Photo taken near Red Rock Canyon, NV. Courtesey of Leah Cook.
The Basin and Range Province of the western United States exhibits characteristics of a rift zone in its infancy. Mantle upwelling has created a buoyant region of stretched and fractured crust that creates a landscape of faulted mountains and down-dropped valleys. If stretching continues, a true rift will form and plate separation will ensue.
|Figure 1 GIS representation of the Basin and Range Province. Areas highlighted in dark brown show elevations greater than 400m above ground level. Image processed by N. Scott using Visual Weather, 4/12.|
|Figure 2 Over-simplified schematic of normal fault-bearing mountain and valley structure. Dikes of intrusive magma may seep into faults and rise to the surface. Image created by N. Scott 4/12.|
The Basin and Range Province is a series of mountain-valley pairs that lie between the Sierra Nevada Mountain Range and the Colorado Plateau, extending to the south into Arizona and reaching as far north as southern Oregon (Thompson & Burke, 1974). This area is commonly referred to in the United States as the Great Basin. The topography consists of north-south oriented ranges that alternate with flat lying valleys. The area's unique geologic origin is what sets the Basin and Range Province apart from other mountainous regions in the western United States. While many mountain belts are formed via subduction volcanism or by uplift driven by converging plates, the Basin and Range Province cannot be attributed to either of these mechanisms. This region of high terrain has been produced by rather counterintuitive measures: by stretching instead of convergence.
The features of the Basin and Range Province are distinct from other global mountain ranges. The alternating terrain is accompanied by vast amounts of normal faulting and heavy sediment accumulation. Normal faults play a critical role in the formation of the Basin and Range’s topography; tectonic stretching causes the crust to give in to extensional stress and crack. On one side of the fault a slab of crust slips downward while the opposing slab is thrust up (see Figure 2), creating the mountain-valley pair (USGS, 2004). The resulting basins and ranges may be tilted in the process, which is evident in exposed stratified sediments (see Figure 3). While tilting may occur, this process creates mountains with steep rises, which contrast sharply with the flat valley floors (see Figure 5).
Over time the uplifted block of crust is eroded by wind and water, transporting sediment into the valleys where they accumulate. This results in deposits that may be thousands of feet thick and creates the sediment-filled valleys that are characteristic of the province (see Figures 3 & 4) (USGS, 2004). The region also displays many alluvial fans, sometimes called bajadas, that consist of poorly sorted sediments which have been transported from rivers originating in the mountains (Coulter, 2005). The climate in the Basin and Range Province is so arid that the amount of runoff water from the high terrain is not significant enough to transport sediment great distances. Alluvial fans are common in the area for this reason, as rivers evaporate and deposit their load soon after descending from the mountains. In fact, much of the Great Basin lacks external drainage channels. Small rivers form and pass through the flat valley floors, however water rarely makes it outside of the basin. The dry air quickly evaporates shallow lakes that accumulate from traversing streams (USGS, 2004).
Extension of the western United States between the Sierra Nevada and the Rocky Mountains is believed to have begun in the Cenozoic Era, approximately 20 million years ago (USGS, 2004; Thatcher et al., 1999; Thompson & Burke, 1974). Thatcher et al. (1999) and Gans & Bohrson (1998) agree that the area has expanded twofold over this time period, possibly in bursts of high activity. The coverage area of the province has presumably doubled and now covers more than 800,000 square kilometers of land (Coulter, 2005).
Tectonic stretching of this magnitude causes significant effects on the lithosphere, as well as on the mantle below. When the lithosphere stretches it relieves pressure on the underlying upper mantle (the asthenosphere), which decreases the melting point of the hot rocks to the point where partial melting begins to occur (Thatcher et al., 1999). A 2010 publication from Incorporated Research Institutions for Seismology (IRIS) describes how this creates regional upwelling of the mantle, which pushes the lithosphere upwards and may further stretch the crust by expanding it over the dome. Eventually fractures occur within the crust, and gravity pulls one block down along a normal fault (Figure 2). Thompson & Burke (1974) find this situation common in rift zones around the world, such as in Africa and in the Rhine region of Europe, but in these cases the mantle is upwelling to the point where it is beginning to break through the surface. While little active volcanism occurs in the Great Basin today, ongoing seismic activity in the region, thermal vents and evidence of previous volcanic activity denote the mantle’s close presence (IRIS, 2010).
