Impact of trench shallowing during aseismic ridge subduction on the stress state and topography of the upper plate: Insights from 2D analytical and numerical force-balance models

When aseismic ridges carried by the subducting oceanic plate enter a subduction zone, they reduce the trench depth and hence the relief of the convergent margin. The trench shallowing increases the deviatoric compression of the upper plate. While the increase in compression can be expected to promote mountain building and surface uplift, these effects have never been quantified. Here we use analytical and two-dimensional finite-element force-balance models to investigate the effects of trench shallowing and evaluate the influence of the initial trench depth, the megathrust dip angle, the slab curvature, the submarine surface slope angle, the density structure of the upper plate, the initial mountain height and the surface topography of the upper plate. Our modelling results indicate that the increase in deviatoric compression mainly depends on the total relief change, the trench depth prior to ridge subduction and the submarine surface slope angle during ridge subduction, with secondary influences from the mountain height and curvature of the plate interface, as well as the average dip angle and the density structure of the upper plate prior to subduction. Additional finite-element models with setups adjusted to the Kamchatka and Central American margins indicate that the surface uplift induced by trench shallowing accounts for nearly all of the maximum uplift in Kamchatka and approximately one-third in Central America. The remaining elevation increase in Central America may indicate passive uplift above the subducting ridge and an increase in shear stress along the plate interface.