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about 2 km, which triggered orographic rainfall from the South American low-level jet
stream.
Garzione et al. (
2008
) used a combination of oxygen and deuterium isotopic ana-
lyses to determine uplift along a line from the Western Cordillera across the Central
Andean plateau to the Eastern Cordillera. They concluded that the elevation of the
Altiplano was less than 2 km until 10 Ma, while the Eastern and Western Cordilleras
were only 2.5 to 3.5 km high until that time. Relatively rapid uplift took place in
the late Miocene after 9 Ma. This inference is consistent with the rapid increase in
sediment accumulation in the Andean foothills between 7.9 and 6 Ma, which Uba
et al. (
2007
) attributed to monsoon intensification and greater climatic variability in
the late Miocene. In this context, we need to bear in mind that while Andean uplift
could have altered climate, climate may also have influenced the morphology of the
Andes (Montgomery et al.,
2001
). Hoke et al. (
2004
) found that along the western
slopes of the Altiplano plateau, a relict pattern of trellised drainage had been cut by
deeply incised canyons, or
quebrados
, some of which were in existence during the
late Miocene and early Pliocene. These canyons were formed by groundwater sapping
processes associated with the drying out of the climate and uplift of the plateau, which
led to a steepened hydraulic gradient.
Let us now consider the desiccation record. Alpers and Brimhall (
1988
), Clark
et al. (
1990
) and Sillitoe and McKee (
1996
) concluded that the supergene oxidation
and enrichment of the Chilean porphyry copper deposits required a climate that was
less arid than the present hyper-arid climate of northern Chile and southern Peru, with
Clark et al. (
1990
) suggesting more than 100 mm of precipitation. All of these authors
inferred that progressive desiccation had taken place since the supergene enrichment
ceased, with somewhat different ages of 35-14 and 15-9 Ma suggested for the timing
of this regional desiccation. Dunai et al. (
2005
) measured the
21
Ne in clasts collected
from surfaces in the Atacama that were deemed sensitive to potential erosion from
surface run-off. The surface exposure ages of the sediments showed minimal evidence
of any erosion in the last 25 Ma, indicating sustained aridity of the Atacama from late
Oligocene-early Miocene times onwards.
Rech et al. (
2006
) investigated a series of fossil soils in the central Andes in order to
determine the timing of uplift and of climatic desiccation. A series of calcic vertisols
with gleyed horizons and calcium carbonate root traces were overlain by gypseous
soils with pedogenic nitrate at elevations between 2,900 m and 3,400 m in the Calama
Basin on the eastern edge of the Atacama. Calcic vertisols with gleyed horizons
develop in poorly drained, vegetated flood-plains under a seasonal rainfall regime
(see
Chapter 15
). Rech et al. (
2006
) suggested that these soils had formed under
a mean rainfall of
200 mm/year. The transition to salic gypsisols with pedogenic
nitrate took place between 19 and 13 Ma. Such soils form today at elevations below
about 2,500 m, where the rainfall does not exceed 5-10 mm/year, but the fossil soils
now occur at elevations 400-900 m higher than this, indicating that amount of uplift in
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