A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate

We present numerical subduction models to investigate overriding plate deformation at subduction zones. All models show forearc shortening, resulting predominantly from shear stresses at the subduction zone interface and opposite-sense mantle shear stresses at the base of the forearc lithosphere. Models dominated by backarc extension show that it results from trench-normal positive velocity gradients in the mantle below the overriding plate. Such gradients result from toroidal mantle flow induced by slab rollback, with velocities below the leading part of the backarc faster than the overriding plate velocity. The velocity gradients induce basal shear stresses that increase trenchward and cause trenchward overriding plate motion at a velocity (v OP⊥) whose spatial average is below the trench retreat velocity (vT⊥). The combination of basal shear stresses and average vOP⊥ vT⊥. This results in trench-normal deviatoric compression and shortening in the leading part of the overriding plate as it collides with the subduction hinge. Ultimately, the geodynamic models demonstrate that backarc extension is favored for narrow slabs and near lateral slab edges and is driven by rollback induced toroidal mantle flow, while backarc shortening is favored for the center of wide slabs and is driven by poloidal mantle flow resulting from downdip slab motion. Key Points Slab rollback driven toroidal mantle flow causes trench-normal velocity gradient The velocity gradient induces basal shear stresses that drive backarc extension Backarc shortening results from overriding plate collision with subduction hing