Biology Reference
In-Depth Information
which contains plant macro- and microfossils. Thus, 40 Ma marked an im-
portant early period of uplift in the Central Andes resulting from a slowing
of convergence between the South American and Nazca plates from 15 to
5 cm a year, possibly in compensation for increased collision of India with
Eurasia. As the crust cooled and hardened along the subduction zone, fric-
tion increased and the highlands were uplifted.
The eroding sandstones and siltstones came from a low ridge in the for-
mer swampland, and by 30 Ma similar sediments were also being deposited
to the east that would form part of the Altiplano. The arc of volcanoes had
now coalesced into a continuous upland to constitute the proto-Cordillera
Occidental. The Altiplano was a river basin near sea level between this
mountain system and the mostly nonvolcanic Cordillera Oriental that was
rising to the east from compression against the subterranean extension
of the Brazilian Highlands. By 15 Ma, half the present altitudes had been
attained, and this was suffi cient to create arid conditions to the west, as
shown by sediments of that age in northern Chile. This was the beginning
of the dry zone along the coasts of Peru and Chile that would later become
the hyperarid Atacama Desert. As noted previously, the remaining altitude
of the Central Andes Mountains has been attained since 10-6 Ma.
The Juan Fernandez Islands lie offshore from the southern Central An-
des at 33°S. They were the setting for Daniel Defoe's novel,
Robinson Cru-
soe
, based on the life of Alexander Selkirk, who spent four years on one
of the islands, from September 1704 to February 1709, before being res-
cued. Selkirk was a buccaneer on board the
Cinque Ports
captained by the
famed pirate William Dampier, whose own life was chronicled by Diana
and Michael Preston in
A Pirate of Exquisite Mind
(2004). Selkirk left the
ship because he was convinced it was not seaworthy (it later sank). In a bit
of rare coincidence, he was rescued on 2 February 1709 by the ship
Duke
,
captained by the same William Dampier.
The Northern Andes
The northern section of the Andes Mountains extends from a megashear
zone called the Amotape Cross in southern Ecuador at the Golfo de Guaya-
quil northward to the Caribbean plate. It consists of two subprovinces or
blocks. The southern block, from Guayaquil to southern Colombia, was up-
lifted by subduction of the Nazca plate in the Late Cretaceous through the
Eocene. In Ecuador the Cordillera Occidental, the Central Valley (or Inter-
andean Depression), and the Cordillera Oriental were elevated at this time
(fi g. 2.35). In the Central Valley there is a rich assemblage of Pleistocene
