Page 312 - Global Tectonics

296 CHAPTER 10

Edge of the Brazilian
Lithosphere
Local compensation
Weak lower crust
Lithospheric flexure
Strong lower crust
Hinterland
Sub-
Andes Foreland
Los
Los
Elevation (km) 6 4 2 Cordillera Altiplano Frailes Chaco 18
Frailes
Western
Chaco
0
Eastern Cordillera
20
100 Slow NS fast Slow EW fast Crust
Depth (km) 200 EW fast Nazca Slab Mantle Brazilian Lithosphere
‘window’

lithosphere
300 Removal of
69 68 67 66 Asthenosphere
65
Change in shear wave 64 63 62 61 60 W
splitting direction
Figure 10.7 Lithospheric-scale cross-section of the Central Andes at latitudes 18–20°S showing interpretations of the
crust and mantle structure (image provided by N. McQuarrie and modified from McQuarrie et al., 2005, with permission

from Elsevier). Fast upper mantle P-wave velocities (dark gray) and slow P-wave velocities (white and gray shades) are
shown. White waves, crustal low-velocity zones. Cross-section shows basement thrust sheets beneath the Eastern
Cordillera, cover rocks are white. White stars are data recording stations.

refl ection profiles can be interpreted only in terms of Puna. Widespread fluid transport and partial melting of

structure and lithologic contrasts. the middle and lower crust during Neogene shortening
Within the crust, seismic velocities indicate the pres- and plateau growth appears to have weakened the crust

ence of a 15- to 20-km-thick zone of low seismic wave sufficiently to allow it to fl ow (Gerbault et al., 2005).
speeds at depths of 14–30 km beneath the Western Cor- Similar features beneath the Tibetan Plateau (Section
dillera and Altiplano-Puna (Yuan et al., 2000, 2002). 10.4.5) suggest that orogenic plateaux in general involve
Average crustal V p /V s ratios of 1.77 beneath the plateau very weak crust.
and peak values of 1.80–1.85 beneath the active volcanic East of the Altiplano-Puna, receiver function deter-
arc suggest the presence of high crustal temperatures minations show a decrease in crustal thickness from 60
and widespread intra-crustal melting. Patches of high to 74 km beneath the Eastern Cordillera to about 30 km
amplitude reflections and zones of diffuse refl ectivity at beneath the Chaco plain (Yuan et al., 2000; Beck &

some 20–30 km depth beneath the Precordillera and Zandt, 2002). Mantle tomography images indicate that
Altiplano suggest the presence of fluids rising through zones of low wave speed at 30 km depth extend through

deep fault zones (ANCORP Working Group, 2003). the lithosphere beneath Los Frailes ignimbrite fi eld (Fig.
One of the largest of these patches is the Quebrada 10.7), suggesting that the volcanism is rooted in the
Blanca Bright Spot (Fig. 10.6). Other low velocity zones mantle and that the mantle lithosphere in this region

and bright reflectors occur at mid-crustal depths beneath has been altered or removed (Myers et al., 1998). East
the Eastern Cordillera and backarc region. The pres- of this low velocity zone, high seismic velocities in the
ence of these features helps to explain differences in shallow mantle, high Q (Section 9.4), and a change in
crustal thickness and in the altitude of the Altiplano and the fast direction of shear-wave anisotropies suggest the