Integrated magmatic, structural, and geophysical data provide a basis for modeling the Neogene lithospheric evolution of the high Central Andean Puna-Altiplano Plateau. Reconstruction of three transects south of the Bolivian orocline in the Altiplano and Puna Plateau shows processes in common, including subduction characterized by relatively shallow and changing slab dips, crustal shortening, delamination of thickened lower crust and lithosphere, crustal melting, eruption of giant ignimbrites, and deep crustal flow. Temporal similarities in events in the three transects can be correlated with changes in the rate of westward drift of South America and slab rollback. Temporal differences between the three transects can be attributed to variations in Nazca plate geometry in response to southward subduction of the aseismic Juan Fernandez Ridge. Subduction of the north-south arm of the ridge can explain an Oligocene flat slab under the Altiplano, and subduction of a northeast arm of the ridge can explain a long period of relatively shallow subduction characterized by local steepening and shallowing. Major episodes of ignimbrite eruption and delamination have occurred over steepening subduction zones as the ridge has passed to the south. Late Miocene to Holocene delamination of dense lithosphere is corroborated by published seismic images. The southern Altiplano transect (17°S–21°S) is notable for high, structurally complex Western and Eastern Cordilleras flanking the Altiplano Basin, the eastern border of which is marked by late Miocene ignimbrites. The broad Subandean fold-and-thrust belt lies to the east. The Neogene evolution can be modeled by steepening of a shallowly subducting plate, leading to mantle and crustal melting that produced widespread volcanism including large ignimbrites. Major uplift of the plateau at 10–6.7 Ma was dominantly a response to crustal thickening related to Subandean shortening and peak lower-crustal flow into the Altiplano from the bordering cordilleras as the ignimbrites erupted, and partly a response to delamination along the eastern Altiplano border. A smaller ignimbrite volume than in the northern Puna suggests the Altiplano lithosphere never reached as high a degree of melting as to the south. An Oligocene flat-slab stage can explain extensive Oligocene deformation of the high plateau region. The northern Puna transect at ~21°S–24°S is notable for voluminous ignimbrites (>8000 km 3 ) and a narrower Subandean fold-and-thrust belt that gives way southward to a thick-skinned thrust belt. The evolution can be modeled by an early Miocene amagmatic flat slab that underwent steepening after 16 Ma, which led to mantle melting that culminated in widespread ignimbrite eruptions beginning at 10 Ma, peaking in the backarc at ca. 8.5–6 Ma, restricted to the near arc by 4.5 Ma, and ending by 3 Ma. The formation of eclogitic residual crust caused periodic lower-crustal and lithospheric mantle delamination. Late Miocene uplift was largely due to crustal thickening in response to crustal shortening, magmatic addition, and delamination. Crustal flow played only a minor role. The high degree of mantle and crustal melting can be explained as a response to steepening of the early Miocene flat slab. The southern Puna transect at ~24°S–~28°S is notable for eastward frontal arc migration at 8–3 Ma, intraplateau basins bounded by high ranges, long-lived Miocene stratovolcanic-dome complexes, voluminous 6–2 Ma ignimbrites, 7–0 Ma backarc mafic flows, and the latest Miocene uplift of the reverse-faulted Sierras Pampeanas ranges to the east. Its evolution can be modeled by a moderately shallow slab producing widespread volcanism with subsequent steepening by 6 Ma, leading to delamination of dense lithosphere culminating in the eruption of the voluminous Cerro Galan ignimbrite at 2 Ma.

Abstract This seven-day field trip is designed to examine the distinctive magmatic, structural, and sedimentological features of the late Oligocene to Recent evolution of the southern part of the high central Andean Puna plateau and the southern Central Volcanic Zone magmatic arc. The stops for Days 1–5 between 23° and 27°S latitude in Argentina emphasize the distinctive magmatic and structural features of the Puna region, which comprises the southern half of the central Andean Puna–Altiplano plateau. Differences between the northern and southern Puna are highlighted. Among the features to be observed are (1) giant Miocene ignimbrites of the northern Puna, (2) distinctive normal and strike-slip faults and associated shoshonitic lavas of the central Puna, (3) the intraplate and calc-alkaline lavas of the southern Puna, (4) the silicic calderas of the southern Puna, and (5) internally drained salar basins. The stops for Days 6 and 7 between 26.5° and 27.5°S latitude in Chile present a view of the Miocene to Recent frontal arc region on the western side of the plateau. The stops particularly highlight the late Miocene to Pliocene displacement of the magmatic arc front from the Maricunga Belt on the western edge of the plateau to its present position in the Central Volcanic Zone. Evidence for the timing of plateau uplift, changes in the angle of the underlying subduction zone, delamination of the underlying continental crust and mantle lithosphere, and forearc subduction erosion are examined throughout the course of the trip. DISCLAIMER: This is not a geologic field guide to be used in the traditional sense of a detailed road log, but rather a survey of the southern Puna region that can be done with the use of Google Earth. Latitude and longitude coordinates are given for all stops in WGS84 (world geodetic system) coordinates that can be used with Google Earth (downloadable from the Web) or any other georeferenced imagery. Driving instructions are given where access is possible by paved or high-quality unpaved roads that are present on road maps that are generally available in the region. Due to the lack of information on available road maps, changing driving conditions, and the need for caution in accessing the sites, precise instructions are not given for others. A significant number of these stops are on primitive, unmaintained roads or tracks that require a serious four-wheel drive vehicle, an experienced off-road driver, and supporting equipment (winch, spare tires, jack, etc.) to navigate safely. Many of the stops are in the Atacama Desert, where there is almost no water. The majority of the stops are at high elevations—most are between 3500 and 4500 m (~11,500–14,750 ft); all are over 2000 m (6500 ft). Attention needs to be paid to the potential for altitude sickness (called the Puna in Argentina). There are no cell phone towers, no service stations, or towing facilities in most of the region to bail you out! The availability of fuel can be questionable at times. Contact a knowledgeable guide to the region before attempting to use this field guide in the more isolated parts of the area.