General geology of the central and southern Andes

The central Andes display two major mountain chains called the Western and Eastern cordilleras (Fig. 10.1b). Where the subducting Nazca plate dips steeply, south of latitude 15°S, the Western Cordillera contains the active volcanic arc. North of this latitude, where an active arc is absent, it is composed of Cenozoic extrusive rock. Paleozoic metasedimentary rock interfolded with Mesozoic-Cenozoic volcanic and sedimentary sequences comprise the Eastern Cordillera.

South of about latitude 15°S, the Western and Eastern cordilleras diverge around a large composite plateau called the Altiplano-Puna (Fig. 10.1b). This oro-genic plateau is 3.8-4.5 km high, 1800 km long, and 350-400 km wide (Isacks, 1988). Only the Tibetan Plateau (Section 10.4.2) is higher and wider. The Altiplano-Puna contains broad, internally drained areas of low relief and records little surface erosion. Its history of uplift began during the Miocene when plate convergence rates were at their peak (Allmendinger et al., 1997). An initial stage of uplift coincided with a major ignimbrite flare-up and a period of intense crustal shortening that initially occurred in the Eastern and Western cordilleras (Allmendinger & Gubbels, 1996) and later migrated eastward into the sub-Andean zone and the Chaco foreland basin (Section 10.3.2). This shortening resulted in very thick, hot continental crust beneath the plateau (Section 10.2.4). Geodetic data indicate that shortening at the leading edge of the orogen now occurs at rates of 5-20 mm a-1 (Klotz et al., 1999; Hindle et al., 2002).

Between the Western Cordillera (volcanic arc) and the Peru-Chile Trench, elevations drop to depths of 7-8 km below mean sea level over a horizontal distance of only 60-75 km. This narrow forearc region suggests that part of the central Andean margin has been removed, either by strike-slip faulting or subduction erosion (von Huene & Scholl, 1991) (Section 9.6). The forearc includes two major belts of rock that are separated by a central valley filled with Cenozoic sediment. East of the valley, the Precordillera exposes Precam-brian basement, Mesozoic sedimentary sequences, and Cenozoic intrusive and extrusive rock. The presence of this belt, which aligns with the Precambrian Arequipa Massif in southern Peru (Fig. 10.1b), indicates that the Andean orogen is founded on Precambrian continental crust. West of the central valley, the Coastal Cordillera is composed of early Mesozoic igneous rock that is a testament to the prolonged history of subduction along the margin. High-angle faults in the Coastal Cordillera, including the Atacama Fault System, record a long, complex history of normal, thrust, and strike-slip displacements (Cembrano et al., 2005).

Near 20°S (Fig. 10.1a), where the orogen is >800 km wide, the backarc region records 300-350 km of Neogene shortening (Allmendinger et al., 1997; McQuar-rie, 2002). Most of this shortening occurs in the sub-Andean zone where combinations of thrust faults and folds deform sequences of Cenozoic, Mesozoic and Paleozoic rock in a foreland fold and thrust belt (see also Sections 9.7 and 10.3.4). East of the sub-Andean ranges, the 200-km-wide Chaco foreland basin is filled with at least 5 km of Neogene sediment on top of the Brazilian Shield. This basin provides an important record of Cenozoic uplift, erosion, and deposition in the central Andes (Section 10.3.2).

The Andean foreland records three different styles of tectonic shortening (Fig. 10.4): (i) thin-skinned fold and thrust belts that are detached within Paleozoic sedimentary sequences at depths of 7-10 km (Lamb et al., 1997); (ii) thick-skinned fold and thrust belts with inferred detachments in Precambrian basement at 1020 km depth; and (iii) foreland basement thrusts that appear to cut through the entire crust (Kley et al., 1999). These different styles owe their origin partly to variations in the pre-Neogene lithospheric structure, temperature, and stratigraphy (Section 10.3.4). In addition, the deep-seated basement thrusts of the Pampeanas

Pilcomayo

Fig. 10.5b

Thin-skinned thrust belts Thick-skinned thrust belts

Foreland basement thrusts

Basement thrusts of the central belts

Figure 10.4 Distribution of the segmented style of foreland deformation in the Andes (after Kley et al., 1999, with permission from Elsevier). Flat slab segments are indicated.

