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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