Concave slab out board of the Tonga subduction zone caused by opposite toroidal flows under the North Fiji Basin

•A revised model is presented for deep seismicity outboard of Tonga slab.•Modelled Vanuatu Arc/Fiji Platform slab matches the extent of seismicity.•Concave geometry is caused by opposite toroidal flows during rollback.•Many slab features match the proposed double saloon door tectonics origin.

An alternative scenario is proposed for the origin of a concave NE-facing slab under the North Fiji Basin between the Tonga subduction slab and Vanuatu Arc. During rollback of the Australian Plate, Vanuatu Arc rotated clockwise, whereas Fiji Platform rotated counterclockwise from 12/10 Ma until 1.5 Ma ago. Thereafter, only Vanuatu Arc rotated until the present. During the period of opposite rotations, toroidal flows entered the mantle around the northwest slab edge of Vanuatu Arc and from the northeast slab edge of Fiji Platform. The latter lies close to the northern end of the Tonga slab where arc-parallel flows are shown by volcanic geochemistry and mantle anisotropy. Opposite toroidal flows with upwelling and downwelling components generate the concave form of the combined Vanuatu/Fiji Platform slab, match its extent to mapped deep seismicity, explain its position overlying the Tonga slab, provide a mechanism for high heat flow in the North Fiji Basin as well as enriched MORB and OIB basalts in the northern NFB, and obviate slab collisions invoked to produce slab curvature.