Dike propagation driven by melt accumulation at the lithosphere–asthenosphere boundary

Abstract

This study examines the combined effects of melt migration by porous flow in the asthenosphere and dike propagation in the lithosphere. Melt collecting at the base of the lithosphere forms a decompacting boundary layer (DBL), in which the overpressure is sufficient to nucleate dikes that propagate buoyantly upward into the lithosphere. The asthenosphere melt flux determines the excess pressure and melt accumulation rate in the DBL, which together with the state of lithospheric stress, control dike growth rate, dike recurrence interval and the height to which dikes propagate. The vertical propagation and subsequent freezing of melt filled dikes heats and thins the lithosphere.Our model couples fundamental aspects of dike propagation and porous flow that are commonly treated separately. Our model allows us to estimate conditions under which vertically propagating dikes can thin the lithosphere, given a melt flux determined by the rate of melt production in the asthenosphere. The model also provides an estimate of the amount of melt present at the base of the lithosphere. We find that a steady state high-porosity boundary layer at the lithosphere–asthenosphere boundary, with a melt fraction about 2.5–4 times higher than the asthenosphere melt fraction. Diking occurs at melt fractions much less than the disaggregation limit, so dikes are only a few km tall and about 1 cm wide. Though dikes are small, their recurrence on the order of days can lead to lithosphere erosion rates on the order of a few km/Myr with a melt fraction of a few percent at the base of the lithosphere. The steady state boundary layer melt fraction is controlled by differences in lithospheric stress state and or asthenosphere melt flux, indicating that seismic discontinuities associated with melt accumulation at the lithosphere–asthenosphere boundary should vary systematically with variations in asthenosphere melt generation and tectonic setting.