Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline

The building of the Andes results fromthe subduction of the oceanic Nazca plate underneath the South American continent1,2. However, how and why the Andes and their curvature, the Bolivian orocline, formed in the Cenozoic era (65.5 million years (Myr) ago to present), despite subduction continuing since the Mesozoic era3 (251.0– 65.5Myr ago), is still unknown. Three-dimensional numerical subduction models demonstrate that variations in slab thickness, arising from the Nazca plate’s age at the trench, produce a cordilleran morphology consistent with that observed1,2. The age-dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate, causing it to thicken. Thus, subducting older Nazca plate below the Central Andes can explain the locally thickened crust and higher elevations. Here we demonstrate that resultant thickening of the South American plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the Bolivian orocline. Additionally, the varying forcing along themargin allows stress belts to form in the upper-plate interior, explaining the widening of the Central Andes and the different tectonic styles found on their margins, the Eastern and Western Cordilleras2. The rise of the CentralAndes and orocline formation are directly related to the local increase of Nazca plate age and an age distribution along themargin similar to that found today; the onset of these conditions only occurred in the Eocene epoch4. Thismay explain the enigmatic delay of the Andean orogeny, that is, the formation of the modern Andes.