The non-collisional subduction margin of South America is characterized by different geometries of the subduction zone and upper-plate tectono-magmatic provinces. The localization of deformation in the southern Central Andes (29°S–39°S) has been attributed to numerous factors that combine the properties of the subducting oceanic Nazca plate and the continental South American plate. In this study, the present-day configuration of the subducting oceanic plate and the continental upper plate were integrated in a data-driven geodynamic workflow to assess their role in determining strain localization within the upper plate of the flat slab and its southward transition to a steeper segment. The model predicts two fundamental processes that drive deformation in the Andean orogen and its foreland: eastward propagation of deformation in the flat-slab segment by a combined bulldozing mechanism and pure-shear shortening that affects the broken foreland and simple-shear shortening in the fold-and-thrust belt of the orogen above the steep slab segment. The transition between the steep and subhorizontal subduction segments is characterized by a 370-km-wide area of diffuse shear, where deformation transitions from pure to simple shear, resembling the transition from thick to thin-skinned foreland deformation in the southern Sierras Pampeanas. This pattern is controlled by the change in dip geometry of the Nazca plate and the presence of mechanically weak sedimentary basins and inherited faults.

Controls on the deformation pattern (shortening mode and tectonic style) of orogenic forelands during lithospheric shortening remain poorly understood. Here, we use high-resolution 2D thermomechanical models to demonstrate that orogenic crustal thickness and foreland lithospheric thickness significantly control the shortening mode in the foreland. Pure-shear shortening occurs when the orogenic crust is not thicker than the foreland crust or thick, but the foreland lithosphere is thin (< 70-80 km, as in the Puna foreland case). Conversely, simple-shear shortening, characterized by foreland underthrusting beneath the orogen, arises when the orogenic crust is much thicker. This thickened crust results in high gravitational potential energy in the orogen, which triggers the migration of deformation to the foreland under further shortening. Our models present fully thick-skinned, fully thin-skinned, and intermediate tectonic styles in the foreland. The first tectonics forms in a pure-shear shortening mode whereas the others require a simple-shear mode and the presence of thick (> ~4 km) sediments that are mechanically weak (friction coefficient < ~0.05) or weakened rapidly during deformation. The formation of fully thin-skinned tectonics in thick and weak foreland sediments, as in the Subandean Ranges, requires the strength of the orogenic upper lithosphere to be less than one-third as strong as that of the foreland upper lithosphere. Our models successfully reproduce foreland deformation patterns in the Central and Southern Andes and the Laramide province.