The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower‐middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian‐Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian‐Devonian shelf‐slope‐oceanic basin assemblage bounded by mélanges on both sides and bearing many stratigraphic similarities to the lower‐middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carboniferous continental, deltaic, and marine strata demonstrates that these three terranes were sutured together and part of South America by the end of the Devonian. Subsequent Permo‐Carboniferous plate interactions more closely resembled the modern Andean margin, with eastward subduction, accretionary prism formation, and minor terrane emplacement exposed along the present coast of Chile and eastward migrating arc magmatism from the present coast of Chile to western Argentina.

Abstract. The southern Andes between 33 ø and 45øS latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18 ø-33øS), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.

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The distribution, petrology, and geochemistry of mid-late Tertiary magmatic rocks in the Main Andean Cordillera over the modern zone of shallow subduction ("flatslab"-28 to 33°S) correlate with the shallowing of the subducted plate and the thickening of the crust in central Chile and Argentina. The evolving characteristics of these "flat-slab" magmatic rocks suggest that crustal thickening occurred earlier in the central (near 30.5°) and northern (near 28°S) regions than in the southern region (near 33°S). Crustal thicknesses approximated by comparing the chemical characteristics (particularly the La/Yb ratios) of the "flat-slab" magmatic rocks with those of similar rocks in the modern Southern Volcanic Zone indicate that the crust thickened from ~35 to 40 km in the late Oligocene to =55 to 65 km in the late Miocene in the northern and central "flat-slab." As the region was under compression, ductile deformation in the lower crust accompanying magmatism was probably responsible for these relatively rapid crustal thickness increases. The mineralogy of crustal residues calculated from basaltic composition parents for the "flat-slab" volcanic rocks changes from a hydrous amphibolegarnet-plagioclase assemblage to an almost anhydrous plagioclase-poor garnet granulite assemblage as the crust thickens. Geochemically, these changes are reflected in the melts by increasing La/Yb ratios and Sr contents associated with decreasing Eu anomalies. A limit to crustal thickening may be the attainment of mantle density by lower crustal rocks as garnet and Al-rich pyroxene replace plagioclase.Early Miocene (»20 Ma) back-arc alkaline basalts at 31°S have relatively low 87 Sr/ 86 Sr ratios (-0.7036) and high eNd (+4.5) compared to "flat-slab" calc-alkaline magmatic rocks. This fact combined with evidence for increasing crustal thickness suggests that progressively higher 87 Sr/ 86 Sr ratios and lower eNd ( 87 Sr/ 86 Sr = 0.7046 to 0.7064; eNd = +1.2 to -3.5) in the "flat-slab" volcanic rocks correlate with an increase

The Patagonian Andes record a period of Cretaceous‐Neogene orogenesis that began with the compressional inversion of a Late Jurassic rift called the Rocas Verdes basin. Detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of ∼148 Ma, suggesting that the basin opened approximately simultaneously along its length during the Late Jurassic. Structural data and U‐Pb isotopic ages on zircon from granite plutons near the Beagle Channel (55°S) show that basin inversion involved two stages of shortening separated by tens of millions of years. An initial stage created a small (∼60 km wide) thrust wedge that placed the basaltic floor of the Rocas Verdes basin on top of adjacent continental crust prior to ∼86 Ma. Structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in Cordillera Darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of ∼35 km beneath a coeval volcanic arc. A subsequent stage of out‐of‐sequence thrusting, culminating in the Paleogene, shortened basement and Upper Jurassic igneous rock in the internal part of the belt by at least ∼50 km, forming a bivergent thrust wedge. This latter period coincided with the exhumation of rocks in Cordillera Darwin and expansion of the fold‐thrust belt into the Magallanes foreland basin. This orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi‐oceanic rift basin.