Introduction

One of the best-studied examples of an orogen that has formed by ocean-continent convergence lies in the

Figure 10.1 (a) Shaded relief map of the central and southern Andes showing topographic features of the Nazca and South American plates. Map was constructed using the same topographic data and methods as in Fig. 7.1. Black dots are active volcanoes. LOFZ is the Liquine-Ofqui fault zone. Box shows location of Fig. 10.1b. ANCORP profile shown in Fig. 10.6. (b) Physiographic provinces of the central Andes (modified from Mpodozis et al., 2005, with permission from Elsevier).

Figure 10.1 (a) Shaded relief map of the central and southern Andes showing topographic features of the Nazca and South American plates. Map was constructed using the same topographic data and methods as in Fig. 7.1. Black dots are active volcanoes. LOFZ is the Liquine-Ofqui fault zone. Box shows location of Fig. 10.1b. ANCORP profile shown in Fig. 10.6. (b) Physiographic provinces of the central Andes (modified from Mpodozis et al., 2005, with permission from Elsevier).

central Andes of Peru, Bolivia, northern Chile, and Argentina (Fig. 10.1). Here, the Andes exhibit the highest average elevations, the greatest width, the thickest crust, and the greatest amount of shortening in the orogen (Isacks, 1988; Allmendinger et al., 1997; ANCORP Working Group, 2003). This central segment illustrates how many of the characteristic features of large orogens may form in the absence of continent-continent collision.

The Andean mountain chain, or cordillera, extends some 7500 km from Venezuela and Colombia in the north to Tierra del Fuego in the south. Along its length, the orogen displays a remarkable degree of diversity in structure, geologic history, and tectonic evolution. This diversity complicates determinations of the factors that control orogenesis within its different segments. Never theless, some common elements are evident that provide important boundary conditions on noncolli-sional orogenic processes. One of these constraints is that the active margin of South America was characterized by either a noncompressive or an extensional regime during the Late Jurassic and Early Cretaceous (Mpodozis & Ramos, 1989). At this time, most of the margin lay below sea level as a series of extensional backarc and marginal basins (Fig. 9.34) formed above a subduction zone (Dalziel, 1981; Mpodozis & Allmendinger, 1993; Mora et al., 2006). This history shows that by itself subduction cannot account for the formation of Andean-type orogens. Rather, mountain building in this setting results only when ocean-continent convergence leads to compression in the overriding plate (Sections 9.6, 10.2.5).

In the Andes, compressional regimes have been established several times since the early Mesozoic, with the most recent phase beginning about 25-30 Ma (Allmendinger et al., 1997). The beginning of this latest phase of compression has been interpreted to reflect two major processes: the trenchward acceleration of the South American plate (Pardo-Casas & Molnar, 1987; DeMets et al., 1990; Somoza, 1998) and strong interplate coupling between the subducting oceanic lithosphere and the overriding continent (Jordan et al., 1983; Gutscher et al., 2000; Yáñez & Cembrano, 2004). One of the principal aims of tectonic studies in the Andes is to determine the origin of the highly variable response of the South American plate to this compression. This section provides a discussion of the first-order physical characteristics of the central and southern Andes that allow geoscientists to make inferences about the genesis of the mountain range.

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