Abstract:

The tectonic evolution of the central Andes is depicted through analysis of the 30° to 33°S segment, which encompasses the highest part of the Andean Cordillera. A period of rifting, starting as early as 600 Ma, was a milestone in the evolution of the different tectonic regimes that are responsible for the present geological composition and structure of the Andes. The early Paleozoic was an important period of continental accretion when allochthonous terranes, such as Chilenia, were incorporated onto the western margin of Gondwanaland. The subduction zone was located about 300 km east of the present trench. An early Paleozoic magmatic arc was developed in the western Sierras Pampeanas. Sedimentary facies of that age record a continental margin between Chilenia and the magmatic arc, which is associated with a disrupted ophiolite sequence. The Famatinides orogeny produced the first deformation of the Andes and the uplift of the Protoprecordillera during middle to late Devonian times. The Gondwanides orogeny is characterized by subduction of an oceanic plate beneath a continental margin, with the accretion of minor exotic terranes in the southern part of the Andes. Magmatism, eastward migration of the volcanic front, sedimentation pattern, and deformation defined an evolving orogenic sequence. This is correlated to a varying convergence history linked to variations in the apparent polar wandering path of western Gondwanaland during late Paleozoic-early Mesozoic times. The Patagonides orogeny was also governed by changes in relative plate motions during middle to late Mesozoic times. The paleotectonic history suggests that two orogenic styles were produced: a stage with little compression and back-arc volcanism, and a stage with high compression without volcanism but with important deformation and emplacement of postorogenic granitoids. The Andean orogenic cycle is distinguished by conspicuous segments of the orogen that are controlled by the segmentation of the subducted oceanic Nazca plate related to the subduction of aseismic ridges. But the compressive phases and mountain building are more closely related to changes of plate motion, which affected thousands of kilometers of the continental margin, exceeding the length of any individual segment. The age of the subducted oceanic slab is an important factor that controls the magmatic activity and the presence or absence of retroarc magmatism. The relative influence of each proposed mechanisms varies substantially among the different segments.