Rheology of the upper mantle; some geodynamic and magmatogenic implications

Geotherms vary between cases where the solidus T of wet peridotite is not reached at any depth (e.g. Precambrian shields), and cases where the solidus is reached not far beneath the Moho (e.g. continental extension zones). Given the exponential T dependence of the rheology, lateral T variations result in viscosities in the upper mantle which vary in the horizontal direction by orders of magnitude. Two factors that affect the rheological properties are degree of partial melting and the volatile content; the presence of volatiles can have a considerable softening effect. If conditions are favourable, melt is produced in sufficient quantities and at a sufficient rate, and migrates upwards; the most likely form of magma transport across the lithosphere is via the propagation of fluid-filled fractures. The rheological properties of both magma and host rock are well within realistic limits for this process to be an efficient mechanism of mass and heat transfer.