Growth of the Central Andean Plateau by Tectonic Segmentation Is Controlled by the Gradient in Crustal Shortening

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Abstract

Crustal deformation, orocline formation, and growth of the central Andean plateau as well as the mutual relations of these processes with climate are a matter of international debate. Earthquake focal mechanism, fault kinematic, paleomagnetic, and cross‐section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model. We offer such a model that approximates simple shear in plan view and that predicts the kinematics of first‐order fault zones in the southern central Andes. The model indicates that segmentation of the upper crust into rhomb‐shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the Andes. An anastomosing network of kinematically coupled, reactivated orogen‐parallel and newly formed NE‐trending deformation zones encompassing components of sinistral and dextral strike‐slip, respectively, defines the domain boundaries. Differential strike‐slip on these boundaries accounts for the presence of both NW‐SE‐ and ENE‐WSW‐directed shortening inferred from kinematic analysis of minor faults as well as Neogene paleomagnetic rotations. Specifically, paleomagnetic rotations are interpreted in terms of both rigid and shear‐induced rotations, the latter being defined as the angular departure from originally orthogonal lines. The net rotation of both rotation components predicted by the kinematic model agrees well with observed paleomagnetic rotations. Differential strike‐slip is an integral part of deformation east of the magmatic arc that led to curvature and longitudinal stretching of the orogen. Crustal segmentation commenced in Eocene time, generated closed basins, and propagated from the central bend to the southern terminus of the plateau at a rate of ca. 26 km/Ma. Along with high sediment accumulation under arid climatic conditions, crustal segmentation into well‐defined domains contributed effectively to the horizontal and vertical growth of the central Andean plateau. This holds important consequences for the structural evolution of the eastern fold‐and‐thrust belt of the central Andes and is akin to the mechanism by which large parts of the NW Tibetan plateau experienced surface uplift.

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