Pilcomayo

Fig. 10.5b

Fig. 10.5d

Pampeanas +30° S

Thin-skinned thrust belts Thick-skinned thrust belts

Foreland basement thrusts

Basement thrusts of the central belts

Figure 10.4 Distribution of the segmented style of foreland deformation in the Andes (after Kley et al., 1999, with permission from Elsevier). Flat slab segments are indicated.

foreland (Fig. 10.5) correspond to a region of flat subduction, suggesting a possible causal relationship (Jordan et al., 1983; Ramos et al., 2002).

Alternations among the different styles of shortening along the strike of the orogen have produced a geologic segmentation of the Andean foreland. One of

Figure 10.5 (a) Sketch map showing the transitions from thick to thin lithosphere under the Central Andes as determined from seismic wave attenuation data. Zone of thick lithosphere correlates with areas of strongest shortening gradient. Santa Bárbara system (SBS) is characterized by thick-skinned deformation. Thin-skinned thrust belt (b) and restored section (c) of the sub-Andean ranges, Bolivia using data from Baby et al. (1992), Dunn et al. (1995), and °

Kley (1996). Thick-skinned thrust belt (d) and restored section (e) of the Santa Bárbara system (all images modified from Kley et al., 1999, with permission from g

Elsevier). Location of profiles (b) and (d) also shown on Fig. 10.4. E

Figure 10.5 (a) Sketch map showing the transitions from thick to thin lithosphere under the Central Andes as determined from seismic wave attenuation data. Zone of thick lithosphere correlates with areas of strongest shortening gradient. Santa Bárbara system (SBS) is characterized by thick-skinned deformation. Thin-skinned thrust belt (b) and restored section (c) of the sub-Andean ranges, Bolivia using data from Baby et al. (1992), Dunn et al. (1995), and °

Kley (1996). Thick-skinned thrust belt (d) and restored section (e) of the Santa Bárbara system (all images modified from Kley et al., 1999, with permission from g

Elsevier). Location of profiles (b) and (d) also shown on Fig. 10.4. E

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w the best-studied transitions occurs south of latitude 24°S. From north to south, a thin-skinned style of shortening in the sub-Andean belt (Fig. 10.5b,c) changes to a thick-skinned style of shortening in the Sierra de Santa Bárbara and northern Sierras Pampeanas (Fig. 10.5d,e). This change is accompanied by a decrease in the amount of shortening. A similar change in shortening magnitude occurs north of 14°S (Fig. 10.5a), implying that the present arcuate shape of the Central Andes either resulted from or has been accentuated by differences in the amount of shortening along the strike of the orogen (Isacks, 1988). The arcuate shape, or orocline, and the gradients in shortening also imply that the Central Andes have rotated about a vertical axis during the Neogene. GPS data, as well as paleomagnetic and geologic indicators, suggest that these rotations are counterclockwise in Peru and Bolivia north of the bend in the Central Andes, and clockwise to the south of the bend (Allmendinger et al., 2005).

Between 40° and 46°S latitude, the age of the subducting Nazca plate decreases from ~25 Ma at 38°S to essentially zero at 46 °S, where the Chile Ridge is currently subducting (Herron et al., 1981; Cande & Leslie, 1986). Along this segment, convergence occurs at an angle of ~26° from the orthogonal to the trench (Jarrard, 1986). The oblique convergence has driven late Cenozoic deformation within a relatively narrow (300-400 km) orogen characterized by average elevations of <1 km (Montgomery et al., 2001). An active volcanic arc occurs north of the subducting ridge where the forearc is undergoing shortening. Inside the arc, dextral strike-slip faults of the 1000-km-long Liquiñe-Ofqui fault zone accommodate the trench-parallel component of relative plate motion (Cembrano et al., 2000, 2002) (Section 5.3). In the backarc region, shortening is relatively minor (<50 km) and controlled by the partial tectonic inversion (Section 10.3.3) of an extensional Mesozoic basin (Ramos, 1989; Kley et al., 1999). The Southern Andes, thus, is characterized by arc volcanism, relatively low relief, and deformation that is focused within a narrow transpressional (Section 8.2) orogen.

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