The Pliocene to Quaternary narrowing of the Southern Andean volcanic arc between 37° and 41°S latitude

Abstract

At the western margin of Neuquén Basin, an arc-backarc system was developed during Cenozoic times. Oligocene fast orthogonal convergence coexisted with extensional conditions that built a wide volcanic arc and marginal basins, followed in the Miocene-Pliocene by tectonic inversion and plutonism in a transpressional setting related to the Liquiñe-Ofqui Fault system on the orogenic front. After that, a wide Pliocene volcanic arc was established along the main axis of the transpressional orogen and extended to the inner backarc region. South of 38ºS, the Plio-Quaternary volcanic front had the same locus of the present arc as can be achieved from Ar-Ar geochronology, geochemical data and proximal facies analysis. Until the Late Pliocene the prevailing conditions were compression at the forearc and arc region although extensional structures can be recognised in the inner back-arc. At the Early Pleistocene, the present architecture of the volcanic arc was established, being the volcanic arc spatial and genetically related to the Quaternary brittle deformation along the Andean orogen. A link between convergence rate and tectonic regimes can be invoked as the cause of volcanic arc dynamics in Plio-Quaternary time. In fact, south of 38ºS no west shift of the volcanic arc occurred at the early Quaternary but a narrowing of the arc maintaining the same front. Then, a steepening of the slab could not be a driving mechanism for arc dynamics as has been invoked before. Dynamics and morphology in the Plio-Quaternary volcanic arc seem to be related to the unique change in the subduction system at this time: decreasing of both orthogonal and trench-parallel subduction velocities. Other mechanisms, at least in this part of the Southamerican margin, are precluded by the data and the inertial properties of the subduction system.