Tectonostratigraphic 3 evolution of the Andean Orogen in Chile REYNALDO CHARRIER (coordinator), LUISA PINTO & MARÍA PÍA RODRÍGUEZ Since the comprehensive synthesis on the Argentine–Chilean parallel to the strike of the range, and oroclinal bends around Andes by Mpodozis & Ramos (1989), important progress has which are major changes in the orientation of the morphology been made on the stratigraphy, palaeogeographic evolution and structure of the range (Fig. 3.1). Two oroclinal bends are and tectonic development of the Andean Orogen in Chile. We present, comprising the Bolivian and the Patagonian oroclines, present here an overview of this evolution considering the new in northernmost and southernmost Chile respectively. The information and interpretations, including some unpublished continuity of the strike-parallel morphostructural units is ideas of the authors. To enable the reader to delve further into interrupted in the regions where the Juan Fernández and the the subjects treated here, we accompany the text with abund- Chile ridges intersect the continental margin, causing segmenta- ant references. In the interpretation of the stratigraphic and tion of the orogen (Fig. 3.2). The region where the passive radioisotopic data we used the timescale of Harland et al. Juan Fernández Ridge is subducting the continental margin (1989). (between c. 27ºS to c. 33ºS) corresponds to a flat-slab sub- During most of its history the continental margin of South duction zone, whereas in the regions north and south of this America was an active plate margin. The Late Proterozoic to flat-slab segment the Wadati–Benioff zone is steeper (Cahill & Late Palaeozoic evolution was punctuated by terrane accretion Isacks 1992). Further south (46–47ºS), the intersection of the and westward arc migration, and can be described as a ‘colli- active Chile Ridge and the continental margin determines the sional history’. Although accretion of some terranes has been existence of the Taitao triple-junction. documented for the post-Triassic history, the evolution during In the Chilean Andes north of the Taitao triple- junction, the post-Triassic times is characterized more by the eastward main morphological change caused by the flat-slab subduction retreat of the continental margin and eastward arc migration, zone is the absence of the Central or Longitudinal Depression, attributed to subduction erosion, and therefore can be a morphological unit that separates the Coastal Cordillera from described as an ‘erosional history’. The intermediate period, the Principal or Main Cordillera (Fig. 3.2a). In this region it is comprising the Late Permian and the Triassic, corresponds to therefore not possible to differentiate between these two cordil- an episode of no, or very slow, subduction activity along the leras. This situation determines the existence of two segments, continental margin, during which a totally different palaeo- one between 18º and 27ºS and the other between 33º and 46ºS, geographic organization was developed and a widely distri- in which the Central Depression is well developed, and an inter- buted magmatism with essentially different affinities occurred. mediate segment which lacks a Central Depression (27–33ºS), It is therefore possible to differentiate major stages in the called the zone of transverse river valleys or Norte Chico. tectonostratigraphic evolution of the Chilean Andes, which can On the Chilean side of the Andes, absence of the Central be related to the following episodes of supercontinent evolu- Depression in the flat-slab segment is associated with absence tion: (1) post-Pangaea II break-up; (2) Gondwanaland assem- of recent volcanic activity, indicating that the subduction of the bly; and (3) break-up of Gondwana. These stages can in turn be Juan Fernández Ridge controls morphology, magmatism and subdivided into shorter tectonic cycles separated from each tectonics. In fact, the existence of the flat-slab subduction also other by regional unconformities or by significant palaeo- causes important along-strike morphologic and tectonic varia- geographic changes that indicate the occurrence of drastic tions on the Argentinian side of the Andes (see Ramos et al. tectonic events in the continental margin. These tectonic events 2002). South of the Taitao triple-junction a drastic change have been related to modifications in the arrangement and occurs in the morphostructural units and the general orogra- dynamics of the lithospheric plates (see James 1971; Rutland phic pattern of the cordillera (Fig. 3.2b). The morphostructural 1971; Charrier 1973a; Aguirre et al. 1974; Frutos 1981; Jordan units in this southern region comprise, from west to east: the et al. 1983a, 1997; Malumián & Ramos 1984; Ramos et al. 1986; Archipelago, the Patagonian Cordillera and the Precordillera. Isacks 1988; Ramos 1988b; Mpodozis & Ramos 1989). 