Page 307 - Global Tectonics

292 CHAPTER 10

plateau (Section 10.2.4). Geodetic data indicate that 80
shortening at the leading edge of the orogen now 60 W
−1
occurs at rates of 5–20 mm a (Klotz et al., 1999; Hindle 10 N
et al., 2002).
Mérida
Between the Western Cordillera (volcanic arc) and
the Peru–Chile Trench, elevations drop to depths of
Magdalena
7–8 km below mean sea level over a horizontal distance
of only 60–75 km. This narrow forearc region suggests
0
that part of the central Andean margin has been Napo
removed, either by strike-slip faulting or subduction
erosion (von Huene & Scholl, 1991) (Section 9.6). The
Huallaga
forearc includes two major belts of rock that are sepa-

rated by a central valley filled with Cenozoic sediment.
Madre
East of the valley, the Precordillera exposes Precam- Flat slab segment de Dios
brian basement, Mesozoic sedimentary sequences, and
Cenozoic intrusive and extrusive rock. The presence of
Beni-
this belt, which aligns with the Precambrian Arequipa 15 S
Massif in southern Peru (Fig. 10.1b), indicates that the
Andean orogen is founded on Precambrian continental Pilcomayo
crust. West of the central valley, the Coastal Cordillera
Fig. 10.5b
is composed of early Mesozoic igneous rock that is a
testament to the prolonged history of subduction along
Fig. 10.5d
the margin. High-angle faults in the Coastal Cordillera,
including the Atacama Fault System, record a long, Pampeanas
complex history of normal, thrust, and strike-slip dis- Flat slab
segment 30 S
placements (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-
Neuquén
rie, 2002). Most of this shortening occurs in the sub-
Thin-skinned thrust belts
Andean zone where combinations of thrust faults and
folds deform sequences of Cenozoic, Mesozoic and
Thick-skinned thrust belts Chubut
Paleozoic rock in a foreland fold and thrust belt (see also
Sections 9.7 and 10.3.4). East of the sub-Andean ranges, 45 S
Foreland basement thrusts
the 200-km-wide Chaco foreland basin is filled with at

least 5 km of Neogene sediment on top of the Brazilian Basement thrusts of the
central belts Magallanes
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 Figure 10.4 Distribution of the segmented style of
and thrust belts that are detached within Paleozoic foreland deformation in the Andes (after Kley et al.,
sedimentary sequences at depths of 7–10 km (Lamb 1999, with permission from Elsevier). Flat slab segments
et al., 1997); (ii) thick-skinned fold and thrust belts with are indicated.
inferred detachments in Precambrian basement at 10–
20 km depth; and (iii) foreland basement thrusts that foreland (Fig. 10.5) correspond to a region of fl at sub-
appear to cut through the entire crust (Kley et al., 1999). duction, suggesting a possible causal relationship
These different styles owe their origin partly to varia- (Jordan et al., 1983; Ramos et al., 2002).
tions in the pre-Neogene lithospheric structure, tem- Alternations among the different styles of shorten-
perature, and stratigraphy (Section 10.3.4). In addition, ing along the strike of the orogen have produced a
the deep-seated basement thrusts of the Pampeanas geologic segmentation of the Andean foreland. One of