Descripción: Located at the transitional southern end (33o S) of the Pampean flat-slab, the Tertiary volcanic belt records the eastward migration of the Andean volcanic arc due to the flattening of the Nazca Plate in Mio-Pliocene times. The tertiary volcanic belt encompasses several very important metallogenetic districts in the San Luis Pampean Ranges. Volcanic rocks and associated ore-deposits crop out following a NW-WNW trending belt at La Carolina, Cañada Honda - Cerros Largos, Cerros del Rosario and El Morro volcanic fields. The available geochronological data indicate that the volcanic activity began early at the western end of the belt and ended ~ 10 Ma later at its eastern border. Cañada Honda district represents the oldest and longest-lived volcanic field of the tertiary volcanic belt. It records eruptive events and related hydrothermal alteration from 12-13 Ma to 7.3 Ma, including lavas and volcaniclastic products. Preliminary structural analysis shows that previous structures have strongly controlled, either directly or indirectly, the emplacement of volcanic rocks and related mineral deposits. Kinematic and strain fabric analyses allow to recognize two volcano-tectonic associations. One of these associations would be related to dextral reactivations of structures parallel to basement foliation. On the other hand, the main one would be linked with the generation of two volcano-tectonic depressions which are aligned in a NW-WNW direction. Their dominant structures trend NW-WNW and reveal sinistral-normal motions, both at local and Tertiary volcanic belt scales.

Descripción: The Payenia basaltic province is a typical retroarc association developed along the foothills of the Andes between 33°40 ́S and 38°00'S. It records two main events, an older, mostly Miocene one (26 to 8 Ma) and a younger Pliocene to Holocene one (younger than 5 Ma). It covers an N-S lowland belt named here the central depression. To the North of 36°20'S, the region was described as Los Huarpes depression, a partially deformed sedimentary basin characterized by a 1000 m-thick, undifferentiated Cenozoic sequence. To the south, Upper Cretaceous to Lower Paleogene layers, instead of the undifferentiated Cenozoic deposits, are covered by basalts. The volcanism additionally covers the western side of the San Rafael Block as well as the fold and thrust belt of the Andean foothills. The Payenia province consists of more than 800 monogenetic basaltic cones, and scarce polygenetic volcanos fed by shallow magmatic chambers. Among the latter, the following examples can be mentioned: Miocene Chachahuén volcano (7 to 5 Ma), composed of andesites, rhyodacites and basalts with high-K and amphibole; Pliocene El Nevado volcano, with calc-alkaline, basaltic trachyandesites, trachytes, dacites and rhyolites; and Upper Pleistocene to Holocene Payún Matrú volcano, composed of trachytes, trachyandesites and trachybasalts. The southernmost area of the central depression is covered by the Pleistocene, Auca Mahuida basaltic shield (1.7 to 0.88 Ma), consisting of trachybasalts, basaltic trachyandesites and trachyandesites. This shield is aligned with Tromen and Domuyo volcanos, defining an NW-SE volcanic belt, oblique to the Andes, whose southern tip corresponds to the Cortaderas lineament. Some of the monogenetic cones are of hydromagmatic origin, whereas a few others exhibit small mantle inclusions. The Payenia retroarc province develops to the south of the Pampean flat slab segment, where the heights of the Andes are smaller. This height difference might have been caused by a higher thermal gradient that softened the crust in the steeper subduction segment, while the colder lithosphere in the flat slab segment allowed greater crustal thickening and height. From the tectonic point of view the height difference has been attributed to a smaller orogenic contraction in the southern segment. Holocene volcanism appears exclusively at Payun Matru volcano. The associated thermal anomaly may explain the segmentation of the central depression, separating the Cenozoic Los Huarpes basin in the north and the section without coeval sedimentation in the south. The only surface evidence of this segmentation is the E-W, La Carbonilla fault, running both sides of the Payun Matru volcano. Other authors have explained this thermal anomaly as a process of crustal thinning and stretching associated with hot injection from the asthenosphere.