The Andes -Tectonic Evolution
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The tectonic evolution of the South Central Andes (18S-40S) segment will be discussed within the four main stages described by Mpodozis and Ramos (1989) and Ramos (2001).

The Gondwana super continent formed by accretion and amalgamation of different terranes of Archaean-Proterozoic age as a result of the Panafrican-Brasiliano Orogeny (Brito Neves et al., 1999). These blocks included part of Antarctica, Australia, India, Africa, and the Guianan-Brazilian shield. The position of this Super continent was near the South Pole and the South American block was composed of three main cratons: Amazonia, Rio de la Plata and San Francisco (Figure 1).

Figure 1: Crustal provinces of South America. AM: Amazonian craton; SF: Sao Francisco craton; RP: Rio de la Plata craton; SL: Sao Luis cratonic fragment; LA: Luis Alves cratonic fragment. From Cordani and Sato (1999).

The Pampia terrane was accreted during late Proterozoic –Early Cambrian to Rio de la Plata craton during the Pampean orogeny (~530 Ma, Rapela et al., 1998) (Fig. 2, 3). Also during the Pampean orogeny the Arequipa-Antofalla terrane collided with the Amazonian craton. During the Cambrian and Early Ordovician this block was separated from the continent and later amalgamated in Late Ordovician times (Ocloyic orogeny) (Bahlburg and Herve, 1997).

Figure 2: Reconstruction of the protomargin of South America (~530 Ma). Accretion of the Pampia terrane marked the onset of the Pampean orogeny during Early Cambrian times. AAC: Arequipa-Antofalla craton; RAC: Rio Apas craton; RPC: Rio de la Plata craton. (From Rapela et al., 1998).

Figure 3: Precambrian to Early Paleozoic evolution of the Sierra the San Luis area, Eastern Sierras Pampeanas. The Pampean terrane is interpreted here as a detached fragment from the Rio de la Plata craton. From Gosen et al. (in press).

Precambrian Basement
In the Andes of Chile and Argentina a series of Precambrian rocks have been described (Figure 4). In order to better understand the geologic evolution of the Central Andes, we follow the subdivisions presented by Mpodozis and Ramos (1990).

Figure 4: Map of major Precambrian and Early Paleozoic outcrops in N. Chile and NW Argentina. AM: Arequipa Massif; Br: Berenguela; B: Belen-Tignamar; U: Uyarani; CC: Cumbres Calchaquies; SA: Sierra de Ancasti; SF: Sierra de Fiambala; SC: Sierra de Cordoba. Modified from Lucassen et al. (2000).

Northern segment (18S-28S)
In the northern segment the Precambrian basement can be divided in two main groups: low grade metasediments (Puncovicana belt) and high-grade metamorphic rocks.

a) Puncoviscana Belt is a thick sedimentary sequence composed mainly of siliciclastic flysh-like turbidites and minor shallow water micritic limestones with local thick conglomerate lenses and affected by low grade metamorphism (Omarini et al., 1999). It continues in Bolivia into the Tucavara belt and separates the Arequipa block from the Amazonian Craton (Ramos, 2000). Omarini et al., (1999) has stated that the Puncoviscan belt formed as an intracontinental basin with bimodal igneous suites as a consequence of the Rodinia Super continent breakup at ~800 Ma. The Puncoviscan belt was intruded by syn- and post-orogenic granites during the Tilcaric orogeny (Early-Middle Cambrian). Radiometric ages for these intrusions range from 564 to 453 Ma.
b) The medium to high-grade metamorphic rocks of Precambrian age are mainly exposed in the Coastal Cordillera but also isolated outcrops can be found in the Puna and Cordillera de Domeyko. Some of these exposures are: Belen, Quebrada Choja (Sierra de Moreno), Peninsula Mejillones and Salar de Navidad in Chile (Damm et al. 1994). Belen and Sierra Moreno are probably extensions of the Arequipa massif and Mejillones and Navidad are probably part of the suspected Mejillonia terrane (Ramos, 1988, 2000) (Figure 6), although Omarini et al (1999) consider Mejillonia and the Precordillera terranes as a single block (Figure 18).

