Effects of glaciations on the tectonics of active continental margins: Insights from force-balance models and implications for the North Patagonian Andes

Force-balance models show 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 changes in mountain topography and mass redistribution by glacial erosion, sediment transport and deposition alter the force balance but the impact on the upper-plate stress state and tectonics remains quantitatively poorly constrained. Here, we use numerical force-balance models (Matthies et al., 2024) and investigate upper-plate stress changes 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 of 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 the deviatoric compression in the arc, and a decrease in megathrust shear force, which reduces the deviatoric compression in the entire upper plate. Smaller stress changes occur for a steepening of the arc front, increased sediment deposition in the trench and the load of an 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 have localized the deformation in the arc interior but did not significantly reduce the mean elevation of the mountain range. Moreover, the reduced activity of thrust faults in the forearc and backarc likely reflect reduced compression of the upper plate due to a decrease in megathrust shear force.   Matthies, F., Dielforder, A., & Hampel, A. (2024). 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. Tectonophysics, 871, 230180. https://doi.org/10.1016/j.tecto.2023.230180.