Geoscience Reference
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dating techniques such as magnetostratigraphy, fission track, zircon uranium-lead and
potassium-argon dating (van der Hammen and Hooghiemstra,
1995
; Uba et al.,
2007
).
Given the many different approaches used and the often unproven assumptions on
which some of them are based, it is no surprise that a detailed consensus on the uplift
history of the Andes is yet to be achieved, although the broad outlines now seem
reasonably clear. Because aridity in the Atacama is in part related to its location in
the western rain shadow of the Andes, some workers have used the history of aridity
in the Atacama as an indirect measure of Andean uplift, while others have relied on
more direct evidence of aridity (Dunai et al.,
2005
). We will now consider the results
of some of these studies.
Hartley et al. (
2005
) postulated that the present-day location of the Atacama Desert
within the dry subtropical belt was the dominant cause of its aridity. They went on
to argue that because there had been very little latitudinal displacement of this region
since the late Jurassic 150 Ma ago, aridity must have prevailed in the Atacama for
the past 150 million years. A contributing factor was the presence offshore of a cold
upwelling current, from at least the early Cenozoic onwards. They concluded that the
Atacama was the oldest desert in existence. Houston and Hartley (
2003
) considered
the more specific question of when the central Atacama became hyper-arid. They
examined the relationship between elevation and precipitation on the eastern and
western slopes of the Andes and concluded that hyper-aridity developed progressively
with uplift of the Andes, especially after the Andes attained elevations of 1-2 km,
which led to a significant rain-shadow effect. In addition, intensification of the cold
Peruvian/Humboldt Current between 15 and 10 Ma ago would have led to enhanced
desiccation inland between latitudes 30
S.
Miocene tectonic uplift in the north-east Andes caused the diversion of drainage
directions from north to east, with the Amazon and Orinoco flowing into the Atlantic
by the late Miocene (Hoorn et al.,
1995
). In the Ecuadorian Andes, a major plana-
tion surface graded to sea level developed at the end of the Lower Pliocene and was
later uplifted to an elevation of 3,500-4,000 m, becoming deeply incised during the
Middle and Late Pleistocene (Coltorti and Ollier,
2000
). Using the clumped isotopic
values in fossil soil carbonates (see
Chapter 7
), Ghosh et al. (
2006b
) estimated that
the Bolivian Altiplano had risen at a mean rate of 1.03
°
S and 15
°
0.12 mm/year between
about 10.3 and about 6.7 Ma. They suggested that uplift of the Altiplano amounted
to 3,700
±
400 m in that time. Both the rate and the amount are probably overes-
timates, given that the isotopic lapse rate (i.e., the change in the
±
18
O content of
precipitation with increasing elevation) would have been lower before the inception
of convective rainfall associated with the uplift (Poulsen et al.,
2010
). These latter
authors used more realistic precipitation isotopic lapse rates for their analysis of
sedimentary carbonate from the Bolivian Altiplano and concluded that the late Mio-
cene
18
O depletion that they identified in the carbonate record indicated the onset
and intensification of convective rainfall once the plateau had attained an elevation of
