Tectonic Regimes of the Central and Southern Andes: Responses to Variations in Plate Coupling During Subduction

Abstract

AbstractConstruction of the Andes has been governed largely by fluctuating contractional, neutral, and extensional tectonic regimes during differing degrees of mechanical coupling along the convergent plate boundary. These three contrasting regimes are likely regulated by geodynamic variations in absolute trenchward motion of South America and episodic regional shifts in the geometry of the subducting oceanic slab. Alternations among these three modes are revealed in syntheses of Mesozoic‐Cenozoic crustal deformation, basin evolution, and arc magmatism along orogen‐perpendicular transects across the central and southern Andes at 23°S, 35°S, and 43°S. Despite considerable along‐strike dissimilarities in the magnitude of deformation, a comparable tectonic regime typified the early Andean history, as defined by a transition from Late Jurassic‐Early Cretaceous postrift thermal subsidence to Late Cretaceous retroarc shortening. A key contradiction is expressed for the late middle Eocene to earliest Miocene, when neutral to extensional conditions affected retroarc to forearc regions, except in parts of the central Andes, which experienced retroarc shortening with only localized forearc extension. This mixed‐mode scenario during Paleogene time may reflect decoupling along the plate boundary (possibly during slab rollback within a retreating subduction system) in which widespread stasis or low‐magnitude extension dominated the southern Andes but was inhibited in the strongly shortened central Andes at 15–25°S where shallow subduction and/or high topography boosted plate coupling. Comparable temporal and spatial shifts in tectonic regime along the Andes and other convergent margins can be related to variable degrees of coupling during first‐order plate‐scale changes in convergence and second‐order regional cycles of slab shallowing and steepening.