The Andes - Introduction
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The Andes is a mountain chain which extends along the western edge of the South American plate. The most prominent features that define this orogen are the subducting Nazca plate and its associated trench, the present day active volcanic arc and the fold-and-thrust belt in the foreland. Overall, the Andes span a total length of about 9000 km extending from Colombia to the Chile triple junction around 46ºS, which defines the interaction point of the South American, Antarctic and Nazca plate. 

Studies of the distribution of earthquakes and volcanoes and other geological and geophysical variables have defined a number of segments in the Nazca plate, whose main difference are convergence direction and dipping angle of the slab. In general, normal or steep dipping slab segments have angles around 30º while in shallow segments this angle does not exceed 15º. 

The tectonic evolution of the Andes extends back into the Paleozoic, when terrane accretion was the dominant process up to the Mesozoic. However, it is generally believed that the present day mountain belt was developed mainly during the Mesozoic to Recent, due to the east dipping slab and the opening of the Atlantic Ocean.

As sugested in Morres and Twiss, 1995 , it is convenient to split the whole length of the Andes in seven N-S aligned segments (zones A-G), each having a characteristic geological signature (adjacent figure). It is understood that the transition of these different environments is in general smooth and the reason to separate them is to have an easier way to explain the complex evolution of the Andes. 

In the next paragraphs we summarize each of these segments, outlining its most important features. The associated cross sections are shown in the adyacent figure. 

This will provide a reference frame, for the  sections of this web page, the southern Peru – central Chile area (segments B-D), that represent an important and well studied part of the Andes with some outstanding features like the highest elevations (large plateaus with average elevations around 3.5-4 km), largest width, the bend near18ºS, among others. 


 


Segment A

On a first order approximation this segment of the Andes can be subdivided in to three areas. The western side consists of an accumulation of collided oceanic blocks, mostly mafic and ultramafic island arcs accreted during the Cretaceous and early Tertiary time, forming the Western Cordillera. The middle section or Central Cordillera is characterized by the intrusion of Mesozoic rocks into a Precambrian basement and Cenozoic volcanic activity. Eastwards, a well developed Paleozoic to Cenozoic fold-and-thrust belt give rise to the Eastern Cordillera.

Segment B

On the eastern rim, Permian and Mesozoic batholitic rocks intrude the Precambrian unit called the Arequipa massif, which in turn thrust eastwards over marginal Mesozoic rocks. It is generally believed that the Arequipa massif has been part of the South American Shield at least since the Paleozoic. This corresponds to the Peru flat slab segment of the subducting Nazca Plate.

Segment C

The most prominent features of this section are the coastal batholiths, which are cut by the 1000 km long strike-slip Atacama fault. The Western Cordillera is characterized by present day volcanic activity, separated from the Eastern Cordillera by the Altiplano plateau of controversial origin. A well developed fold-and-thrust belt extends from the Eastern Cordillera over the South American Shield.

Segment D

The same batholithic structure observed in the previous section form the Coastal Range. A well defined Central Valley separates the Main Cordillera which in turn gives rise in the west to a Precordillera and finally to the Pampean Ranges, a series of basement uplifts. This section contains the transition zone from where the Nazca plate changes from a normal dip of ~30o to subhorizontal configuration and back to normal dip. It is characterized by a lack of volcanic activity.

Segment E

The Coastal Range is characterized by a late Paleozoic accretionary prism complex which is intruded by Paleozoic plutons. The Main Cordillera corresponds to the present day volcanic arc and gives rise in the east to the Agrio fold-and-thrust belt. Even further east, the presence of back-arc basaltic outcrop is considered anomalous since no significant spreading have been recorded so far.

Segment F

Composed essentially of a late Paleozoic accretionary complex intruded by Jurassic-Cretaceous plutons. These are bisected by a mayor left-lateral strike-slip fault, the Liquiñe-Ofqui fault, which forms a tectonic boundary along the western edge of the present day volcanic arc. To the east of the arc, no significant deformation is present.

Segment G (cross section trends from south-west to north-east!)

In the south, the coastal batholith is overthrust by Cretaceous volcanic rocks. Further north, a Cretaceous ophiolite belt overthrusts a metamorphic unit which in turn gives rise to a continent-vergent fold-and-thrust belt in the north of late Cretaceous and early Tertiary age.



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Authors: Fernando Barra, Robert Fromm, Victor Valencia