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