Biology Reference
In-Depth Information
and they are being pushed from the east by spreading of the Mid-Atlantic
Ridge. The crustal shortening of the Cordillera Oriental was at least 100 km
between 30 and 20 Ma, and at least another 100 km from 10 Ma to the
present. The eastern margin is now riding up and over the subterranean
western edge of the shield, producing the distinctive ridge and valley topo-
graphy of the Yungas.
Another key feature of the Andes Mountains is their westward bend (the
“elbow of the Andes,” or the Bolivian Orocline) near Santa Cruz. This is a
consequence of two geologic processes operating in the Central Andes. One
is the lithosphere thinning already noted, which produces a more heated
fl exible crust there than along segments to the north and south. Subduction
since 10 Ma has forced this pliable part of the Andes to the east, while the
more rigid segments on either side have remained relatively stationary, or
have been pushed westward, producing the distinctive curvature. In addi-
tion to being a signature feature of the Andes Mountains, the orocline af-
fects the biology of the adjacent Amazon Basin. As winds blowing from the
northeast to the southwest across the lowland basin rise along the slopes
of the Andes Mountains, they lose moisture and create zones of high rain-
fall. The orocline provides a slope that faces more directly at right angles
to these winds, and the region below receives an even greater amount of
rain. It may have been a refugium for rain forest during cold dry periods of
the Quaternary (chap. 9). Farther north the east-facing concave confi gura-
tion of the Andes between about 15°S to 5°N has created another regional
concentration of moisture, augmenting precipitation from winds off the
Amazon Basin. (In addition to the 1997
Oxford Atlas of the World
, 150, good
regional maps can be found online at the Google Earth and Worldmapper
sites.)
Timing and sequence of events in the Central Andes
At about 100 Ma, the western coast of central South America was swamp-
land and lakes without bordering highlands, as shown by the horizontal bed-
ding of the limestone strata and the absence of eroded terrestrial sediments.
Preserved in the limestone is one of the most extensive displays of dinosaur
footprints in the world. To the west in the Pacifi c Ocean there was an arc
of volcanoes. Beginning at 40 Ma, eroded sandstone and siltstone covered
the limestone, indicating the early presence of highlands. These highlands
are slightly younger than those to the south (Southern Andes) and older
than those to the north (Northern Andes). In the latest Eocene and early
Oligocene, sedimentation increased and deposited the Potoco Formation,
