Rapid mantle flow beneath the Tonga volcanic arc

Flow in the Earthʹs asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates, suggesting flow rates comparable to plate velocities. However, arc-parallel shear-wave splitting and geochemical observations imply along-strike flow in the Lau basin, presenting a challenge to geodynamic models of plate-driven subduction systems as plate-driven flow models predict arc-perpendicular fast direction with simple (A-type) mineral texturing. Although B-type mineral texturing would result in the fast directions developing 90° relative to A-type and likely occurs in the forearc mantle, such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail. By tracking mineral texturing development within geodynamic flow models patterned after the Tongan system in the Lau basin that include along-strike flow, we find that the observations are best explained by models including rapid along-arc flow within the subarc mantle. The effect of melt on the anisotropy does not appear to be a primary control, but needs to be investigated more fully. The best fit to the observations is a model with along-strike flow within a low-viscosity channel beneath the arc at a rate of nearly 50 cm/yr, several times the plate motion velocities.