Andes of South America

Andes of South America The Andes form a sinuous mountain chain, 100–700 km wide and extending along the entire western margin of South America for over 5000 kilometres from 10° N in Venezuela to 55° S in southernmost Chile and Argentina. They pass northwards into the subduction systems of Central America and the Caribbean, and southwards into those of the Scotia Arc and the Antarctic Peninsula. They are conventionally divided into the northern (10° N–10° S), central (10° S–35° S), and southern (35° S–55° S) Andes, separated by marked swings in trend: the Patagonian orocline at c. 55°S; the Bolivian orocline at c. 18°S, and the Peru–Ecuador orocline at c. 5° S. The Andes can be divided into a number of physiographic provinces which parallel the topographic grain, with generally distinct geological histories: the Western Cordillera (Cordillera Occidental or Principal Cordillera) forms a high spine (often rising to over 5km) of mountains (usually volcanoes) on the western margin; the Eastern Cordillera (Cordillera Oriental) is a rugged region on the eastern margin, usually rising to between 3 and 4.5 km. In the Central Andes, the Altiplano or Puna is a high plateau or region of more subdued relief between the Eastern and Western Cordilleras; and the Subandean zone or Precordillera, which is the site of active thrusting today, lies in the foothills on the eastern margin. Satellite ranges occur as far as 500 km east of the main Andean range only in central Peru (Shira, Contaya, and Moa uplifts) and in northern Argentina (Sierras Pampeanas). To the east of the Andes, the core of South America, through which most of the major Andean rivers (Orinoco, Amazon, Putumayo) drain, is generally only a few hundred metres above sea level. Mt. Aconcagua (6959 m) at c. 33°S is the highest point in the Andes, but the mountain range as a whole is widest and highest between 12° S and 27° S, forming a mountainous zone nearly 700 km wide, with the Altiplano– Puna plateau on average c. 4000 m high. Overall, the average elevation and width of the Andes decreases more or less symmetrically north and south from central Bolivia.

Plate-tectonic setting

The Andes are a wide zone of convergent continental deformation between the South American plate and oceanic lithosphere of the Nazca and Pacific plates (Fig. 1). Shallow (0–70km) and deep-focus (70–650 km) earthquakes occur along the entire Andean margin. The shallow earthquakes are predominantly the result of thrust faulting, concentrated offshore in the trench and on the eastern margin of the Andes (Subandean zone or Precordillera). The deep-focus earthquakes define a slab-like zone (Benioff zone) inclined at 30–50° beneath the Andes, reaching depths greater than 600km in the Central Andes. The Benioff zone lies in oceanic lithosphere that is being subducted at the oceanic trench. This trench runs the length of the Andes, about 75 km offshore and up to 7 km deep off the coast of northern Chile. The relative plate convergence, averaged over the past 3million years, is about 80 mm per year in a ENE direction. However, at 47°S, near the Taitao peninsula in southern Chile, the active Chile Ridge spreading centre intersects the trench. South of this, the Pacific plate is being subducted beneath the Andes at about 20 mm per year in a more easterly direction, but the southern termination is a sinistral transform. The northern termination of the Andes is dominated by E–W-trending dextral strike-slip motion between the Caribbean Plate and northern Colombia and Venezuela.

Plate convergence is accommodated both by slip at the plate interface in the subduction zone and by shortening in the continental lithosphere of the South American plate, resulting in crustal thickening and mountain building. It has been estimated that during the Cenozoic less than 20 per cent of the total plate convergence between the Nazca and South American plates was absorbed by continental shortening, giving rise to a maximum of about 300–350 km of tectonic shortening in the Central Andes. This has resulted in a crustal thickness ranging from c. 34 km east of the Andes to up to 80km beneath the Altiplano–Puna. The thickness of the lithosphere also varies markedly, and seems to be relatively thin beneath the Central Andes. The present large-scale topography is almost entirely the result of deformation during the past 50Ma, in the Cenozoic, and substantial increases in height have occurred in the past 10 Ma.

