CENOZOIC MAGMATISM AND DEFORMATION IN NORTHERN AND CENTRAL CHILEAN ANDES: TWO PROCESSES FOLLOWING DIFFERENT PATHS IN THE CONSTRUCTION OF THE MODERN OROGEN

In northern and central Chile, a major tectonic phase occurred in Mid-Eocene. During this phase, deformation concentrated along the previous arc (Late Cretaceous - Early Paleogene), probably due to crustal weakening after magmatic advective heat transport into this area. This resulted in formation of the Incaic Range, a major topographic relief derived from tectonic inversion of the intra-arc.

North of ~27ºS, arc activity remained along the Incaic Range with minor eastward shift until present, but contractional deformation progressed uninterruptedly eastward until present through the Altiplano, Eastern Cordillera, and Subandean Belt. Thus, after initial association, magmatism and deformation evolved separately.

Further south, the area between 27º-39º S evolved differently. Renewed arc activity concentrated to the east of the Incaic Range in a ~80 km wide extensional intra-arc (Abanico) basin. Extension caused considerable crustal thinning, evidenced by the tholeiitic affinity and unradiogenic nature of coeval magmas.

A new tectonic pulse in Late Paleogene caused rejuvenation of the Incaic relief in northern Chile and inversion of the Abanico basin in central Chile. Deformation, again affected here the arc domain, whereas in northern Chile magmatism and deformation were already dissociated. The great amount of deposits in the basin restrained tectonic inversion and new thrust systems developed to the east. Since then the deformation front progressed, like in northern Chile, uninterruptedly towards east at a higher rate than the eastward shifting of the magmatic arc.

There is a 20 m.y. time span during which extension and crustal thinning occurred in central Chile, while contractional deformation and crustal thickening occurred without interruption in the north. This might represent an essential factor to explain the differences in the Andean orogen in both regions. Whatever the evolution, deformation zones were initially concentrated within the arc domain being probably magmatic-assisted. Evolved contractional deformation, though, overcomes such control to further development subjected to the regional crustal architecture.