Home > Basin and Range

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 RidgeCourtesy USGS The Mid-Atlantic Ridge is an underwater mountain range of the Atlantic Ocean that runs from Iceland to Antarctica, and is the longest mountain range on Earth. The ridge was discovered by Bruce Heezen in the 1950s. The discovery of this ridge or in failed rifting areas such as the Western Rift of the Great Rift ValleySinai Peninsula is in center and the Dead Sea and Jordan River valley above The Great Rift Valley is a vast geographical and geological feature of east Africa and southwest Asia created by the rifting and separation of the African and Arabian tectonic pla in East AfricaEast Africa is a region generally considered to include the countries of Djibouti, Eritrea, Ethiopia, Kenya, Somalia, Tanzania, and Uganda. Madagascar, is also included. Some parts of East Africa have been renowned for their concentrations of wild animals. 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 shadowA rain shadow (or more accurately, precipitation shadow is a dry region on the surface of the Earth that is leeward or behind a mountain with respect to the prevailing wind direction. A rain shadow area is dry because, as moist air masses rise to top a mo effect resulting in exceedingly dry conditions in this province).

1 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 PlateThe Pacific Plate is an oceanic tectonic plate beneath the Pacific Ocean. To the north the easterly side is a divergent boundary with the Explorer Plate, the Juan de Fuca Plate and the Gorda Plate forming respectively the Explorer Ridge, the Juan de Fuca moving north relative to the North American PlateThe North American Plate is a continental tectonic plate covering the continent of North America and extending eastward to the Mid-Atlantic Ridge. The easterly side is a divergent boundary with the Eurasian Plate to the north and the African Plate to the (this is the same force behind the creation of the San Andreas FaultCalifornia The San Andreas Fault is a right-lateral strike-slip fault that spans a length of roughly 800 miles (1287 kilometers) through California. The San Andreas Fault marks a transform boundary between the Pacific Plate and the North American Plate.) 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.