One of the longest mountain chains on Earth is the Andes of South America, extending from northern Venezuela nearly 9000 km down the west side of the continent to Tierra del Fuego, at the southern tip. The Andes formed by southward subduction of the Caribbean Plate at the South Caribbean Deformed Zone (discussed in Chapter 4) and eastward subduction of the oceanic Nazca and Antarctica plates beneath the western edge of the continent south of 6°–7º N (subduction of the Antarctica Plate south of 46º S). For parts of the plate boundary, subduction is oblique, giving a right-lateral or left-lateral strike-slip component to Andean crustal faults (Dewey and Lamb, 1992). Subduction is accompanied by great earthquakes, including the largest earthquake ever recorded, Mw 9.5, in southern Chile, in May 1960 (located on Figure 5.1a). The Andes encompasses the Altiplano, which, together with the Puna of Argentina, comprises the second largest and second highest plateau on Earth, after Tibet (Figure 5.1a).
Some of its active volcanoes are the highest on Earth: Chimborazo in Ecuador is 6310 m high, Sajama in Bolivia is 6542 m high, and Ojos del Salado on the Chile–Argentina border is the highest of all, 6880 m. But of greater tectonic significance is the absence of active volcanoes in parts of the Andes (Figures 5.1b, 5.2). This is due to the dip of the subducting oceanic Nazca Plate (Figure 5.3). Active volcanoes are present where the Nazca Plate dips into the mantle at a moderate angle of 30º, as it does in southern Colombia and northern Ecuador, beneath the Altiplano of southern Perú, southwestern Bolivia, northwestern Argentina, and northern Chile, and in the southern Andes of Chile and Argentina. Northern Perú between 4º S and 14º S and the Andes of Argentina and Chile between 27º S and 33º S are underlain by a subducting slab dipping 5º to 10º (flat-slab subduction) at depths greater than 80–100 km (Stauder, 1973; Barazangi and Isacks, 1976). These areas lack active volcanoes, and the active tectonics differs from the zones of normal subduction (Gutscher et al., 2000).