Located wholly within southwestern Bolivia, Pastos Grandes is the northernmost complex of the APVC. Cerro Pastos Grandes (5,800 m) was first recognized as the resurgent centre of a major caldera by Baker (1981) on Landsat MSS images. The caldera covers an area ~50 km in diameter with a base elevation of about 4,500 m and is bounded by a well developed arcuate fault zone forming a prominent scarp approximately 400 m high on its northern and eastern sides. Two generations of faults can be discerned which appear to be related to two separate collapse events. The southern and western sides of the caldera are not as well defined structurally, but appear to lie among the parallel chains of volcanoes known as the Cordon de Inacaliri just within Chile. Extrusive lava domes of varying age surround the resurgent centre. Lagunas Pastos Grandes, Khara and Cachi may be remnants of a much more extensive lake which once occupied much of the moat of Pastos Grandes caldera. Our preliminary field observations suggest that another, older, caldera may exist on the eastern part of the present Pastos Grandes caldera.
Pastos Grandes is believed to have been the source of at least two major ignimbrites, an unnamed 3.2 Ma old ignimbrite, the 5.6 Ma old Chuhuilla ignimbrite, and possibly a third, the 8.1 Ma old Sifon ignimbrite, which extends up to 150 km from the caldera (de Silva & Francis, 1989). Young rhyolite lava flows of Cerro Runtu Jarita and the lava dome or torta, Cerro Chascon q.v. were the most recent activity from the caldera. Their excellent preservation and morphology indicate a Holocene age. Of the several hot springs associated with the caldera, the largest is in the Laguna Pastos Grandes. These provide evidence for continuing geothermal activity within the underlying plutonic system.
Some smaller ignimbrite centers found just outside the structural limits of main caldera complex are probably parts of the larger system. On the northern flanks of Pastos Grandes, about 10 km to the southeast of the pueblo of Alota, is the Cerro Juvina dome (21º28'S; 67º32'W) which is the source of a sequence of small rhyolitic ignimbrites. These are largely confined to the area between the Pastos Grandes caldera and a range of older volcanoes to the north, known informally as the "Alota Basin", which was the site of a large lake during the 10,000 yr BP post-glacial regression. Ages of 3.9 and 3.2 Ma have been obtained from the Alota ignimbrites by Baker (1981), although he suggested that they were from the Pastos Grandes Caldera.
Between Pastos Grandes and the Cerro Guacha caldera to the south is the Laguna Colorado ignimbrite center (22º15'S; 67º34'W). Little is known about this center except that it consists of an apron of ignimbrites dipping to the west and east from a summit source area (5,300 m) covered by a mass of andesitic and dacitic cumulovolcanoes and domes (Cerros Panizos and Viscachillas). In many respects this ignimbrite center is similar to the "ignimbrite shield" of Cerro Purico (q.v) and does not appear to be a typical collapse caldera, although it was the source of at least two extensive ignimbrites. According to Francis & Baker (1978) the youngest ignimbrite from this center (on the western flanks of the center on the shores of Laguna Colorado) is ~1.7 Ma old. Preliminary mapping indicates that this ignimbrite extends further westward into Chile where it is known as the Tatio ignimbrite.
Baker, M.C.W., 1981. The nature and distirbution of upper Cenozoic ignimbrite centres in the Central Andes. J. Volcanol. Geotherm. Res. v11, p293-315.
de Silva, S.L., & Francis, P.W., 1989. Correlation of large ignimbrites - two case studies from the Central Andes of N. Chile. J. Volcanol. Geotherm. Res. v37, no.2, p.133-149.
Francis, P.W., & Baker, M.C.W., 1978. Sources of two large-volume ignimbrites in the Central Andes: Some Landsat evidence. J. Volcanol. Geotherm. Res. v.4, p81-87.