Figure 5: Detail map of major Precambrian and Early Paleozoic units in NW Argentina. 1: Salar Centenario; 2: Salar Ratones and Salar Diablillos; 3: Salar Hombre Muerto; 4: Laguna Blanca; 5: Cerro Blanco; 6: Salar de Antofalla; 7: El Jote and El Peon; 8: Cafayate; 9: Rio Anchillo – Quebrada Quilmes. >From Lucassen et al. (2000).

Figure 6: Precambrian and Early Paleozoic basement units in Northern Chile. Radiogenic ages are indicated in box. V: volcanic intrusion; M: metamorphic overprint, p denotes peak metamorphism and r retrograde metamorphism; I: intrusive event; Z: zircon fission track age; A: apatite fission track age. From Damm et al. (1994).

Central segment (28S-36S)
The basement in this area includes the composite Cuyania terrane and the Chilenia terrane. The Cuyania terrane is formed by the Precordillera and the Pie de Palo terranes.

FAMATINIAN OROGENIC CYCLE (Early Ordovician – Early Carboniferous)
The main characteristic of this orogenic cycle is the amalgamation of several terranes to theGondwanian protomargin. The nature of these terranes is in some cases exotic, probably derived from Laurentia, and para-autochthonous. The existing evidence for terrane accretion during this time is the presence of disrupted ophiolitic sequences and calc-alkaline rocks within the continent and more than 500 km from the present trench. The disrupted ophiolites suggest the presence of an ocean basin between the margin of Gondwana and these terranes.

Two diastrophic events are part of the Famatinian Cycle:
a) Ocloyic Orogeny (Middle to Late Ordovician)
b) Chanic Orogeny (Early to Middle Devonian)

Cambrian - Early Ordovician
This period is characterized by extension, subsidence, magmatic activity and deformation (Figure 7).

Figure 7: Global reconstruction for mid-Late Cambrian. A: Arequipa-Antofalla; Fa: JPGatina; P: Puna; C: Chilenia; Cu: Cuyania-Precordillera; O: Oaxaquia; Ch: Chortis. From Keppie and Ramos (1999).

Northern segment (18S-26S)
Two magmatic arcs have been described in the Puna
a) Faja Eruptiva Occidental (FEW): western magmatic arc developed on the Arequipa-Antofalla block, and
b) Faja Eruptiva Oriental (FEE): eastern magmatic arc developed on the western margin of the Pampia terrane.

Figure 8: Generalized profile showing the Paleozoic evolution at 23-25S. >From Ramos (1988).

An extensive Ordovician basin formed to the west of the Arequipa Antofalla block and was filled with turbidites and to a lesser degree with volcanic rocks from the FEE (Figures 9, 10).

Figure 9: Global reconstruction for Early Ordovician. A: Arequipa-Antofalla; Fa: Famatina; P: Puna; C: Chilenia; Cu: Cuyania-Precordillera; O: Oaxaquia; Ch: Chortis. From Keppie and Ramos (1999).

Figure 10: Global reconstruction for Early-Middle Ordovician. A: Arequipa-Antofalla; Fa: Famatina; P: Puna; C: Chilenia; Cu: Cuyania-Precordillera; O: Oaxaquia; Ch: Chortis. From Keppie and Ramos (1999).

All these rocks are intensively deformed as a consequence of the Ocloyic deformation at the end of the Ordovician and represents the final amalgamation of the Arequipa-Antofalla terrane.

Central segment (26S-38S)
Three main features characterizes this segment
1) Formation of a granitoid belt in the western part of the Sierras Pampeanas (Pampean Ranges) of Argentina, which represents the southern extension of the Faja Eruptiva de la Puna (FEP), and is probably related to the accretion of the Cuyania-Precordillera terrane during Middle Ordovician times (Ramos, 2000).