There are marked variations in the style and amount of deformation along the length of the Andes. Features of the oceanic Nazca plate may have played a role in this, such as a change in age at the trench, between 6° N and 47° S, from Recent (in the north and south) to Palaeocene (at the latitudes of the Central Andes) with a number of prominent ridges that intersect the trench (Nazca Ridge at c. 16° S, Juan Fernandez Ridge at c. 33° S). Two ‘flat-slab’ regions have been recognized: beneath central Peru (3° S–16° S) and beneath Chile and Argentina (27° S–33° S). In these regions the subducted lithosphere is nearly horizontal in a wide zone before plunging steeply into the Earth's mantle further east. The flat-slab zones also coincide with the presence of outlying ranges far to the east of the Andes.

A volcanic arc running along the western margin of the Andes follows the 90–150-km depth contour of the Benioff zone. However, the ‘flat-slab’ regions are associated with a gap in the active volcanic arc, although there is evidence for Miocene and older volcanic activity. The gaps in active arc volcanism are often used to divide the arc into north-ern (NVZ), central (CVZ), and southern (SVZ) volcanic zones. These are the type location for the characteristic intermediate-composition andesitic volcanism, although mafic-rhyolitic volcanism is also common. Widespread ignimbrites, deposited during explosive and ash-rich volcanic eruptions, are an important feature of the CVZ.

Geological evolution

The core of the South American continent is underlain by Proterozoic and older basement, which is the result of episodes of welding of continental blocks during the Pan-African and Brazilian orogenic cycles (700 ± 100 Ma), now separated into two main shield areas, referred to as the Guyanan and Brazilian Shields. The western margin of South America has been the site of continental accretion, crustal growth, and both compressional and extensional deformation throughout the Phanerozoic, situated during the Palaeozoic and early Mesozoic at the edge of the giant landmass of Gondwana. In the Cambro-Ordovician this region seems to have evolved from a zone of continental collision to a passive continental margin associated with the collision and subsequent rifting of the Laurentian (North American) continental mass. It later evolved as a complex subduction margin with associated island-arc systems, rifted continental slivers, and back-arc basins. Thick sequences (up to 10 km) of shelf and continental slope deposits were deposited in the Central Andes from the Cambrian to Devonian. Palaeozoic subduction and accretion also resulted in the amalgamation of various terranes, associated with regional compressive events. Notable terranes include Arequipa terrane in coastal southern Peru and northern Chile; Chilena, Precordillera, and Patagonia terranes in Chile and Argentina. However, since the early Triassic (c. 250 Ma), when the supercontinent of Pangea was fully assembled, the central and southern Andes appear to have formed a classic continental-type subduction margin, with eastward subduction of oceanic lithosphere (Nazca and previous oceanic plates), and no further terrane accretion. However, the northern Andes, in Venezuela, Columbia, and Ecuador, have had a more complex history, partly influenced by Caribbean tectonics and relative motion of North and South America. Here, a series of allocthonous terranes (island-arc systems) were accreted in the latest Jurassic to early Cretaceous (Ecuador) and early Tertiary (Columbia).

In the Jurassic and Cretaceous there was widespread marine and lacustrine sedimentation along much of the length of the Andes, associated with rifting in forearc and behind arc basins such as the West Peruvian trough at 5°–14° S; the Aimara basin at 16° S–28° S; Neuquen basin near 38° S; and the Megallanes basin south of 47° S, covering much of southern Patagonia. Early Cretaceous back-arc spreading in southern Patagonia was also sufficient to create new ocean floor, which was subsequently obducted in the Middle Cretaceous, forming the Rocas Verdes Ophiolite. Jurassic to Cretaceous magmatic activity, along the entire length of the Andes, was associated with the emplacement of huge granite batholiths which crop out today in the coastal regions of Peru and Chile.

The relative plate motions along the Andean plate boundary are well documented only since the latest Cretaceous (68 Ma), by means of ocean-floor magnetic anomalies. The rate of convergence increases markedly at about 50 Ma, marking the start of the Cenozoic phase of compressional deformation in the Central Andes. The rate of relative plate convergence subsequently decreased in the late Eocene (c. 35 Ma), before increasing markedly again in the latest Oligocene (c. 27 Ma). The two phases of increased relative plate motion correlate well with major phases of compressive deformation in the Andes, sometimes referred to as Incaic (early Cenozoic) and Quechua (Middle–Late Cenozoic). The South American lithosphere, east of the region of mountain-building, has flexed downwards in response to the deformation, resulting in a series of Cenozoic ‘foreland’ basins which contain a thick sequences, usually less than 5 km thick, of mainly fluvial sediments.

Simon Lamb and and Lorcan Kennan