Abstract
Several of Earth’s intra-plate volcanic provinces are difficult to
reconcile with the mantle plume hypothesis. Instead, they exhibit
characteristics that are better explained by shallower processes
involving 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 their associated melting to
several key controlling parameters. Our simulations demonstrate that the
spatial and temporal characteristics of EDC are sensitive to the
geometry and material properties of the lithospheric step, in addition
to the depth-dependence of upper mantle viscosity. These simulations
also indicate that asthenospheric shear can either enhance or reduce
upwelling velocities and predicted melt volumes, 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.