Alternating contraction and extension in the southern central andes (35°-37°s)

Abstract

The Andes are thought to be formed through discrete contractional stages separated by periods of little to no orogenic construction. This paper analyzes the intervals between the main contractional phases that built the Southern Central Andes between 35° and 37°S in order to determine whether they were characterized by neutral, contractional or extensional conditions. During an interruption in orogenesis between the Late Cretaceous and the Miocene shortening phases, two extensional stages are recorded through the opening of a series of intra- and retro-arc basins. U-Pb dating of detrital zircons in a sample collected from the Los Á ngeles unit, a synextensional volcano-sedimentary succession located at ∼35°40'S along the Chile and Argentina international border, provided a maximum depositional age of 67.1 +2.4/ -0.9 Ma. This age, in association with evidence of regional crustal thinning, suggests a previously unrecognized extensional phase during latest Cretaceous times. Limited shortening succeeded this extensional event and was followed by a second extensional episode during late Oligocene and earliest Miocene times. While the first extensional event was restricted to the core of the Late Cretaceous orogen, the second episode affected a wide area ranging between the present forearc and retroarc areas. A structural section across the Malargüe fold-thrust belt at ∼36°S indicates inversion of normal faults where extension was focused and new thrust generation in areas not affected by extensional deformation. Our data reveal that the growth of the Southern Central Andes is the product of a complex alternation of contractional and extensional phases, with inherited structures playing a role in their tectonic evolution. A comparison with other Cordilleran orogenic systems such as the Puna-Altiplano plateau, the northern Peruvian Andes and the North American Sevier-Laramide orogenic belt, suggests that extensional deformation in the Southern Central Andes responds better to changes in plate kinematics, rather than to localized events within a continuous contractional setting.