Shear-driven upwelling induced by lateral viscosity variations and asthenospheric shear: A mechanism for intraplate volcanism

Volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation, but instead must arise from sub-lithospheric processes that generate upwelling flow and decompression melting. Several convective processes, such as mantle plumes, convective instability, edge-driven convection, and Richter rolls, produce upwelling via the action of gravity on density heterogeneities in the mantle. Here we investigate an alternative mechanism, the shear-driven upwelling (SDU), which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity. Using a numerical flow model, we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer. We demonstrate that for certain geometries and viscosity ratios, circulatory flow develops within a “cavity” or “step” embedded into the lithospheric base, or within a low-viscosity “pocket” embedded within the asthenospheric layer. For asthenosphere shearing at 5 cm/yr, we estimate that SDU can produce upwelling rates of up to ∼0.2 cm/yr within a continental rift, ∼0.5 cm/yr along the vertical edge of a craton, or ∼1.0 cm/yr within a “pocket” of low-viscosity asthenosphere. In the last case, the pocket must feature an aspect ratio of more than 5, occupy ∼20-60% of the asthenosphere's thickness, and be at least 100 times less viscous than the surrounding asthenosphere. Such viscosity heterogeneity may be associated with thermal, chemical, melting, volatile, or grain-size anomalies, and is consistent with tomographic constraints on asthenospheric variability. We estimate that SDU may generate up to 2.5 km/Myr of melt that is potentially eruptible as surface volcanism; this is faster than eruption rates observed at some locations of continental basaltic volcanism. We conclude that SDU could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere, for example in the Basin and Range region of North America.