Abstract There are over 200 Pleistocene and Holocene Andean arc volcanoes along western South America, occurring in four distinct segments ( Fig. 5.1 ) called the Northern (NVZ; 2°N–5°S), Central (CVZ; 14°S–28°S), Southern (SVZ; 33°S–46°S) and Austral (AVZ; 49°S–55°S) volcanic zones. In the Andes of Chile alone there are more than 100 Pleistocene and Holocene stratovolcanoes, as well as a number of large volcanic fields and giant caldera complexes, of which 60 have documented Holocene eruptive activity ( Simkin & Siebert 1994 ; González-Ferrán 1995 ). These are located in the CVZ of northern Chile, the SVZ of central-south Chile, and the AVZ of southernmost Chile. Pleistocene and Holocene backarc volcanic centres, the westernmost part of the Patagonian plateau basalts, also occur in southern Chile along the border with Argentina. In addition, intraplate oceanic volcanoes form Chilean islands in the Pacific Ocean, submarine volcanism takes place along the Chile Ridge, and slab-window volcanic activity occurs above the region where the Chile Ridge is currently being subducted, both along the west coast of Chile and in the submarine environment near the trench. Pleistocene and Holocene volcanoes of the Chilean Andes provide a natural laboratory for the study of volcanism, magma genesis and volcanic hazards in the context of oceanic– continental plate collision. Andean volcanic activity results from subduction of the Nazca and Antarctic oceanic plates below the continental lithosphere of western South America ( Fig. 5.1 ). Volcanoes in the CVZ of northern Chile and the SVZ of central-south Chile occur where the angle of

A complex arc-backarc system developed at the western margin of the Neuquén Basin north of 37°S during the late Cenozoic. Both transpressional deformation on the western orogenic front (Liquiñe-Ofqui fault system) and contractional deformation of the eastern Andean foothills occurred during the middle to late Miocene. South of 38°S, a physiography dominated by uplifted blocks and elongated basins was the site of intense volcanism from the early Pliocene to the Holocene. A wide volcanic arc was established from the western orogenic front to the eastern foothills of the Andes during the Pliocene. This volcanic phase was coeval with both extended transpressional deformation along the frontal arc and transient extensional episodes in the inner retroarc. Arc-front geochemical signatures of the magmas occurred further east of the front showing an increased subduction input in the subarc mantle. A decrease in plate convergence velocity 2–3 m.y. ago, along with a stable arc front caused a progressive westward narrowing of the Quaternary volcanic arc, probably since ca. 1.6 Ma. From the middle to late Pleistocene, volcanism was mainly centered around the Liquiñe-Ofqui fault system with minor Holocene activity in the eastern Andean region. Arc-front geochemical signatures are now restricted to the present volcanic front. Morphologically, this has resulted in paired volcanic belts that reflect different stages of arc narrowing rather than separate arc fronts.