Time dependence of intraplate volcanism caused by shear‐driven upwelling of low‐viscosity regions within the asthenosphere

摘要

Although volcanism far from tectonic boundaries is likely due to upwelling near the lithospheric base, the convective processes that induce upwelling are unclear. Numerical models show that asthenospheric shear can be deflected upward by lateral viscosity variations within the asthenosphere, producing “shear‐driven upwelling” (SDU). To constrain the rate, duration, and surface expression of intraplate volcanism caused by SDU, we simulate 2‐D flow and peridotite melting in the upper 200 km of the mantle. Asthenospheric shear is driven by lithospheric plates with different thicknesses moving at 3–9 cm/yr, and the initial low‐viscosity region is a rectangularly shaped pocket with an imposed viscosity that is 2 orders of magnitude smaller than the surrounding asthenosphere. Melting decreases as the pocket deforms and reaches steady state after 3–12 Myr. The age progression of surface volcanism is nearly stationary in the reference frame of the plate, which distinguishes SDU from hot spot volcanism. Similar behavior occurs if the viscosity heterogeneity is induced by variations in the water content of mantle peridotite. If the pocket’s low viscosity is caused by excess temperature, buoyant upwelling of the entire pocket dominates volcanism. Differences in the time dependence of volcanism associated with damp and warm pockets may help identify which type of mantle heterogeneity and associated dynamic process best explains weak, intermittent, intraplate volcanism with no obvious age progression. We suggest that asthenospheric shear induced by plate motions and global mantle flow, by exciting SDU, drives some of the non–hot spot small‐scale volcanism that occurs away from plate boundaries.