The Cura Mallín basin is part of a chain of sedimentary basins that formed within the Andean volcanic arc between 33° and 43°S during the late Oligocene and early Miocene. Most previous studies of these basins have suggested that they are pull-apart–type basins, produced by strike-slip deformation of the Liquiñe-Ofqui fault zone and other structures, all of which are currently active. However, no direct evidence has been cited for a correlation between formation of the Oligocene-Miocene basins and concurrent strike-slip faulting. The Cura Mallín basin lies more than 100 km north of the modern Liquiñe-Ofqui fault zone and is one of the largest and best exposed of the Southern Andean Oligocene-Miocene basins, making it a promising study area for distinguishing between Oligocene-Miocene tectonic activity that produced the basin and subsequent tectonic activity. Stratigraphic and structural data presented here from the Cura Mallín basin and its surroundings include facies variations, stratal thickness patterns, internal and external structural features, ⁴⁰Ar/³⁹Ar radiometric ages, and apatite and zircon fission-track ages. Based on the distribution of sedimentary facies and their relation to geologic structures, we conclude that the Cura Mallín basin formed as a result of normal faulting, with little or no significant strike-slip deformation in the area. Due to the lack of supporting evidence for interpretations of the other Oligocene-Miocene basins as pull-apart basins, we suggest that the entire chain of Oligocene-Miocene sedimentary basins formed in response to extensional tectonics on the Southern Andean margin.