2) The presence of the Cuyania composite terrane. This terrane contains the Precordillera, which is an allochthonous carbonate platform with Laurentian affinities. The Cuyania-Precordillera terrane was accreted to the Famatina terrane in late Ordovician times (Fig. 12).

3) Presence of an oceanic realm between the Cuyania Terrane and Chilenia terrane that was amalgamated during the Late Devonian to Early Carboniferous.

Figure 11: Global reconstruction for Middle Ordovician. A: Arequipa-Antofalla; Fa: Famatina; P: Puna; C: Chilenia; Cu: Cuyania-Precordillera; O: Oaxaquia; Ch: Chortis. From Keppie and Ramos (1999).

Figure 12: Schematic tectonic evolution of western proto-margin of Gondwana showing the relationships between Cuyania-Precordillera and Famatina terranes. >From Ramos (2000).

The accretion of the Cuyania-Precordillera terrane has been extensively debated. Two general models have been proposed. One model proposed that the Precordillera terrane detached from Laurentia during Early Cambrian time and accreted to the Famatinian terrane during the Middle to Late Ordovician (Figure 10, 11, 12).

The second model states that the Precordillera formed as a consequence of the collision between Laurentia and Gondwana in Early to Middle Ordovician time, from 487-467 Ma (Taconic- Famatinian Orogeny) (Dalla-Salda et al., 1992) (Figures 13, 14).

Figure 13: Present day location of Taconic and Famatinian orogens.

Figure 14: Global reconstruction for ~520 Ma showing southern Iapetus ocean between Laurentia and Gondwana. Asterisks indicate truncated ends of Taconic and Famatinian belts. From Dalla Salda et al. (1992).

The Chilenia terrane was accreted to the proto-margin of Gondwana at ~420-390 Ma (figure 15).

Figure 15: Global reconstruction for 420 Ma and 390 Ma. From Keppie and Ramos (1999).

The only major terrane accreted in late Devonian times is the Patagonia Terrane (Fig. 16). A minor allochthonous terrane, the Madre de Dios Archipielago has also been recognized in the southern part of Chile (50-52S).

Figure 16: Global reconstruction for 374 Ma; O: Oaxaquia; Ch: Chortis. >From Keppie and Ramos (1999).

By the end of the Paleozoic the configuration of the South American continent is similar to the present day configuration with the exception of some terranes accreted at the Northern Andes in Late Tertiary times.

There is still a lot of controversy regarding the formation of the South American western margin. Mainly two different models have been proposed: The terrane model (Ramos, 1988, 2001; Bahlburg and Herve, 1997; Omarini et al., 1999) and the mobile belt model (Becchio et al., 1999; Lucassen et al., 2000). Within the terrane model different terrane configurations have been proposed (see figures 17, 18 and 19). Evidence that supports the terrane model are the presence of disrupted ophiolitic sequences interpreted as remnants of an oceanic realm between the Gondwanian proto-margin and the terranes and the existence of calc-alkaline magmatic arcs more than 500 km from the present trench (Ramos, 2000). On the other hand, Lucassen et al. (2000) favor a mobile belt model to explain the Paleozoic evolution of the western margin of South America between 18 and 26S. This model is mainly based on metamorphic crystallization ages and model ages for the Late Precambrian – Early Paleozoic metamorphic basement rocks.

Figure 17: Allochthonous terranes of southern South America (Modified from Ramos, 1988)

Figure 18: Terrane map according to Omarini et al (1999). C: Cuyania-Antofalla-Belen-Arequipa terrane; P: Precordillera-Mejillonia terrane; R: rio de la Plata craton; Pa: Pampia terrane; Ch: Chilenia terrane; CP: Cordoba-Paraguay arc.

Figure 19: Terrane map of South America according to Ramos and Aleman (2000). This map does not include the Coastal Cordillera terranes (Mejillonia, Pichidangui, Chanaral, Chiloe terranes) as shown in the first map of Ramos (1988).

