Page 306 - Global Tectonics

OROGENIC BELTS 291

(a) 78°W 74°W 70°W 66°W 62°W (b)
South American A A'
Plate
A' W E
18°S 0
Nazca Altiplano 100

Plate
A E. Cordillera 200
22°S Subandean Ranges Depth (km) 300
79.4
400 Arc normal distance
66.6 Puna (c) 100 km
26°S
B B'
W E
0
66.5
30°S 100
B B' Depth (km) 200
66.7
300
JFR
34°S
66.6 400
100 km
NUVEL-1 GPS Arc normal distance
80.3
Figure 10.3 (a) Shaded relief map of the central Andes showing the distribution of large to moderate earthquakes
(crosses) and volcanoes (triangles) active since the Pliocene. Topographic databases are from Hastings & Dunbar (1998)
and Smith & Sandwell (1997). Earthquakes are from the USGS National Earthquake Information Center, Preliminary
Determination Epicenter catalogue (1973 to present) for shallow (≤70 km) events. Black and white arrows show relative
convergence of the Nazca and South American plates from NUVEL-1 (DeMets et al., 1990, 1994) and continuous GPS
−1
observation (Kendrick et al., 1999; 2003) in mm a . (b,c) Cross-sections showing depth distribution of relocated
earthquakes (Engdahl et al., 1998) (images provided by L. Siame and modified from Siame et al., 2005, by permission of

the American Geophysical Union. Copyright © 2005 American Geophysical Union). JFR, Juan Fernández Ridge.

10.2.3 General geology of the plateau called the Altiplano-Puna (Fig. 10.1b). This oro-
genic plateau is 3.8–4.5 km high, 1800 km long, and
central and southern Andes 350–400 km wide (Isacks, 1988). Only the Tibetan
Plateau (Section 10.4.2) is higher and wider. The Alti-
The central Andes display two major mountain chains plano-Puna contains broad, internally drained areas of
called the Western and Eastern cordilleras (Fig. 10.1b). low relief and records little surface erosion. Its history
Where the subducting Nazca plate dips steeply, south of uplift began during the Miocene when plate conver-
of latitude 15°S, the Western Cordillera contains the gence rates were at their peak (Allmendinger et al.,
active volcanic arc. North of this latitude, where an 1997). An initial stage of uplift coincided with a major
active arc is absent, it is composed of Cenozoic extru- ignimbrite flare-up and a period of intense crustal short-

sive rock. Paleozoic metasedimentary rock interfolded ening that initially occurred in the Eastern and Western
with Mesozoic-Cenozoic volcanic and sedimentary cordilleras (Allmendinger & Gubbels, 1996) and later
sequences comprise the Eastern Cordillera. migrated eastward into the sub-Andean zone and the
South of about latitude 15°S, the Western and Chaco foreland basin (Section 10.3.2). This shortening
Eastern cordilleras diverge around a large composite resulted in very thick, hot continental crust beneath the