Abstract

In Ordovician time, Gondwana in the area of northwestern Argentina and northern Chile had a west-facing active margin. The evolution of this margin culminated in the Oclóyic orogeny at the end of Ordovician time. This orogeny was caused by the collision of the allochthonous Arequipa-Antofalla terrane with this margin. The early Paleozoic evolution of northwestern Argentina and northern Chile contrasts markedly with the accretionary history of central Argentina and central Chile, where the Precordillera and Chilenia terranes docked in the Late Ordovician and Late Devonian periods, respectively. An inspection of the available stratigraphic and geochronological data on sedimentary, volcanic, and plutonic units of the southern Central Andes of northern Chile and northwestern Argentina reveals a lull in magmatic and metamorphic activity lasting for ≈100 m.y., from Early Silurian to early Late Carboniferous time. This is interpreted as corresponding to a tectonic scenario in which the present Andean margin was a passive margin of Gondwana. This passive margin developed in response to the rifting off of a part of the Arequipa-Antofalla terrane; the present location of this block is unknown. Late Carboniferous time marks the renewed onset of subduction, initiating the Andean plate tectonic setting still prevalent today. Recently proposed models explain the Late Ordovician orogeny by the collision of Laurentia with western South America during Laurentia's clockwise motion around South America and away from its position in the Neoproterozoic supercontinent. In its present form, this hypothesis is difficult to reconcile with the Paleozoic tectonostratigraphic evolution of the southern Central Andean region.

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