Geoscience Reference
In-Depth Information
of the Andes and is flanked offshore by a cold ocean current that further accentuates
its aridity. Finally, it lies several thousand kilometres inland with respect to moist air
masses from the north-east, which bring heavy rainfall to the eastern flanks of the
northern Andes and to the lowlands of Amazonia.
21.4 Cenozoic tectonism, desiccation and cooling
The inception of aridity in the Atacama, Patagonia and the Bolivian Altiplano has
its roots in the Mesozoic and Cenozoic tectonic history of South America. South
America was once part of the supercontinent of Pangea, or Gondwana, which was in
existence for several hundred million years before the initial Jurassic break-up some
180 Ma ago. Gondwana split up in several stages, with the eastern portion (comprising
South America and Africa) separating some 180 Ma ago from the western portion
(comprising Antarctica, Australia and Greater India) to form the Indian Ocean (Smith
et al.,
1981
; Owen,
1983
; Kearey and Vine,
1996
; Williams et al.,
1998
). South
America and Africa began to move apart in the Early Cretaceous from about 125 Ma
onwards, giving rise to the South Atlantic Ocean (
Chapter 3
;
Figure 3.2
). Movement of
the Nazca tectonic plate eastwards away from the East Pacific Rise and its subduction
beneath the South American Plate (
Chapter 3
;
Figure 3.1
) resulted in uplift of the
Andes and creation of the deep Peru-Chile trough along the west coast of South
America. The existing pattern of ocean circulation around South America was largely
established by the end of the Cretaceous some 65 Ma ago. The former flow of
the warm equatorial current between North and South America from the Atlantic
to the Pacific ended with closure of the Panama Isthmus around 3.5 Ma ago. The
Antarctic circumpolar current came into being as a result of the opening of the Drake
Passage between Antarctica and the southern tip of South America by around 25 Ma.
Unlike Australia and Greater India, which moved rapidly away from Antarctica,
South America moved only very slowly northwards, amounting to about ten degrees
of latitude. As a result, South America has been within the global circulation system
of the Hadley Cell in the north and the westerlies in the south for at least 90 Ma
(Houston and Hartley,
2003
).
The uplift history of the Andes is complex and was not uniform from north to south.
Uplift took place in a series of discrete stages separated by intervals of tectonic stability
and sustained erosion (Garzione et al.,
2008
). A variety of different methods have been
used to determine the degree of uplift and when such uplift occurred. These methods
include geomorphology (Coltorti and Ollier,
2000
; Montgomery et al.,
2001
;Hoke
et al.,
2004
), sediment analysis (Hoorn et al.,
1995
; Hartley et al.,
2005
; Uba et al.,
2007
), pollen analysis (Helmens and van der Hammen,
1994
; Hooghiemstra and Ran,
1994
; Hooghiemstra,
1995
; Markgraf et al.,
1995
; Wijninga et al.,
2003
), analysis of
fossil soils (Rech et al.,
2006
), stable isotope geochemistry (Ghosh et al.,
2006b
;Gar-
zione et al.,
2008
; Poulsen et al.,
2010
) and thermochronology, backed up by traditional
