New views on the structure and evolution of the Andes-Altiplano orogenic system: Are collision- and subduction-type mountain belts mechanically different?

Abstract:

The Andes-Altiplano orogenic system, one of the most significant topographic feature on Earth, is the case example of subduction-type mountain belts. This latter conceptual model considers that the overall structure of the mountain belt forms antithetic to the subduction zone which marks the main plate interface, and as such poses several mechanical issues. Recent studies challenged this view, by proposing a model of the Andes involving an embryonic west-vergent intracontinental subduction, synthetic to the subduction zone and thus geometrically comparable to alpine-type collision belts. Here, we compare these two models of the Andes, by re-evaluating a cross-section of the range at the latitude of Santiago del Chile (33.5 °S). A particular structure, the east-vergent Aconcagua fold-and-thrust belt (AFTB), appears critical in discussing and discriminating between these two views. We revise the structural geometry of the AFTB based on satellite imagery, digital elevation models, existing geological maps and field observations. We propose a new 3D cartography and cross-section of the AFTB and conclude that the AFTB develops over a 2-3 km deep décollement, with a finite shortening of ~13 km. We integrate the AFTB to the well-documented fold and thrust belt forming the western flank of the belt and find that it is a relatively secondary feature of the Andes. From kinematic modelling using Move software (Midland Valley Ltd), we build a crustal-scale evolutionary model showing the progressive shortening of the Andean margin. We find that the AFTB can be modelled as a secondary east-verging structure, passively transported westward atop a basement ramp anticline forming the Frontal Cordillera. This evolutionary model is discussed and constrained in perspective of available thermochronology data. Our model therefore suggests that the Andes orogen can be mechanically modelled as an alpine-type collision belt, challenging our long-standing view of subduction-type mountain-belts.