Force-balance modelling of the impact of glacial erosion, trench sedimentation, megathrust weakening and glacial loading on the stress state of the crust at active continental margins

Abstract

Force-balance analyses showed that the stress state at active continental margins is largely controlled by the gravitational force and the megathrust shear force and remains unchanged as long as subduction proceeds undisturbed. Glacially induced topographic changes and mass redistribution by glacial erosion, sediment transport and deposition may affect the force balance but the impact on the stress state and the style of deformation in the upper plate has not been investigated so far. Here, we use numerical force-balance models to investigate the stress changes in the upper plate resulting from (i) a reduction in mountain height in the arc by glacial erosion, (ii) a steepening of the arc front, (iii) a decrease in the megathrust shear force due to increased sediment subduction and fault weakening, (iv) an increase in sediment thickness in the trench, and (v) the load by an ice cap. Our model results show that each process causes distinct stress changes that affect different parts of the upper plate. The largest stress changes result from a reduction in mountain height, which increases compression in the arc interior, and fault weakening by increased sediment subduction, which decreases the megathrust shear force and hence deviatoric compression in the forearc and backarc. Smaller stress changes occur for a steepening of the arc front, increased sediment deposition in the trench and the load of the ice cap. The different stress changes may promote or suppress faulting in different parts of the upper plate. Application of our model to the North Patagonian Andes indicates that glacial erosion during late Cenozoic cold periods may explain the localization of deformation in the arc interior, whereas the reduced activity of thrust faults in the forearc and backarc may reflect a decrease in deviatoric compression caused by a decrease in the megathrust shear force.