DEFINITION OF BASIN AND RANGE PROVINCE, UNITED STATES

The Basin and

Range Province of the United States extends from eastern California

to central Utah, and from southern Idaho into the state

of Sonora, Mexico. The region is arid, characterized by steepsided

linear mountain ranges separated by generally flat, alluvial

fan-bounded basins. The Earth’s crust in the entire region

has been extended by up to 100 percent of its original width,

thinning the crust and forming large extensional faults. The

faults are generally north-south trending, bound the uplifted

mountain ranges, and accommodate the intervening basin’s

down-dropping and subsidence, forming the distinctive alternating

pattern of linear mountain ranges and basins.

Although there are many faults of different ages in the

Basin and Range Province, the extension and crustal stretching

that shaped the modern landscape produced mainly normal

faults. On the upthrown sides of these faults are

enormous mountain ranges that characteristically rise abruptly

and steeply, whereas the down-dropped sides create distinct

low valleys. Most of the fault planes dip about 60°, and many

have throws or displacements of more than 2 miles (3 km).

As the mountains rise, they are subjected to weathering

and erosion, being attacked by water, ice, wind, and other

erosional agents. These physical and chemical weathering

processes are very important in the erosion of the mountains

and the formation of sediments in the intervening basins.

Extreme climate conditions, ranging from cold winters with

heavy snowfall to long hot summers with little or no rainfall,

enhance weathering processes, with frost and heat causing

large boulders to spall off of cliff faces and fall to the basin

floor, where they are further reduced by chemical processes.

The Basin and Range Province is very active tectonically.

The northern part of the province is an actively deforming

intercontinental plateau lying between stable blocks of the

Sierra Nevada and the Colorado Plateau. The Basin and Range

Province is experiencing rapid extension and active tectonics

along its predominant faults. The province has extended by a

factor of about two in the last 20 million years, and extension

continues, with ongoing seismic activity and slip along numerous

faults distributed across a zone 500 miles (800 km) wide.

The internal part of the province is generally free from such

deformation, with most occurring along the outer edges of the

province. Space geodetic measurements broadly define movement

across the province, whereas local surveys have mapped

concentrated deformation in several seismically active zones.

Determining the detailed pattern of active faulting is important

since it defines the current seismic hazard zones, with zones of

high-velocity gradients having more frequent damaging earthquakes

than regions of low gradient.

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