|Figure 3 This range has been tilted slightly, seen in the darker strata in its mid-section. Sediment built up at its flanks can also be seen. Photo taken by Leah Cook 4/12|
Upwelling and the close proximity of the asthenosphere causes the lithosphere beneath the Basin and Range Province to be thinner than most other mountainous regions (Thompson & Burke, 1974). To gather information on the depth of the lithosphere, geologists use seismic wave methods to study crustal properties beneath the surface. Thatcher et al. (1999) used this method to determine the crustal thickness beneath the Basin and Range, which they found to be approximately 30 kilometers. This is widely inconsistent with other mountainous terrains, which are generally associated with crust that is 33 percent thicker (Thatcher et al., 1999). Observed high heat anomalies in the region also suggest a shallow lithosphere, as the upwelling mantle brings heat much closer to the surface than in surrounding areas. The anomalous heat values cease sharply where the Great Basin meets the Sierra Nevada mountain chain, which implies that this phenomena is exclusive in the United States to the Basin and Range Province (Thompson & Burke, 1974).
The intense amount of heat beneath the range not only creates a mechanism for normal faulting, but also contributes to the high elevation of the region (IRIS, 2010). The increased amount of heat beneath the crust in the Great Basin creates a buoyant plume on which the lithosphere rests. This lifts the North American plate between the Sierra Nevada and Rocky Mountains, causing elevations that average around 1400 meters above sea level (Coulter, 2005).
|Figure 4 Near vertical rises are typical of basin and range topography. Photo taken by Leah Cook 4/12 near Red Rock Canyon, NV.|
|Figure 5 A wider angle shows the steep inclines contrasting with the flat valley floor. Photo taken by Leah Cook 4/12.|
Movement in the Basin and Range Province can be attributed to two things: external plate movement and internal deformation (Thatcher et al., 1999). The Pacific and North American plates bound the province and exert pressures on it from its periphery. Divergence between the two plates is a significant player in deformation of the province due to tectonic stretching and the faulting that occurs to accommodate the stress (Thompson & Burke, 1974). The characteristics of the base of the lithosphere also have an impact on deformation; the base of the plate is weakened due to partial melting by the mantle below, and therefore is more susceptible to gravitational and buoyant forces. Thompson & Burke (1974) found that movement in the range is generally in an east-west direction, perpendicular to the normal faults, due to continued stretching and divergence of the North American and Pacific plates in that direction. Smaller movements within the region may be attributed to more local effects, i.e. interactions between down-dropped crustal slabs and their uplifted or stable neighbors (Thatcher et al., 1999).
Thatcher et al. (1999) studied subtle movements in the range by using GPS trackers or by taking space geodetic measurements. Velocity vectors calculated from these measurements helped determine the root cause of the movement and deformation at many locations. Sources of stress may be inferred by determining the direction of motion relative to bordering geological features that are considered stable, such as the Colorado Plateau. In some cases deformation is due largely to internal forces, including the stability of the base of the lithosphere, whereas others are driven by large-scale plate movements. Thatcher et al. (1999) found that stretching velocities in the Basin and Range Province typically fall in the range of 0.2 to 0.6 centimeters per year. In some cases areas under high extensional stresses, such as the Great Basin region of southeast California, movement may approach 5 centimeters per year (Coulter, 2005).
The Basin and Range Province of the southwest United States is actively stretching. If large-scale plate movements and mantle upwelling remain unchanged, normal faulting of much greater magnitude will occur and eventually lead to rifting of the North American Plate. In 20 million years the Great Basin has already been extended to two times its original width, but it is difficult to project what the breaking point will be. Regardless, the mantle lies just beneath the surface, and future volcanism and seismic activity in the Great Basin should be expected.
Coulter, P. Volcanoes of the Eastern Sierra Nevada: Geology and Natural Heritage of the Long Valley Caldera, updated 2005. The Basin and Range Province. http://geology.fullerton.edu/Jfoster/Paleoseismology%20575/Basin%20and%20Range%20Geology.htm [retrieved March 31, 2012].
Gans, P. B. and Bohrson, W. A. 1998. Supression of Volcanism During Rapid Extension in the Basin and Range Province, United States. Science 279, 66 p.
Incorporated Research Institutions of Seismology. Basin and Range Province, updated January 19, 2010. http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/15/BasinRange_Background.pdf [retrieved April 3, 2012].
Thatcher, W. et al. 1999. Present-Day Deformation Across the Basin and Range Procince, Western United States. Science 283, 1714 p.
Thompson, G. A., and Burke, D. B. 1974. Regional geophysics of the Basin and Range Province. Annual Reviews, p. 213-238.
USGS Geology in the Parks, updated January 12, 2004. Geologic Provinces of the United States: Basin and Range. http://geomaps.wr.usgs.gov/parks/province/basinrange.html [retrieved April 1, 2012].