Payenia volcanic province in the Southern Andes: An appraisal of an exceptional Quaternary tectonic setting

Abstract

The Southern Volcanic Zone of the Andes has a Quaternary basaltic province along the retroarc which has a unique tectonic setting. The Payenia volcanic province covers an area larger than 40,000 km 2 between 33°30′ and 38° South latitudes, with an estimated volcanic volume of about 8387 km 3 erupted through more than 800 volcanic centers in the last ~ 2 Ma. The mainly basaltic province developed above the San Rafael Block is subdivided in three segments characterized by the Cerro Nevado, Llancanelo, Payún Matru, Tromen and Auca Mahuida volcanic fields, together with hundreds of minor monogenetic basaltic centers. The analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2.0 and 1.7 Ma, which reached the 35°S to the north. Above this plateau monogenetic centers as Nihuil Vn. 1.433 Ma and Cerro Chato at 1.352 Ma are developed, followed by the large polygenetic center of Cerro Nevado (3980 m a.s.l.) at 1.320 Ma. This plateau was broken by a series of normal faults that produced volcanic cone alignments such as the NNW-trending Mancha Jarilla lineament in the central part at about 1.0 Ma. Extension shifted to the eastern margin of the San Rafael Block, which concentrates tens of monogenetic centers between 0.9 and 0.7 Ma. Extension then migrated towards the foothills in the west, where many monogenetic cones were erupted through NW-trending normal faults between 0.5 and 0.435 Ma. The collapse of the large Diamante Caldera at 0.445 Ma coincides with that period. Subsequent volcanism was concentrated in (1) the Payún Matru volcanic field, with the eruption of Cerro Payén between 0.272 and 0.261 Ma; the Payún Matru shield volcano, with polygenetic eruptions at least since the last 0.233 Ma and with the caldera formation bracketed between 0.168 ± 0.004 Ma and 0.082 ± 0.001 Ma, followed by several eruptions until 7000 yrs, and even historical ones; and in (2) the Tromen volcano, where younger than 0.2 Ma eruptions took place and historical eruptions were reported. The understanding of these eruptions in time and space, combined with geophysical data, indicates the geometry of an important crustal attenuation beneath Payenia, associated with a hot sublithosphere. The Late Miocene uplifted San Rafael Block collapsed in the Early Pleistocene as a consequence of the steepening of the subducted slab, and the injection of hot asthenosphere produced the Quaternary Payenia volcanic province. Melts of the lower crust along the Principal Cordillera at these latitudes are responsible for the Quaternary calderas, ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the Andes. The Payún Matru volcanic field concentrates this asthenospheric flow in the Present.