In contrast to the Famatinian cycle, which is strongly marked by accretion of terranes the Gondwanan cycle is characterized by an important extensive felsic magmatism and extensional events, that are the precursor of the Pangea break-up (see Early Jurassic; Late Jurassic global paleogeographic reconstructions) (Ramos and Aleman, 2000).
The beginning of this tectonic cycle is characterized by simple subduction of an oceanic plate under the continental margin. The subduction process created an accretionary prism along the Coastal Cordillera during Late Carboniferous to Late Triassic (Figure 20). Tectonic erosion removed most part of the Late Paleozoic subduction complex N of 32S.

Figure 20: Schematic cross section of the South American margin at 29 –33S during Carboniferous to Early Triassic times. From Mpodizis and Ramos (1990).

The extensive felsic magmatism that characterizes this cycle was originally interpreted to be the product of crustal extension (Zeil, 1979). Later studies have shown that the felsic magmatism is the result of an early cycle of subduction followed by acid-non-orogenic magmatism associated with active extensional faulting (Ramos, 2000).
The volcanic activity is mainly represented by the Choiyoi rhyolite group in the Cordillera Frontal of Chile and Argentina between 27 –34S, but the volcanic activity extended from ~20S to 42S (Figure 21). The Triassic rift system has a general NW-SE trend, heavily controlled by basement fabrics, and followed the rifting that began with the Choiyoi volcanics. The rifts were filled with red-beds and volcanic rocks of bimodal composition (e.g. Cuyo basin in Argentina).

Figure 21: Rhyolitic provinces of South America. Red areas are exposures; blue areas are equivalent volcanic arc rocks. Circles represent oil wells in which the rhyolites have been intersected. Yellow and green areas represent possible extension of the rhyolitic provinces of Choiyoi (Late Carboniferous-Early Permian to Triassic) and Chon Aike (Middle Jurassic), in Patagonia. From Ramos (2000).

The volcanic rocks of Choiyoi are associated to shallow level plutonic rocks of the rhyolitic and dacitic composition.. The main batholiths are shown in figure 22. The plutonic rocks of the Elqui-Limari batholith in Chile have been subdivided in two main complexes: The Elqui Complex (Late Carboniferous to Permian) and the Ingaguas Complex (Permian to Late Triassic) (Mpodozis and Kay, 1992).

Figure 23: Main batholiths and stocks of Late Paleozoic - Early Mesozoic age. From Ramos (2000).

The Elqui and Ingaguas complexes have been further subdivided in four units. The Elqui Complex is composed of the following units: Guanta (tonalitic and granodioritic composition), Montosa (granodioritic), Cochiguas (granodioritic and granitic composition) and El Volcan (granitic). The Ingaguas Complex is formed by El Colorado (granites and rhyolitic porphyries; El Leon (monzogranites; Chollay (monzogranites and Los Carricitos (granodiorites). Similar suites have been recognized in Argentina (Colanguil, in La Ramada and Aconcagua areas and in Cordon del Portillo).
The two complexes mention above are separated by the San Rafael orogenic phase which is responsible for the unconformity between turbidite beds of Late Carboniferous to Early Permian age and Permo-Triassic volcanic rocks. This compressional event has been interpreted as the result of the accretion of an unknown terrane (X terrane, Mpodozis and Kay, 1992) or to an increase in the convergence rate (Ramos, 1988).

This period is also characterized by the development of a series of sedimentary basins that formed as the result of the continuous extensional regime. According to Ramos (2000) these sedimentary basins can be grouped in two different sedimentary cycles characterized by different tectonic settings. The first cycle was deposited in retro-arc basins developed during active subduction in Carboniferous–Early Permian times. These basins were short-lived and did not evolved in marginal basins. A link between these basins and the Chaco-Parana and Paganzo intracratonic basins has been proposed.
The second cycle is characterized by rift basins developed along the continental margin in Middle to Late Triassic times. This second cycle was deposited in angular unconformity (San Rafael orogenic phase) over Paleozoic and Precambrian rocks.
These basins were developed along sutures of the Paleozoic terranes as seen for example for the Cuyo basin, which was developed along the suture between the Precordillera and Chilenia terranes.

