long hiatus separates them from the recent volcanic activity which started in the Pliocene (5.333–2.58 million years ago). At that time, the basaltic El Cenizo Formation and the andesitic Cerro El Zaino volcanics were emplaced. This kind of calcalkaline volcanic activity is interpreted to be the consequence of flat slab subduction during the Miocene (23.03-5.333 million years ago) and Pliocene, and took place between twenty and five million years ago. Later during the Pliocene and Quaternary the slab steepened, and probably as a consequence volcanism in the land above increased, reaching a peak between eight and five million years ago. Local The basement rock underneath Payún Matrú is formed by Mesoproterozoic (1,600–1,000 million years ago) to Triassic rocks of the San Rafael Block, Mesozoic (251.902 ± 0.024 to 66 million years ago) to Paleogene sediments of the Neuquén Basin and Miocene lava flows such as the Tertiary Patagonian basalts. The Andean orogeny during the Miocene has folded and deformed the basement, creating basins and uplifted basement blocks, and the Malargüe fold and thrust belt underlies part of the volcanic field. Oil has been drilled close to the volcanic field from sediments of Mesozoic age. Payún Matrú is part of the backarc volcanic province, east of the Andes and east of the Peru-Chile Trench. The volcanic activity still relates to the subduction of the Nazca Plate beneath the South America Plate, however; one proposed mechanism is that a Miocene change in the subduction regimen led to the development of extensional tectonics and of faults that form the pathways for magma ascent, while other mechanisms envisage changes in mantle characteristics. Other volcanic fields in the region are the Llancanelo volcanic field, the Nevado volcanic field and Salado Basin volcanic field; the first two lie north of Payún Matrú and the last south. These fields are subdivided on the basis of geochemical differences and consist of two stratovolcanoes (Payún Matrú itself and Nevado) and many monogenetic volcanoes. The volcanic field is part of the larger Payunia volcanic province, which covers an area of about in the Provinces of La Pampa, Mendoza and Neuquén and is also known as the Payenia or Andino-Cuyana volcanic province. Monogenetic volcanism of mainly basaltic composition has been active here for millions of years accompanied by the formation of several polygenetic volcanoes, generating more than eight hundred monogenetic cones although historical eruptions have not been observed. Further south are