THE BASIN AND RANGE PROVINCE

= Subduction

The San Andreas fault exerts frictional drag against the

zone

western margin of North America. Figure 20–15 shows

how such a force stretches and fractures the brittle upper

East

Pacific

crust. Large blocks of crust have dropped as grabens

Rise

along the faults, leaving other blocks elevated as mountain ranges between the grabens.

For the past 30 million years, this faulting has cre-

(a)

ated the northeast–southwest-oriented mountain ranges and valleys of the Basin and Range (Fig. 20–16).

Miocene

25 my

Igneous activity accompanied the faulting. Many geolo-

= Spreading

gists who work in the Basin and Range believe that the

centers

stretching and faulting have at least doubled the width of

= Transform

the region. The tectonic forces associated with the San

faults

Andreas fault continue today, and Basin and Range faulting and magmatism are still active.

= Subduction

zone

THE COLORADO PLATEAU

A large block of western North America, known as the Colorado Plateau, remained strangely immune to the faulting and igneous activity, although it is surrounded on three sides by the Basin and Range (Fig. 20–9).

(b)

Perhaps because it has a thicker and stronger crust, the

Pliocene

3 my

= Spreading centers

= Transform faults

Cape Mendocino San Andreas

Fault zone

= Subduction

zone

San Andreas

western North America overran the East Pacific rise, beginning

Fault

30 million years ago. The fault has grown longer as more of California has hit the rise. The subduction zone, shown in light green, once extended the entire length of the coast, but it has become inactive in the region of the San Andreas fault.

Figure 20–15 Friction along the San Andreas fault stretches Nevada and nearby regions in a northwest–southeast direction (arrows), forming normal faults (red lines) in the Basin and Range.

Figure 20–16

A radar image of the United States with the Basin and Range outlined in red. Notice the parallel valleys and mountain ranges caused by normal faulting.

entire Colorado Plateau simply rotated clockwise in re- Later, one additional event sculpted the northern parts of sponse to the tectonic forces that created the Basin and

our continent.

Range. Then between 5 and 10 million years ago, the At least five major episodes of glaciation have oc- Colorado Plateau rose without much internal deforma-

curred in the Earth’s history. The most recent is the tion to become a high, nearly circular topographic fea- ture (Fig. 20–17). As the Colorado Plateau rose, the Colorado River cut the Grand Canyon into the rising

Figure 20–17 Sedimentary rocks of the Colorado Plateau bedrock.

in Grand Canyon have been uplifted but show little folding.

THE COLUMBIA PLATEAU The Columbia Plateau is one of the largest basalt plateaus

in the world (Fig. 20–9). It formed by rapid extrusion of flood basalt magma about 15 million years ago (Fig. 20–18). Volcanic activity then migrated eastward along the Snake River plain and occurred as recently as a few thousand years ago in Yellowstone National Park at the eastern end of the Snake River plain. Here the story may not be over, because active magma still underlies por- tions of the Park. The vast Snake River plain and Yellow- stone magmatic systems are described in Chapter 5.

THE BASIN AND RANGE PROVINCE