Dynamic modelling of post-collisional magmatism

Abstract

This study addresses the question of post-collisional magmatism and its production mechanisms, addressing especially the mantle processes involved. Numerical experiments are conducted to examine the effects of viscosity weakening by subduction related water content increase in the upper mantle and the resulting sub-lithospheric small-scale convection. The models presented incorporate parameterized and thermodynamic melting models, and take into account variable relationships between mantle water content, mantle strength, water extraction by partial melting and related depletion stiffening.

The results demonstrate the possible importance of so called ”hydrous activation” of the lithosphere-asthenosphere boundary: The post-collisional loss of the lithospheric mantle can be initiated and augmented by the elevated upper mantle water contents that enhances the sub-lithospheric small-scale convection, increases heat flow into the lithosphere, and produces localized lithosphere thin- ning. The irregular spatial and temporal melting patterns and the mantle melt volumes correspond to typical post-collisional mantle-derived magmatism. The small-scale convection can be localized into an edge-driven convection by significant lithosphere thickness gradients, e.g. craton edges. This helps to understand the uplift and volcanism observed in intraplate orogenic settings and implies the importance of these processes at other locations of lithosphere thickness gradients, e.g. recent collision zones.

The lithospheric thinning produced by small-scale convection can initiate whole lithosphere mantle loss via positive feedback mechanisms: gradual thinning of the lithosphere causes partial melting in the lowermost crust, weakening the crust-mantle boundary and providing a detachment mechanism for the lithospheric mantle, leading to stronger lithosphere thinning and, finally, exposure of the lower crust to the hot asthenosphere.
Small-scale convection and processes related to or initiated by it offer new insight and future research possibilities in studies of continental collision magmatism.