Thinning and Localization of Deformation During Rayleigh-Taylor Instability and its Implication for Intracontinental Magmatism

Thinning of the lower lithosphere due to Rayleigh--Taylor instability can be a source for continental magmatism near active or recently active plate boundaries. We consider whether it is also plausible as a mechanism for intra-continental magmatism, several hundred kilometers from active subduction or rift zones. For depth varying viscosity, Rayleigh--Taylor instabilities with a shear-stress free top can grow more rapidly than we expect when wavelengths are greater than approximately three times layer thickness (> 3h) and localized thinning can occur far from the loci of downwelling. We perform 2D Rayleigh--Taylor experiments and find that the combination of a shear-stress free top and non-Newtonian flow splits the deformation field of a growing instability into two groups, largely dependent on how viscosity varies with depth and with a transition zone in between. For small variation with depth, with the e-folding depth scale as large as a third to a half of the thickness of the unstable layer, deformation concentrates at the ends of the layer in zones of localized upwelling and downwelling, and the middle part of the layer moves horizontally towards the downwelling as a coherent block undergoing minimal strain rate. When viscosity varies more rapidly with depth, the pattern of deformation is such that the similarities between upwelling and downwelling diverge. These zones develop with different growth rates so that the localized upwelling deformation is minimal, and thinning of the layer is instead distributed laterally over a wide zone. Between the regions of thickening and thinning, shear strain and vertical gradients in horizontal velocity prevent this area from ever moving as a coherent block. The rheological exponent in the relation of strain rate varying as a power of stress, n, controls the degree of localization, or width, of the downwelling and upwelling. In geologic settings where a shear-stress free top condition could be applicable, a mix of rheological properties could provide a mechanism for the narrow zones of thinning and upwelling, which would facilitate decompression related volcanism.