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0
40
0 km
Puget
Trough
Coast
Range
Interior
Lowlands
Cascade
Range
Columbia
Plateau
New England
Klamath
Mts.
Black
Hills
Idaho
Batholith
Snake River
Plain
Basin
and
Range
Coast
Range
Appalachian
Highlands
Sierra
Nevada
Colorado
Plateau
Great
Valley
Ouachita
Mountains
Atlantic
Coastal Plain
San Andreas
Fault
Ozark and
Interior
Low Plateaus
Gulf Coastal Plain
Cenozoic basins
of Pacific Coast
Pliocene-Pleistocene
volcanics
Cenozoic volcanics
Mountains
Plains
Volcanoes of
Cascade Range
Lowlands
Mesozoic batholiths
Plateaus
◗
Figure 23.4
The North American Cordillera The Cordillera in western North America is a segment
of the circum-Pacifi c organic belt.
much further inland from a convergent plate boundary, and
neither volcanism nor emplacement of plutons was very
common. Second, deformation was mostly vertical, fault-
bounded uplifts rather than the compression-induced fold-
ing and thrust faulting typical of most orogenies.
To account for these differences, geologists have modi-
fi ed their model for orogenies at convergent plate boundaries.
During the preceeding Nevadan and Sevier orogenies, the
oceanic Farallon plate was subducted beneath North Amer-
ica at about a 50-degree angle, and volcanism and pluto-
nism took place 150 to 200 km inland from the oceanic
trench. Most geologists now agree that, by Early Paleogene
time, there was a change in the subduction angle from steep
to gentle, and the Farallon plate moved nearly horizontally
beneath the continent. However, they disagree on what may
have caused this change in angle of subduction.
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