CONTRASTING TECTONIC REGIMES AND BASIN EVOLUTION IN RESPONSE TO VARIABLE PLATE COUPLING ALONG THE ANDEAN MARGIN

A long complex history of Andean basin evolution can be reconciled with geodynamic models relating tectonic regime to variations in coupling between the subducting and overriding plates. Emerging results from the central and southern Andes suggest that Mesozoic-Cenozoic construction of the Andean orogenic belt and associated forearc and retroarc basins has been governed by fluctuating contractional, neutral, and extensional tectonic regimes during contrasting degrees of mechanical coupling along the subduction plate boundary. These contrasting regimes are likely regulated by (1) first-order geodynamic variations in absolute trenchward motion of the South American plate and (2) second-order episodic regional shifts in the geometry of the subducting oceanic slab (i.e. phases of slab shallowing and steepening). Alternations among these three modes are revealed in reconstructions of deformation, arc magmatism, and basin subsidence for three east-west transects across the Chile-Argentina segment of the Andes at 23°S, 35°S, and 43°S. Despite considerable along-strike variations in the magnitude of deformation, a relatively uniform tectonic framework defined the early Andean history, with a regional transition from Late Jurassic-Early Cretaceous postrift thermal subsidence to Late Cretaceous retroarc shortening (Rio Mayo, Cañadón Asfalto, San Jorge, and Neuquén basins). However, a key contradiction is revealed for the late Eocene to early Miocene, when neutral to extensional conditions during slab retreat/rollback affected most forearc/intraarc regions (Abanico, Cura Mallín, and Loncopué basins) and retroarc regions (El Maitén and Malargüe foreland basins), yet principally retroarc shortening characterized the central Andes at 15-25°S. This scenario may reflect transient phases along the plate boundary in which selected segments of shallow subduction and inboard arc magmatism (particularly in the southern Andes) or concentrated crustal thickening (such as the highly shortened central Andes) boosted plate coupling and inhibited extension. Comparable temporal and spatial shifts in tectonic regime along the Andes and other convergent margins can be related to variable plate coupling during first-order changes in plate convergence and second-order cycles of slab shallowing and steepening.