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| We present time-dependent three-dimensional (3D) fully-dynamic simulations of subduction to study the influence of a non-uniform overriding plate on the evolution of slab geometry and induced mantle flow. The equations of conservation of mass, momentum and energy are solved for a 3D incompressible fluid. Time-dependent subduction is driven by thermal density anomalies prescribed by the initial thermal structure. We find that along-strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate; the variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones, where both slab dip and overriding plate thermal state are strongly variable and correlated, and thus may be an important mechanism in other subduction zones. The development of hydrodynamic instabilities in the mantle wedge eventually leads to complete coupling between plates and subduction cessation. We further emphasize that the lithospheric structure of the overriding plate should be taken into account in analysis and modelling studies of subduction zones. |
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