THE TECTONIC FORCES CHANGE

ie r

Baja

The western margin of the continent was compressed

California

while convergence of the two tectonic plates and dock-

Sierra

a d ing of the accreted terrains continued at high speed.

re Occidental O

Madre

However, the rate of convergence suddenly slowed about

rie

45 million years ago for an unknown reason. Then the

n ta

compressive forces weakened and the warm, thick crust of the Cordillera began to spread out like a mound of honey on a tabletop.

Figure 20–9 The Cordilleran mountain chain includes sev- Only the deeper, hotter part of the crust could spread eral subdivisions forming the vast mountainous region of west-

plastically, however. The upper crust was cold and brit- ern North America.

tle. It fractured and faulted as the spreading deeper rocks pulled it apart. The brittle upper rocks can be likened to

a layer of frosting coating the mound of honey. As the docking, because it is similar to a ship tying up along-

honey flows outward, the frosting breaks into segments, side a dock.

which separate as spreading continues. In this way, North America grew westward from

As the shallow rocks rifted and faulted in this man- about 180 to 80 million years ago. Geologists have iden-

ner, a second magmatic episode began about 45 million tified about 40 accreted island arcs and microcontinents,

years ago, and it continues today. Plutonic and volcanic called accreted terrains, in the Cordillera. Accreted ter-

rocks formed during the last 45 million years are abun- rains are identified in the same way as the provinces of

dant in western North America (Fig. 20–13).

364 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

Granitic intrusions Metamorphic rocks

(a)

(b)

Figure 20–10 (a) Granitic batholiths and related metamorphic rocks of western North America formed as a result of subduction along the western margin of the continent, beginning about 180 million years ago. (b) The granite of El Capitan in Yosemite Valley is part of the Sierra Nevada batholith, which formed in this way.

THE TECTONIC FORCES CHANGE AGAIN: subduction stopped where California touched the Pacific THE SAN ANDREAS FAULT

plate.

As North America moved westward, the west coast drew But the Pacific plate was moving in a slightly dif- closer to a portion of the mid-oceanic ridge called the

ferent direction from the continent. A new fault devel- East Pacific rise (Fig. 20–14). By 30 million years ago,

oped to accommodate the small difference in direction. southern California had arrived at the East Pacific rise.

The Pacific plate began sliding northwestward against At the time, the Pacific plate was moving northwest—

the California coast along the new strike-slip (transform) nearly parallel to the westward movement of North

fault (Fig. 20–14b). The new fault was the beginning of America. Since these two plates were moving in nearly

the San Andreas fault. As North America continued to the same direction, they were not converging; therefore,

move westward and reached greater lengths of the East

Building of the Western Mountains 365

Figure 20–11 Folded limestone in the Lizard Range, British Columbia.

ille ra n

Figure 20–12 The Cordilleran fold and thrust belt is a zone of thrust faults and folded rocks that extends for the entire length of the Cordilleran chain.

366 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

ille ra n

Igneous rocks younger than

45 million years

(a)

(b)

Figure 20–13 (a) Volcanic and plutonic rocks in the United States formed from 45 million years ago through the present. They are shown as if none had eroded away. (b) Lizard Head Peak is a volcanic plug in the San Juan Mountains, Colorado.

Pacific rise, the San Andreas fault grew longer and mi- grated inland (Fig. 20–14c).

THE MODERN CASCADE VOLCANOES Figure 20–14c shows that the San Andreas fault now

veers westward in northern California, where it runs out into the Pacific Ocean to connect with the remnants of the East Pacific rise spreading center. This spreading center extends along the coasts of northern California, Oregon, Washington, and southwestern British Columbia. The Juan de Fuca oceanic plate, east of the spreading center, is sinking beneath the westward-moving continent. The subduction creates an active volcanic zone

Building of the Western Mountains 367

Eocene

60 my

from northern California to southern British Columbia. Several of the volcanoes, including Mounts Lassen,

= Spreading

Shasta, Rainier, St. Helens, and Baker, have erupted re-

centers

cently, some within the past 100 years.

= Transform faults

THE TECTONIC FORCES CHANGE