Figure 23: Triassic paleogeography of the Gondwana margin showing location of the main rift systems. From Ramos and Kay (1991).

Figure 24: Cross section of the Cuyo basin showing possible tectonic setting palinspastically restored at the end of the Triassic. From Ramos and Kay (1991).

Although extension continued during the Late Triassic to Jurassic, Franzese and Spalletti (2001) have suggested that the Triassic-Jurassic (T-J) extension event (Figures 25, 26) should be considered as an independent extensional episode based on different tectonic frameworks for each extensional episode (Lower Triassic, T-J, Upper Jurassic). The T-J extension and bimodal magmatism were the result of margin mechanical interaction between the lithospheric plates at the Gondwana margin. In this setting the cessation of subduction paired with a dextral strike-slip regime at the continental margin caused the collapse of the slab and generation of an asthenospheric window (Fig. 27). The Upper Jurassic extensional episode, which developed the Patagonian basins, is directly related to break-up of Gondwana (see below).

Figure 25: Location of depocenters in South Central Chile formed during Triassic-Jurassic extension. Numbers refer to depocenters in Figure 25. >From Franzese and Spalletti (2001).

Figure 26: Main stratigraphic units generated during syn-rift stage of the T-J continental extension event. Numbers refer to depocenters in Figure 24. From Franzese and Spalletti (2001).

Figure 27: Schematic cross section of the South American margin at ~37S during Late Carboniferous to Middle Jurassic times. From Franzese and Spalletti (2001).

During Jurassic and Cretaceous times a series of complex basins (fore-arc, intra-arc and retro-arc type) developed in the Andes (figure 28). This extension is also present in the central Andes and is related in its early stages to the Pangea break-up but later on is associate to a peculiar type of subduction that developed a poorly evolved magmatism (La Negra Formation) with intra and retroarc extension. In Central Andes extension was active from the volcanic arc in Chile to central Argentina. This extension is coeval with the Salta rift basin and is also responsible of the retroarc basins of Neuquen, Rio Mayo and Magallanes Basin. No oceanic terranes accretion characterizes this period in the central Andes as compared with the Northern Andes.
Oblique subduction is responsible for the development of the Atacama strike-slip fault. This fault is parallel to the trench and affected the Coastal Cordillera. It is also responsible of the ascent and emplacement of the arc granitoid of the Coastal Batholith during Jurassic and Early Cretaceous.

Figure 28: Early Cretaceous rifting in South America coeval with the Atlantic opening. From Ramos and Aleman (2000).

One of the main features of the Mesozoic evolution of the Andes of Chile and Argentina is the development of a tectonic segmentation. Mpodozis and Ramos (1990) described five segments (A-E) with different geologic characteristics. Here we will describe the first three segments that are part of the Central Andes from Late Triassic times.

Segment A: Northern segment (22-27S)
Triassic – Early Cretaceous
Late Triassic: marine transgression forms a small basin at the site of the present Cordillera de Domeyko.
Early Jurassic: expansion of the marine basin (Tarapaca basin). Subsidence of the basin is controlled by extensional tectonics. Development of the La Negra magmatic arc which extends from Arica (18S) to Chanaral (26S).
Early Cretaceous: development of the Atacama fault (1000 km strike-slip fault).
Oxfordian time: retreat of sea in northern part of the basin. In the south marine conditions were maintain until Tithonian and Neocomian time. During the Early Cretaceous the basin is filled with continental red beds and locally with lava flows from the La Negra magmatic arc.

Middle Cretaceous: cessation of the La Negra magmatic activity, uplift of the Tarapaca basin to form the Proto Cordillera de Domeyko (PCD). As a consequence of this uplift and subsequent erosion of this positive topographic element a sequence of continental red beds are deposited in a basin to the east of PCD (Purilactis Formation).

