The foothills of the northern segment of the Magallanes basin between 47° and 49°S latitudes record the development of a complex fold-and-thrust belt in the Patagonian Cordillera during the late Miocene. Upper Paleozoic sedimentary and metasedimentary rocks and thick sequences of Lower Cretaceous deposits with well-defined source and reservoir rocks are deformed by the Andean events. The structure is characterized by a triangle zone and a series of related underthrusts, which define a nonemergent backthrust system at the mountain front. Differences in the degree of deformation from south to north reveal the evolution of the triangle zone.

The stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early Tertiary, although the final structure was developed during the late Miocene. These deformed deposits are covered by extensive late Miocene horizontal retro-arc basalts.

Analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro-arc basalts are related to the collision of a segment of the Chile Ridge during the late Miocene. The final deformation and uplift of the Patagonian Cordillera fold-and-thrust belt at these latitudes are closely linked to this collision.

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