Basin and Range

The Basin and Range Province is a particular type of topography that covers much of the southwestern United States and northwestern Mexico that is typified by elongate north-south trending arid valleys bounded by mountain ranges which also bound adjacent valleys. Death Valley is a good example of a modified basin and range valley.

The basins are down-fallen blocks of crust and the ranges are up-thrust slabs (actually the arrangement is a bit tilted to the east - in profile this would look similar to an encyclopedia leaning to one side - like so ///). The normal arrangement in the basin and range system is that each valley (i.e. basin) is bounded on each side by a normal fault that runs parallel to the range.

This arrangement is very similar to the horsts and grabens seen in divergent plate boundaries such as the Mid-Atlantic Ridge or in failed rifting areas such as the Western Rift of the Great Rift Valley in East Africa. However the extent of the rifting in the Basin and Range is not concentrated into a single valley but is spread out over a very large area creating much smaller grabens laying roughly parallel to each other in a north-south direction (which leads to a rain shadow effect resulting in exceedingly dry conditions in this province).

Contents

1 Geography 2 Geology 3 References 4 Further reading

Geography

The province extends east from the Sierra Nevada all the way to the Colorado Plateau and extends south over northern parts of the Baja California peninsula. This covers parts of the U.S. states of Arizona, California, Idaho, New Mexico, Texas, and Utah and almost all of Nevada. Basin and Range topography also dominates large parts of the Mexican states of Sonora, Chihuahua, and Baja California. The arid Great Basin is part of this province as well as the Sonoran Desert and the Mexican Highlands.

Geology

This unique topography has formed as the result of extension (literally the Earth's crust is being pulled apart) that is thought to be caused by the effect of the Pacific Plate moving north relative to the North American Plate (this is the same force behind the creation of the San Andreas Fault) and by other forces (see below). The crust here has been stretched up to 100% of its original width. Along the roughly north-south-trending faults mountains were uplifted and valleys down-dropped, producing the distinctive alternating pattern of linear mountain ranges and valleys of the Basin and Range province.

Although there are other types of faults in the Basin and Range province, the extension and crustal stretching that have shaped the present landscape produce mostly normal faults. The upthrown side of these faults form mountains that rise abruptly and steeply, and the down-dropped side creates low valleys. The fault plane, along which the two sides of the fault move, extends deep in the crust, usually an angle of 60 degrees. In places, the relief or vertical difference between the two sides is as much as 10,000 feet (just over 3000 m).

Subsequent to the mountain building episode a large part of the mountain belt created in the Laramide orogeny and previous orogenies (the Sevier, and part of the Nevadan) went through a long period of extension that persists today.

As the rocky ranges rise, they are immediately subject to weathering and erosion. The exposed bedrock is attacked by water, ice, wind and other erosional agents. Rock particles are stripped away and wash down the mountain sides, often covering young faults until they rupture again. Sediment collects in the adjacent valleys, in some places burying the bedrock under thousands of feet of rock debris.

There are several hypotheses trying to explain how the continental crust of North America responded to the great deal of compaction it went through with the Laramide orogeny. There is at least some evidence to support all of these ideas but it is very possible that more than one is correct:

1. Movement of the Pacific Plate is stretching the North American Plate toward the West.
2. As the spreading center (divergent plate boundary) of the subducting Farallon Plate moved beneath the North American plate, it formed a "slab gap", which caused heat from the mantle plume feeding the spreading zone to thin out the continental crust above it and cause the spreading (see slab gap hypothesis).
3. After the Laramide orogeny, the crust under the Rockies got overthickened and the Great Basin spread out in response.
4. The continental root of the proto-Rockies was so deep that the bottom part broke off and was incorporated into the asthenosphere.

References

• Steven M. Stanley, Earth System History (W.H. Freeman and Company; 1999) pages 537, 540-543, 545 ISBN 0-7167-2882-6
• Plummer, McGeary, Carlson, Physical Geology, Eight Edition (McGraw-Hill: Boston, 1999) pages 321, 513, 514 ISBN 0-697-37404-1
• USGS - Geologic Provinces of the United States: Basin and Range Province (http://wrgis.wr.usgs.gov/docs/parks/province/basinrange.html) (adapted public domain text)

Further reading

• Basin and Range by John McPhee, Noonday Press, 1990. ISBN 0374516901ja:ベイズン・アンド・レンジ