Basin and Range Province

The basins are down-fallen blocks of crust and the ranges are up-thrusted 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 horst 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 an excellated rain shadow effect[?] resulting in very dry conditions in this province).

Death Valley is a good example of a modified basin and range valley.

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).

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 extention that persists today.

There are several hypotheses trying to explain how the continental crust[?] of North America responded to the great deal of campaction 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

Further reading

• Basin and Range by John McPhee, Noonday Press, 1990. ISBN 0374516901