Recent geophysical and geochemical studies suggest that some subduction zones have a significant component of trench-parallel flow that is difficult to reconcile with conventional 2D corner-flow models. Mapping of different mantle Pb-isotopic signatures provides an opportunity to assess the role of along-strike flow beneath the Vanuatu region. Young back-arc spreading centres in the southern segment of the subduction zone have high 208Pb/204Pb for a given 206Pb/204Pb, similar to the Indian-type mantle signatures of present-day volcanoes in the central segment of the arc, but distinct from adjacent southern-arc volcanoes with Pacific-like mantle traits. This configuration indicates that convection in the mantle wedge beneath Vanuatu is dominated by southward flow parallel to the trench, rather than pure corner flow induced by viscous drag along the upper surface of the subducting plate. We propose that this southward flow is related to pressure gradients developed during asymmetric rollback of the Australian plate. Trench-parallel flow rates in Vanuatu are estimated at 10–28 cm a−1, further highlighting that along-strike flow can exceed the convergence-parallel mantle motions predicted from conventional 2D corner-flow models.