A PULL APART VOLCANIC RELATED TERTIARY BASIN, AN EXAMPLE FROM THE PATAGONIAN ANDES

The Oligocene-Miocene Nirihuau basin is located along the eastern foot hills of the Patagonian Andes (41 degrees-42 degrees S.L.) to the east of an Oligocene volcanic belt formed during a slow period of oblique convergence between the South American and the Farallon plates. The ba sin is a pull-apart wrench fault furrow depression located to the east of the active (Pacific) continental margin, between the volcanic chai n (magnetic arc) and the cratonic area. the sedimentary fill was mainl y controlled by tectonism and synchronous volcanic activity. Most sedi ments in the Nirihuau Basin are continental volcaniclastics, associate d to primary pyroclastites, lava flows and carbonate sediments. Three tectosedimentary stages of evolution have been defined for the Nirihua u 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 i ncreasing and more uniform subsidence, and consequent enlargement of t he basin along its major axis. A longitudinal fluvial system progradin g from the north, and associated to proximal laharic facies and distal lacustrine facies is inferred. 3) Final stage, characterized by sligh t subsidence and enlargement of the basin both along and across its ma jor axis. The sedimentary fill is composed of primary and reworked pyr oclastics deposited as aeolian dust (loess) sediments as well as in hi gh sinuosity fluvial systems, flood-plains and shallow lakes. Copyrigh t (C) 1996 Elsevier Science Ltd & Earth Sciences & Resources Institute