Lithospheric thinning and localization of deformation during Rayleigh-Taylor instability with nonlinear rheology and implications for intracontinental magmatism

Thinning of mantle lithosphere due to Rayleigh-Taylor instability can be a mechanism for triggering continental magmatism near active or recently active plate boundaries. We consider whether it is also plausible as a mechanism for intracontinental magmatism, several hundred kilometers from active subduction or rifting. We perform two-dimensional Rayleigh-Taylor experiments and find that a shear stress-free top and non-Newtonian flow permit two types of instability to develop, largely dependent on how the viscosity coefficient varies with depth. For small variation with depth, with the e-folding depth scale (the interval across which the coefficient changes by a factor of e) greater than a third to a half of the thickness of the unstable layer, deformation concentrates at the ends of the layer in localized thinning and thickening zones; the middle part moves horizontally toward the region of thickening as a coherent block undergoing minimal strain. When the viscosity coefficient decreases more rapidly with depth, thinning of the layer is 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 moving as a coherent block. The rheological exponent, n, that relates strain rate to stress in the constitutive equation controls the degree of localization of the downwelling and upwelling: the width varies as approximate to n(-1/2). In intraplate settings where a shear stress-free top condition could be applicable, high-stress crystalline plasticity could provide a mechanism for the narrow zones of thinning and upwelling, which would facilitate decompression related volcanism.