Variability of the shortening rate in Central Andes controlled by subduction dynamics and interaction between slab and overriding plate.

The nature of the shortening of the Central Andes has been a matter of debate. The South American plate is advancing westwards forcing the subducting Nazca plate to roll back and the trench to retreat. But as the trench slowed its retreat the Andean mountain belt formed. This decrease of trench velocity has been attributed to the anchoring of the slab, but this process cannot explain the observed pulsatile behaviour of the shortening rate. Indeed, whereas the formation of the Central Andes started ~50 Ma ago, most of the shortening and elevation growth, including the formation of the Altiplano-Puna plateau, took place in two pulsatile steps at 15 Ma and 7 Ma as recognized from geological data. Thus we hypothesize that the deformation of the Central Andes is controlled by the subduction dynamics and a complex interaction between the overriding and subducting plates.

We used the FEM geodynamic code ASPECT to develop a self-consistent subduction E-W-oriented 2D high-resolution geodynamic model along the Altiplano-Puna plateau (21°S). This model incorporates the flat slab subduction episode at 35 Ma and follows the evolution of the lithospheric deformation. Our model results reproduced the observed spatial and temporal variations of tectonic shortening in Central Andes.

Three main conditions related to the plate interaction are of key importance to explain the observed shortening rate evolution in Central Andes. Firstly, the subduction dynamics affects the trench migration: each episode of slab steepening is followed by the blocking of the trench. The steepening occurs after the flat slab and at the end of two slab-buckling instabilities at 15 Ma and at 7 Ma. The second relevant process is the weakening of the overriding plate. This is ensured by the partial removal of a part of the lithospheric mantle after the re-steepening of the flat slab at 35 Ma and by weakening of the sediments in the Subandean Ranges after 10 Ma. Thirdly, a relatively high interplate friction coefficient (~0.05) is needed to ensure the stress transfer from the slab to the overriding plate, which is further enhanced by the delaminated mantle lithosphere eventually blocking the subduction corner flow.

The pulses of shortening rate occur at the end of each slab-buckling cycle when the trench is blocked. The deformation of the overriding plate is intensified by the eclogitization of the lower crust and the subsequent delamination of the sublithospheric mantle. Finally, at ~10 Ma, the deformation switches from pure-shear to simple-shear shortening, after the underthrusting of the Brazilian craton in presence of weak foreland sediments.