Re-examining Slab Pull: Trench Topography and Trailing Plate Force Balance in Numerical Subduction Models

Slab pull has come to be widely regarded as a dominant driver of plate motions. The nature of slab–plate coupling is typically conceptualised in terms of the Orowan–Elsasser stress-guide model, in which the capacity of the slab to support a differential stress results in a tension-like force transmitted through the subduction hinge, providing an edge force on the trailing plate. Meanwhile, advances in geodynamic modelling now allow subduction to be simulated using increasingly Earth-like constitutive behaviour and, critically, permit the internal force balance to be examined explicitly. While the forces driving tectonic plates on Earth remain debated, the force balance within any given numerical model should be unambiguous.  I discuss results from a vertically integrated horizontal force balance applied to a suite of numerical subduction models. I focus on a particularly useful decomposition that highlights the role of topographic (or gravitational potential energy–related) forces, including ridge push, plate tilting driven by asthenospheric pressure gradients, and—critically—the influence of non-isostatic trench topography. Each of these topographic forces can be expressed in terms of differences in the integrated vertical normal stress - a proxy for the topographic-related pressure gradients in the boundary layer. The trench topographic force,   or trench pull force, is of special interest because it mediates the coupling between predominantly vertical loading imparted by the slab and a horizontal force (pressure gradient) acting on the trailing plate.  Numerical models suggest that a tension-like formulation of net slab pull plays at most a secondary role. Instead, it is primarily through the trench topographic force (trench pull) that the slab induces a net horizontal force on the trailing plate. Numerical models provide a direct means to isolate, compare, and quantify the trench topographic force relative to a tension-like edge force, and to establish quantitative bounds that can guide future analytical investigation of trench topographic forces.