A pull apart volcanic related tertiary basin, an example from the Patagonian Andes

The Oligocene — Miocene Ñirihuau basin is located along the eastern foothills of the Patagonian Andes (41 °–42 ° S.L.) to the east of an Oligocene volcanic belt formed during a slow period of oblique convergence between the South American and the Farallón plates. The basin is a pull-apart wrench fault furrow depression located to the east of the active (Pacific) continental margin, between the volcanic chain (magnetic arc) and the cratonic area, the sedimentary fill was mainly controlled by tectonism and synchronous volcanic activity. ediments in the Ñirihuau Basin are continental volcaniclastics, associated to primary pyroclastites, lava flows and carbonate sediments. Three tectosedimentary stages of evolution have been defined for the Ñirihuau Basin infill: 1) Initial stage, characterized by a deep, narrow and elongated trough formed by rotation and translation along very active strike-slip fault-systems. The sedimentary record is composed of two (fan-delta and lake) facies associations. 2) Intermediate stage, with increasing and more uniform subsidence, and consequent enlargement of the basin along its major axis. A longitudinal fluvial system prograding from the north, and associated to proximal laharic faciés and distal lacustrine facies is inferred. 3) Final stage, characterized by slight subsidence and enlargement of the basin both along and across its major axis. The sedimentary fill is composed of primary and reworked pyroclastics deposited as aeolian dust (loess) sediments as well as in high sinuosity fluvial systems, flood-plains and shallow lakes.