temperature of wet peridotite is not reached at any depth (e.g ., Precambrian Shields), and cases where the solidus is reached not fa r below the Moho (e.g., continental extension zones). Given the expone ntial temperature dependence of the theology, lateral temperature vari ations result in viscosities in the upper mantle which vary in the hor izontal direction by orders of magnitude. Two factors that affect the rheological properties are degree of partial melting and volatile cont ent. While a small (less than a few percent) amount of melting may not significantly change the steady-state theology, the presence of volat iles can have a considerable softening effect. If conditions are favou rable, melt is produced in sufficient quantities and at a sufficient r ate, and migrates upwards. The most likely form of magma transport acr oss the lithosphere is through the propagation of fluid-filled fractur es. The theological properties of both magma and host rock are well wi thin realistic limits for this process to be an efficient mechanism of mass and heat transfer.