7 BUILDING OF THE WESTERN MOUNTAINS
䊳 20.7 BUILDING OF THE WESTERN MOUNTAINS
The Cordilleran mountain chain is the long, broad mountain chain of western North America. It reaches from the western edge of the Great Plains to the west coast, and from Alaska to Mexico (Fig. 20–9). The name Cordillera is taken from the Spanish word for “chain of mountains.” It includes the Rocky Mountains, the Coast Ranges, the Sierra Nevada, and all other mountains and intermountain regions in the western part of our conti- nent. The Cordillera formed as Pangea III rifted apart.
As the Atlantic Ocean opened, the lithospheric plate carrying North America began moving westward. To ac- commodate this new movement, tectonic plates of the Pacific Ocean sank beneath the western edge of the con- tinent, creating a subduction zone and an Andean-type continental margin. From about 180 to 80 million years ago, great granite batholiths rose into the crust over this subduction zone (Fig. 20–10). Later, tectonic forces raised the batholiths and erosion exposed the rocks, cre- ating the beautiful granite mountains of the Sierra Nevada and many other parts of the Cordillera.
ACCRETED TERRAINS The land that was to become western Alaska, the Yukon,
British Columbia, Washington, Oregon, western Idaho, and western California started to join the continent shortly after Pangea III broke up. At that time, numerous island arcs and microcontinents dotted the Pacific Ocean—much like the southwestern Pacific today. As
Building of the Western Mountains 361
Figure 20–7 Flat-lying platform sedimentary rocks are exposed along the banks of the Iowa River near Bluffton, Iowa. (D. Cavagnaro/ Visuals Unlimited)
362 CHAPTER 20 T H E G E O L O G I C A L E VO L U T I O N O F N O RT H A M E R I C A
143.8 million years ago (beginning of Cretaceous)
(a)
84 million years ago (late Cretaceous)
(b) Figure 20–8 Pangea began to break up about 180 million years ago. The rifting followed
the lines of the earlier collision zone that had formed the Appalachians. However, bits and pieces of Africa or Eurasia remained stuck to North America, and fragments of North America went off with Africa and Eurasia. (a, b) Positions of continents at two times from early Cretaceous through early Oligocene. (From Bally et al., The Geology of North America—An Overview, GSA, 1989)
the tectonic plates of the Pacific sank beneath western they were too buoyant to sink into the mantle, they col- North America, these island arcs and microcontinents
lided with the western margin of the continent and arrived one by one at the subduction zone. Because
became parts of the continent. This process is called
Building of the Western Mountains 363
the craton are recognized. Geologic relationships within
Brooks Range
each terrain are continuous, but relations across terrain boundaries are discontinuous. The terrain boundaries are
Yu k
on
intensely deformed as a result of the collisions, and ra-
Alas
P la
diometric ages change abruptly across the boundaries.
ka
The terrains originated in many different parts of the
an g
e Pacific Ocean.
Eastern ranges
The subduction that brought the accreted terrains to
(Rocky Mts.)
western North America also continued to form granitic magma that rose into the continental margin. As a result
Interior ranges
of this process, many of the accreted terrains contain granite batholiths and related volcanic rocks that formed
Interior
C as the terrains docked.
plateaus
Some of the collisions between the accreted terrains
Western
a Tintina
and the continent were not direct head-on collisions.
ranges
Trench
Instead, the Pacific plates carrying some terrains were
Deformed
moving northward while the continent was moving west.
Cenozoic of
Rocky
As a result, many of the accreted terrains were smeared
Pacific margin
Mountain
Trench
northward for several hundred kilometers along huge strike–slip faults as they docked.
Columbia
FOLDING AND THRUST FAULTING
Plateau
IN THE CORDILLERA
a Northern Rockies
e g d va As terrains crashed into the continent, they created com-
pressive forces like those of a continent–continent colli-
sion. The resulting zone of folded and thrust-faulted
e i o Range a
rocks is called the Cordilleran fold and thrust belt (Fig. 20–11). It is only a few hundred kilometers wide
Colorado
but extends north–south for the entire length of the Cordillera (Fig. 20–12).
Plateau