Why short‐term crustal shortening leads to mountain building in the Andes, but not in Cascadia?

Abstract

Subduction‐induced crustal shortening, now measured by the GPS across many subduction zones, has led to mountain building in the Andes but not in Cascadia and some other Andean‐type convergent plate boundaries. Here we use a two‐dimensional viscoelastoplastic finite element model to explore how the GPS‐measured short‐term strain relates to long‐term mountain building. We show that previously proposed causative factors of mountain building can be represented by two model parameters: the strength of mechanical coupling on the plate interface, and the yield strength of the overriding plate. The critical condition for producing permanent (plastic) crustal shortening, hence mountain building, is for the plastic yield strength of the plate interface to be higher than that of the overriding plate. Strong trench coupling and a weak lithosphere explain the Andean mountain building, whereas weak trench coupling in Cascadia allows short‐term crustal shortening to be restored periodically by trench earthquakes and aseismic slips.