The Rı́o Lácteo Formation of Southern Chile. Late Paleozoic orogeny in the Andes of southernmost South America

Abstract

The late Paleozoic Rı́o Lácteo Formation forms the metamorphic basement of the Mesozoic and Cenozoic Andean orogenic belt in the Lago General Carrera area (about 45°S) of southern Chile. The rocks were emplaced during a late Paleozoic orogeny on the active continental margin of southernmost South America. They form part of a SW to NE striking fold belt with dominant overthrusting towards the SE. The metamorphic grade and structural style are indicative of deformation in the crystalline core of an orogenic belt, probably resulting from microplate interactions, rather than fore-arc accretion. The Rı́o Lácteo Formation represents metamorphorphosed marine turbidites, limestones and pyroclastic rocks which accumulated in an intraplate setting. The strata were intensely deformed and subjected to medium grade, greenschist to epidote-amphibolite facies metamorphism in pre-late Carboniferous times. Four phases of deformation have been identified. The first phase produced a strong planar fabric. This was followed by subhorizontal SW to NE stretching and NW to SE compressive shortening. The third phase formed a spaced cleavage related to E-W extension. The fourth episode produced broad shear zones, characterised by chevron folding and intrusive dykes of crush breccia, related to thrusting towards the southeast.