3 Tectonostratigraphic evolution of the Andean Orogen in Chile REYNALDO CHARRIER (coordinator), LUISA PINTO & MARÍA PÍA RODRÍGUEZ Since the comprehensive synthesis on the Argentine–Chilean Andes by Mpodozis & Ramos (1989), important progress has been made on the stratigraphy, palaeogeographic evolution and tectonic development of the Andean Orogen in Chile. We present here an overview of this evolution considering the new information and interpretations, including some unpublished ideas of the authors. To enable the reader to delve further into the subjects treated here, we accompany the text with abund- ant references. In the interpretation of the stratigraphic and radioisotopic data we used the timescale of Harland et al. (1989). During most of its history the continental margin of South America was an active plate margin. The Late Proterozoic to Late Palaeozoic evolution was punctuated by terrane accretion and westward arc migration, and can be described as a ‘colli- sional history’. Although accretion of some terranes has been documented for the post-Triassic history, the evolution during post-Triassic times is characterized more by the eastward retreat of the continental margin and eastward arc migration, attributed to subduction erosion, and therefore can be described as an ‘erosional history’. The intermediate period, comprising the Late Permian and the Triassic, corresponds to an episode of no, or very slow, subduction activity along the continental margin, during which a totally different palaeo- geographic organization was developed and a widely distri- buted magmatism with essentially different affinities occurred. It is therefore possible to differentiate major stages in the tectonostratigraphic evolution of the Chilean Andes, which can be related to the following episodes of supercontinent evolu- tion: (1) post-Pangaea II break-up; (2) Gondwanaland assem- bly; and (3) break-up of Gondwana. These stages can in turn be subdivided into shorter tectonic cycles separated from each other by regional unconformities or by significant palaeo- geographic changes that indicate the occurrence of drastic tectonic events in the continental margin. These tectonic events have been related to modifications in the arrangement and dynamics of the lithospheric plates (see James 1971; Rutland 1971; Charrier 1973a; Aguirre et al. 1974; Frutos 1981; Jordan et al. 1983a, 1997; Malumián & Ramos 1984; Ramos et al. 1986; Isacks 1988; Ramos 1988b; Mpodozis & Ramos 1989). Morphotectonic features and subdivision of the Andean Orogen in Chile Within the Andean Orogen, which is the first-order mor- phologic element in this region, it is possible to differentiate two other types of features: morphostructural units orientated parallel to the strike of the range, and oroclinal bends around which are major changes in the orientation of the morphology and structure of the range (Fig. 3.1). Two oroclinal bends are present, comprising the Bolivian and the Patagonian oroclines, in northernmost and southernmost Chile respectively. The continuity of the strike-parallel morphostructural units is interrupted in the regions where the Juan Fernández and the Chile ridges intersect the continental margin, causing segmenta- tion of the orogen (Fig. 3.2). The region where the passive Juan Fernández Ridge is subducting the continental margin (between c. 27ºS to c. 33ºS) corresponds to a flat-slab sub- duction zone, whereas in the regions north and south of this flat-slab segment the Wadati–Benioff zone is steeper (Cahill & Isacks 1992). Further south (46–47ºS), the intersection of the active Chile Ridge and the continental margin determines the existence of the Taitao triple-junction. In the Chilean Andes north of the Taitao triple- junction, the main morphological change caused by the flat-slab subduction zone is the absence of the Central or Longitudinal Depression, a morphological unit that separates the Coastal Cordillera from the Principal or Main Cordillera (Fig. 3.2a). In this region it is therefore not possible to differentiate between these two cordil- leras. This situation determines the existence of two segments, one between 18º and 27ºS and the other between 33º and 46ºS, in which the Central Depression is well developed, and an inter- mediate segment which lacks a Central Depression (27–33ºS), called the zone of transverse river valleys or Norte Chico. On the Chilean side of the Andes, absence of the Central Depression in the flat-slab segment is associated with absence of recent volcanic activity, indicating that the subduction of the Juan Fernández Ridge controls morphology, magmatism and tectonics. In fact, the existence of the flat-slab subduction also causes important along-strike morphologic and tectonic varia- tions on the Argentinian side of the Andes (see Ramos et al. 2002). South of the Taitao triple-junction a drastic change occurs in the morphostructural units and the general orogra- phic pattern of the cordillera (Fig. 3.2b). The morphostructural units in this southern region comprise, from west to east: the Archipelago, the Patagonian Cordillera and the Precordillera. Based on the morphologic and tectonic differences occurring at the intersections of the continental margin with oceanic ridges, Aubouin et al. (1973b) and Gansser (1973) subdivided the Andean Range into three main regions: the Northern, the Central and the Southern Andes (Fig. 3.1a). The Chilean Andes form part of the southern Central Andes, north of the Taitao triple-junction, and the Southern Andes, south of the triple-junction. North of the Taitao triple-junction, convergence between the Nazca and South American plates is essentially orthogonal,