Abstract
Several of Earth’s intra-plate volcanic provinces are hard to reconcile
with the mantle plume hypothesis. Instead, they exhibit characteristics
that are more compatible with shallower processes that involve the
interplay between uppermost mantle flow and the base of Earth’s
heterogeneous lithosphere. The mechanisms most commonly invoked are
edge-driven convection (EDC) and shear-driven upwelling (SDU), both of
which act to focus upwelling flow and the associated decompression
melting adjacent to steps in lithospheric thickness. In this study, we
undertake a systematic numerical investigation, in both 2-D and 3-D, to
quantify the sensitivity of EDC, SDU, and the associated melting to key
controlling parameters. Our simulations demonstrate that the
spatio-temporal characteristics of EDC are sensitive to the geometry and
material properties of the lithospheric step, in addition to the
magnitude and depth-dependence of upper mantle viscosity. These
simulations also indicate that asthenospheric shear can either enhance
or reduce upwelling velocities and the associated melting, depending
upon the magnitude and orientation of flow relative to the lithospheric
step. When combined, such sensitivities explain why step changes in
lithospheric thickness, which are common along cratonic edges and
passive margins, only produce volcanism at isolated points in space and
time. Our predicted trends of melt production suggest that, in the
absence of potential interactions with mantle plumes, EDC and SDU are
viable mechanisms only for Earth’s shorter-lived, lower-volume
intra-plate volcanic provinces.