Figure 29: Tectonic evolution of Segment A (22-27S) since Late Jurassic to Miocene times. From Mpodozis and Ramos (1990).

This period is characterized by a discontinuous migration of the magmatic arc (Fig. 30). Volcanism started again in Late Cretaceous until Eocene, decreases during the Oligocene. Although volcanic activity decreases in the Oligocene intrusive activity is widespread and important during the Late Eocene and Oligocene (48-28 Ma). Most of the world-class chilean porphyry copper deposits are emplaced during this period. The emplacement of these porphyries occur along the axis of the PCD and through strike-slip faults such as the West Fissure in Chuquicamata. This period of waning volcanism coincides with a period of highly oblique convergence of the Nazca plate and is responsible of these strike-slip structures.

Figure 30: Eastward migration of the volcanic front related to tectonic erosion of the continental margin up to Paleogene times. Miocene volcanic front migration and magmatic arc expansion were controlled by changes in the Wadati-Benioff geometry. From Ramos and Aleman (2000).

Segment B: Central segment (27-33S)
This segment is characterized by a Jurassic-Early Cretaceous magmatic arc in the Coastal range (inner arc), a coeval outer arc in the present Chile-Argentina border (Ramos and Aguirre-Urreta, 1992), the Aconcagua Platform, which is as sedimentary backarc basin and the development of an aborted marginal basin during the Early Cretaceous. Also plutons of Jurassic-Cretaceous age are intruded along the Coastal region.
The inner arc is represented by the Bandurrias Formation and the shallow marine environment is represented by the Chaarcillo Group. South of 29S the volcanic rocks are represented by the Arqueros Formation. This Formation and the Pelambres and Lo Prado Formations (Early Cretaceous) farther south, are composed of thick sequence of andesitic rocks interbedded with marine deposits. The presence of limestones suggested a submarine environment for the volcanic eruptions. A marine transgression during the Early and Middle Jurassic developed the clastic-carbonate platform of the Aconcagua Platform.
Subaereal volcanism (Horqueta Formation) was caused by uplift during Upper Jurassic times, which is also evident from the marine regression in the back arc region.
Early Cretaceous is marked by a new marine transgression in the Aconcagua Platform, a decrease of plutonism in the Coastal Cordillera and extensive volcanism associated with an aborted marginal basin (Aberg et al, 1984). The extensional regime that characterizes this segment during the Jurassic and Early Cretaceous is the result of an elevated thermal gradient that causes crustal spreading and as is responsible for the thick pile of volcanic rocks with burial metamorphism.
During the Late Cretaceous changes in the tectonic regime, from a low-stress Mariana-type subduction to a high-stress Chilean-type subduction caused closure of the intra- and back-arc basins and eastward migration of the magmatic foci. The new stress conditions are responsible for the development of marine regression and a continental retro-arc basin. The closure of basins and associated shortening produce the Aconcagua fold-thrust belt (AFTB) along the eastern margin of the Cordillera Principal in the Late Cretaceous-Miocene interval.
This segment does not present Quaternary volcanism.

Figure 31: Different tectonic regimes in Southern Central Andes, showing the change between and extensional and compressional regime at about 115 Ma (Ramos and Aleman (2000).

Segment C: Central Southern segment (27-33S)
This segment is characterized by a stationary Jurassic-Quaternary magmatic arc along the present Cordillera Principal, lack of magmatism in the Coastal Ranges, active Quaternary volcanism and the back-arc Neuquen basin. The Neuquen Basin is a wide ensialic and asymmetric foreland basin that developed as a marine embayment. It became filled with marine and continental sediments during the Jurassic-Early Cretaceous and was subsequently deformed during Middle Cretaceous, Late Eocene and Middle Miocene. The Neuquen basin is one of the most important oil producing areas of Argentina.

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