Page 310 - Global Tectonics

294 CHAPTER 10

the best-studied transitions occurs south of latitude 400-km-long seismic refl ection profile across the central
24°S. From north to south, a thin-skinned style of Andes at 21°S latitude (Fig. 10.6). This profi le, together
shortening in the sub-Andean belt (Fig. 10.5b,c) changes with the results of geologic (Allmendinger et al., 1997;
to a thick-skinned style of shortening in the Sierra de McQuarrie, 2002) and other geophysical studies
Santa Bárbara and northern Sierras Pampeanas (Fig. (Patzwahl et al., 1999; Beck & Zandt, 2002), forms part
10.5d,e). This change is accompanied by a decrease in of a >1000-km-long transect between the Pacifi c coast
the amount of shortening. A similar change in shorten- and the Brazilian craton (Fig. 10.7). Below the central
ing magnitude occurs north of 14°S (Fig. 10.5a), imply- Andean forearc, the seismic refl ection profi le shows

ing that the present arcuate shape of the Central Andes east-dipping (∼20°) packages of reflectors that mark the
either resulted from or has been accentuated by differ- top of the subducting Nazca plate (ANCORP Working
ences in the amount of shortening along the strike of Group, 2003). Above and parallel to the slab are thick,
the orogen (Isacks, 1988). The arcuate shape, or orocline, highly reflective zones that indicate the presence of

and the gradients in shortening also imply that the trapped fluids and sheared, hydrated mantle at the top
Central Andes have rotated about a vertical axis during of descending slab. Some diffuse seismicity in this
the Neogene. GPS data, as well as paleomagnetic and region is probably related to dehydration embrittlement

geologic indicators, suggest that these rotations are (Section 9.4). Sub-horizontal reflectors below the
counterclockwise in Peru and Bolivia north of the bend Coastal Cordillera may represent ancient intrusions that
in the Central Andes, and clockwise to the south of the were emplaced during Mesozoic arc magmatism.
bend (Allmendinger et al., 2005). East of the forearc, converted (compressional-to-
Between 40° and 46°S latitude, the age of the shear) teleseismic waves indicate that crustal thickness
subducting Nazca plate decreases from ∼25 Ma at 38°S increases from about 35 km to some 70 km beneath the
to essentially zero at 46°S, where the Chile Ridge is Western Cordillera and Altiplano (Yuan et al., 2000;
currently subducting (Herron et al., 1981; Cande & Beck & Zandt, 2002). Crustal thickness also varies along
Leslie, 1986). Along this segment, convergence occurs the strike of the orogen, reaching a maximum of 75 km
at an angle of ∼26° from the orthogonal to the trench under the northern Altiplano and a minimum of 50 km
(Jarrard, 1986). The oblique convergence has driven under the Puna Plateau (Yuan et al., 2000, 2002). The
late Cenozoic deformation within a relatively narrow lithosphere is 100–150 km thick below the Altiplano
(300–400 km) orogen characterized by average eleva- (Whitman et al., 1996) and several tens of kilometers
tions of <1 km (Montgomery et al., 2001). An active thinner beneath the Puna. Lithospheric thinning
volcanic arc occurs north of the subducting ridge beneath this latter segment explains the high elevation
where the forearc is undergoing shortening. Inside (∼4 km) of the Puna above a relatively shallow Moho.
the arc, dextral strike-slip faults of the 1000-km-long The southward transition from thin-skinned to thick-
Liquiñe–Ofqui fault zone accommodate the trench- skinned thrust faulting in this same region (Fig. 10.5)
parallel component of relative plate motion (Cembrano suggests that the removal of excess mantle lithosphere
et al., 2000, 2002) (Section 5.3). In the backarc region, accommodates the westward underthrusting of the
shortening is relatively minor (<50 km) and controlled Brazilian Shield (McQuarrie et al., 2005).
by the partial tectonic inversion (Section 10.3.3) of Across the ANCORP ’96 seismic refl ection profi le
an extensional Mesozoic basin (Ramos, 1989; Kley (Fig. 10.6), a distinct Moho is conspicuously absent. A
et al., 1999). The Southern Andes, thus, is character- broad transitional zone of weak reflectivity occurs at its

ized by arc volcanism, relatively low relief, and defor- expected depth. The cause of this diffuse character of
mation that is focused within a narrow transpressional the boundary appears to be related to active fl uid-
(Section 8.2) orogen. assisted processes, including the hydration of mantle
rocks and the emplacement of magma under and into
the lower crust. Most of the reflectivity across the

10.2.4 Deep structure of the profile is linked to petrologic processes involving the
release, trapping, and/or consumption of fl uids
central Andes (ANCORP Working Group, 2003). These processes
have produced a seismic refl ection profile whose char-

In 1996, geoscientists working on the Andean Conti- acter contrasts with those collected across fossil moun-
nental Research Project (ANCORP ‘96) completed a tain belts (Figs 10.33b, 10.34b, 11.15b,c) where seismic