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, Morphotectonic features and subdivision of the Andean the Central and the Southern Andes (Fig. 3.1a). The Chilean Orogen in Chile Andes form part of the southern Central Andes, north of the Taitao triple-junction, and the Southern Andes, south of the Within the Andean Orogen, which is the first-order mor- triple-junction. phologic element in this region, it is possible to differentiate North of the Taitao triple-junction, convergence between the two other types of features: morphostructural units orientated Nazca and South American plates is essentially orthogonal, 22 R. CHARRIER ET AL. South American Plate Nazca Plate WC ALTIPLANO Pacific- T AB CC CD FP EC SS Antarctic Scotia Southern A. Plate Plate Bolivian Arica Orocline lquique Antofagasta Central Volcanic T CC PC FC P PR Chanaral Zone Copiapo La Serena Flat-slab Zone Santiago Concepcion PC CC South CD Volcanic Valdivia Zone Puerto Montt Puerto Aisén Coihaique Chile Chico Cochrane Patagonian Orocline Puerto Natales APAC P Punta Arenas Fig. 3.1. Location of the Central and Southern Andes relative to South America, and major morphologic elements of the Andean Cordillera and oceanic plates facing the western margin of southern South America. (a) Location of the Andean Cordillera relative to South America, major subdivisions along the mountain range, and major tectonic and morphologic elements in the oceanic plates facing western South America: the Nazca and Pacific–Antarctic plates, separated by the Chile Ridge, and from north to south the Carnegie, Nazca and Juan Fernández passive ridges (in light grey). (b) Major morphological features of the Central and Southern Central and Southern Andes in Chile (17º45p to 56ºS): Bolivian and Patagonian Oroclines, Andean segmentation, and morphostructural units: 1, Coastal Cordillera (Coastal Ranges, between 27º and 33ºS); 2, Central Depression; 3, Forearc Precordillera and Western Cordillera, between 18º and 27ºS, High Andean Range, between 27º and 33ºS (flat-slab subduction segment), Principal Cordillera, between 33ºS and c. 42ºS; 4, Patagonian Cordillera; 5, Andean foreland in the southern Patagonian Cordillera. (c) Schematic section across the northern high-angle subduction segment (Central Volcanic Zone) showing distribution of the morphostructural units. (d) Schematic section across the flat-slab segment. (e) Schematic map and section across the high-angle subduction segment (Southern Volcanic Zone) showing distribution of the morphostructural units. (f) Schematic section across the Southern Andes showing distribution of the morphostructural units. Abbreviations: AB, Arica Basin; CC, Coastal Cordillera; CD, Central Depression; EC, Eastern Cordillera; FC, Frontal Cordillera; FP, Forearc Precordillera (northern Chile); P, Precordillera (in Argentina between 27º and 33ºS); PA, Patagonian Archipelago; PAC, Patagonian Cordillera; PC, Principal Cordillera; PR, Pampean Ranges; SS, Subandean Sierras; T, Trench; WC, Western Cordillera. ANDEAN TECTONOSTRATIGRAPHY 23 Fig. 3.2. Plate configuration of the SE Pacific Ocean with the Nazca, Pacific–Antarctic and Scotia plates. Chile Ridge (not to scale) separates the Nazca and the Pacific–Antarctic plates. The Scotia Plate is separated from the South America Plate by a left-lateral structure connected to the west with Magallanes Fault, the North Scotia Margin, and separated from the Pacific–Antarctic Plate by the Shackleton Fracture Zone. (a) Location of the Juan Fernández hot-spot and ridge, outlines of the Nazca Plate and segmentation of the orogen, with the flat-slab segment (Norte Chico region) without Central Depression and Plio-Pleistocene volcanic activity, and with development of the Pampean Ranges on the Andean foreland, in Argentina (inset corresponds to the region represented in Fig. 3.61). (b) Location of the triple-junction region (inset corresponds to a region illustrated in Figs 3.57 & 3.70). Key: 1, Plio-Pleistocene volcanism; 2, Central Depression; 3, Pampean Ranges (PR). whereas south of the triple-junction convergence gradually (Puncoviscana Basin) located to the east, with accretionary changes from nearly orthogonal to oblique, and finally to events occuring further south (Lucassen et al. 2000). parallel to the continental margin, in southernmost Chile. Geological units Stratified rocks belonging to this cycle have not been found Pampean tectonic cycle (Late Proterozoic–Early in Chile. Proterozoic U–Pb ages (see Chapter 2) have been