Impact of decreasing trench depth during aseismic ridge subduction on the forearc stress state: Insights from analytical and finite-element force-balance models

When aseismic ridges carried by the subducting oceanic plate enter a subduction zone, the trench depth and hence the margin relief is reduced, which increases the compression of the upper plate. The increase in compression may be relevant for understanding surface uplift and mountain building in response to ridge-subduction, but detailed effects remain to be explored. Here we use analytical and two-dimensional finite-element force-balance models to investigate the effects of relief changes and other parameters that may change during ridge subduction, including 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 modeling results indicate that the increase in upper-plate compression mainly depends on the total relief change, the trench depth prior to ridge subduction and the submarine surface slope angle during ridge subduction. Secondarily, the increase in compression also depends on the average dip angle and curvature of the plate interface, as well as on the density structure of the upper plate and the mountain height prior to subduction. The enhanced upper-plate compression due to ridge subduction promotes mountain building in the upper plate until the increase in elevation leads to stress conditions comparable to those before the entrance of the ridge. We investigate this aspect for the subduction of the Cocos Ridge, based on additional finite element models that approximate the setting along the Central American margin near Costa Rica before and after the entrance of the ridge. The models indicate that the mere decrease in trench depth of ~3.3 km due to ridge subduction promoted an increase in mountain height of ~0.6 km. This corresponds to one-third of the maximum uplift inferred for Costa Rica. We further find that the remaining elevation increase of up to 1.4 km cannot be explained by changes in the slab dip angle or upper-plate density structure but may indicate an increase in shear stress along the plate interface. Taken together, our analysis shows that the decrease in trench depth during ridge subduction increases the compression of the upper plate, which promotes surface uplift and mountain building even at greater distances to the ridge.