128 ANDES The height of the Main Cordillera decreases from $6600 to < 2000 m southward as the fore-arc Central Valley widens and back-arc shortening related to both thin- and thick-skinned thrusting decreases from $100 to < 35 km at 37 S Between 38 S and 40 S, the arc front has migrated westward since the Pliocene in conjunction with mild extension in the region of the Loncopue´ Graben From $38 S southward, the modern volcanic arc overlaps with the > 900 km long Liquin˜ e–Ofqui Fault system (Figure 6), which has had dextral displacement since at least the Early Miocene The Mesozoic and Tertiary arc fronts are in the modern fore-arc north of 36 S and near the modern arc to the south The giant Late Miocene El Teniente Cu deposit occurs in the westernmost Cordillera near 34 S The foreland has important basins with both rift and foreland histories that surround the pre-Mesozoic Somuncura and Deseado massifs They include the Jurassic through Tertiary Neuque´ n Basin north of $38 S, the Early Cretaceous Rio Mayo Embayment near 45 S and the Palaeogene to Early Neogene Nirihuau Basin near 41 S A series of smaller segmented basins occur in the fore-arc Andean magmatism is well developed in the foreland in two regions The first is in the Payenia area between $35 S and 38 S where extensive volcanic sequences cover the northern half of the Neuque´ n Basin These volcanic sequences, which can occur more than 500 km east of the modern trench, consist of Miocene andesitic/dacitic volcanic centres with arc geochemical signatures, and Early Miocene and Late Pliocene to Recent mafic alkaline flows The second region is between $41 S and 44 S where Palaeogene and Miocene andesitic to rhyolitic sequences, Eocene alkaline basalts, and the extensive Late Oligocene to Miocene mafic plateau flows associated with the Mesetas de Somun Cura and Canquel occur Chile Triple Junction and Austral Volcanic Zone Segment (47 to 56 S) Important changes occur south of the Chile Triple Junction near $47 S where the Chile Ridge is colliding with the trench (Figure 7) The modern volcanic arc disappears between $47 S and 49 S and resumes as the Austral Volcanic Zone in response to subduction of the Antarctic Plate The high Andes at this latitude are dominantly composed of post-Triassic magmatic rocks with the principal part of the Jurassic to Miocene Patagonian Batholith forming the backbone of the Cordillera Late Palaeozoic/Early Mesozoic fore-arc accretionary complexes occur in the fore-arc The Patagonian Cordillera reaches a maximum elevation of $4000 m near $47 S east of where the Chile Ridge is currently colliding and decreases in elevation to the south The 30 000 km2 Patagonian ice-field, the world’s third largest, occurs at the higher elevations The importance of back-arc crustal shortening abruptly increases at $47 S as does the amount of Tertiary foreland sedimentary deposits To the south, Mesozoic normal faults have largely been inverted by Tertiary compression whereas similar age normal faults are preserved to the north The Patagonian region east of where the Chile Ridge has collided is notable for Neogene fore-arc volcanism (Taitao Ophiolite), abundant Late Neogene mafic plateau flows east of where ridge collision occurred at $12 and Ma, and widespread Pleistocene to Recent mafic flows Extensive Eocene mafic retroarc plateau lavas also occur in this area The southernmost part of Patagonia includes the Jurassic to Tertiary Magallanes (austral) Basin whose axis coincides with the Early Cretaceous Tortuga and Sarmiento ophiolite complexes The Andes of Tierra de Fuego contain the Cretaceous metamorphic complexe of the Cordillera Darwin They include an east to west trending fold–thrust belt cut by a major northwest to south-east trending left-lateral fault system that delimits the northern boundary of the Scotia Plate The SVZ Mount Cook volcanic centre also occurs in this region Jurassic to Recent Evolution of the Andean Chain The evolution of the Andes that began with the Mesozoic breakup of Pangaea can be divided into a Late Triassic to Early Cretaceous stage dominated by rifting processes and extensional arc systems, a Late Cretaceous to Early Oligocene stage in which the Andes evolved from an extensional dominated to a compressional regime, and a latest Oligocene to Recent stage in which most of the main Andean range was uplifted Stage 1: Rifting and Extensional Arc Systems The Late Triassic to Early Cretaceous stage began with rifting associated with the breakup of Pangaea The geometries of the rifts that began all across western South America in the Triassic and Jurassic reflect extensional directions, basement fabrics and old terrane boundaries The north-east to south-west rifting en echelon pattern in the Northern Andes matches the counterclockwise rotation associated with rifting from the conjugate North America Yucatan Block North-west trending rifts in the rest of the Northern, Central and Southern Andes are aligned with Gondwana Palaeozoic sutures The north-west trending dextral shear pattern of rifts in Patagonia could have