How does trench coupling lead to mountain building in the Subandes? A viscoelastoplastic finite element model

Subduction-induced crustal shortening is believed to be the primary cause of the Andean mountain building. The present-day crustal shortening in the Andes is clear from the GPS measurements, but the rate (30-40 mm/a) is higher than the geological shortening rate (< 15 mm/a), suggesting that much of the present-day crustal shortening may recover during future earthquakes in the subduction zone. Using a two-dimensional viscoelastoplastic finite element model, we investigate here how the cyclic trench coupling leads to long-term mountain building, which has been concentrated in the Subandes in the past few million years. Our results show that much of the GPS-measured crustal shortening in the Andes can be explained by the interseismic and postseismic viscoelastic strain that does not contribute to mountain building. The apparently discordant GPS and geological data can be reconciled in the geodynamic model with a weakened crust in the Subandes, which allows accumulation of plastic strains that leads to mountain building. The localized crustal weakening and mountain building in the Subandes may have been a consequence of the uplifted High Andes, the accelerated westward drift of the South American plate, the possible delamination of mantle lithosphere under the Eastern Cordillera, and the increasing trench coupling.