TheBasin and Range Province is a vastphysiographic region covering much of the inlandWestern United States andnorthwestern Mexico. It is defined by uniquebasin and range topography, characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. Thephysiography of the province is the result oftectonic extension that began around 17 million years ago in the earlyMiocene epoch.
The numerous ranges within the province in the United States are collectively referred to as the "Great Basin Ranges", although many are not actually in theGreat Basin. Major ranges include theSnake Range, thePanamint Range, theWhite Mountains, and theSandia Mountains. The highest point fully within the province isWhite Mountain Peak inCalifornia, while the lowest point is theBadwater Basin inDeath Valley at −282 feet (−86 m).[1] The province's climate is arid, with numerousecoregions. MostNorth American deserts are located within it.
The Basin and Range Province should not be confused with theGreat Basin, a region defined by its unique hydrological characteristics (internal drainage) that overlaps much of the greater Basin and Range physiographic region. Nor should it be confused with theBasin and Range National Monument, located in Southern Nevada, which is one small part of the much larger province.
The Basin and Range Province includes much of westernNorth America. In the United States, it is bordered on the west by the easternfault scarp of theSierra Nevada and spans over 500 miles (800 km) to its eastern border marked by theWasatch Fault, theColorado Plateau and theRio Grande Rift. The province extends north to theColumbia Plateau and south as far as theTrans-Mexican Volcanic Belt inMexico, though the southern boundaries of the Basin and Range are debated.[2] In Mexico, the Basin and Range Province is dominated by and largely synonymous with theMexican Plateau.
Evidence suggests that the less-recognized southern portion of the province is bounded on the east by theLaramide Thrust Front of theSierra Madre Oriental and on the west by theGulf of California andBaja Peninsula with notably less faulting apparent in theSierra Madre Occidental in the center of the southernmost Basin and Range Province.[3]
Common geographic features include numerousendorheic basins, ephemeral lakes, plateaus, andbolson valleys alternating with mountains (as described below). The area is mostly arid and sparsely populated, although there are several major metropolitan areas, such asReno,Las Vegas,Salt Lake City,Phoenix,Tucson,El Paso—Ciudad Juárez,Mexicali, andHermosillo.
It is generally accepted that basin and range topography is the result of extension and thinning of thelithosphere, which is composed ofcrust andupper mantle. Extensional environments like the Basin and Range are characterized bylistric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing ahorst andgraben geometry, where horst refers to the upthrown fault block and graben to the down dropped fault block.
The average crustal thickness of the Basin and Range Province is approximately 30–35 km and is comparable to extendedcontinental crust around the world.[4] The crust in conjunction with the upper mantle comprises thelithosphere. The base of the lithosphere beneath the Basin and Range is estimated to be about 60–70 km.[5] Opinions vary regarding the total extension of the region; however, the median estimate is about 100% total lateral extension.[6] Total lateral displacement in the Basin and Range varies from 60 to 300 km since the onset of extension in the EarlyMiocene with the southern portion of the province representing a greater degree of displacement than the north. Evidence exists to suggest that extension initially began in the southern Basin and Range and propagated north over time.[7]
Clarence Dutton famously compared the many narrow parallel mountain ranges that distinguish the unique topography of the Basin and Range to an "army of caterpillars crawling northward."[8]
The tectonic mechanisms responsible for lithospheric extension in the Basin and Range province are controversial, and several competing hypotheses attempt to explain them. Key events preceding Basin and Range extension in the western United States include a long period of compression due to thesubduction of theFarallon plate under the west coast of the North American continental plate which stimulated the thickening of the crust. Most of the pertinent tectonic plate movement associated with the province occurred in theNeogene period (23.03-2.58 million years ago) and continues to the present. By the EarlyMiocene sub-epoch (23.03-15.97 million years ago), much of the Farallon plate had been consumed, and theseafloor spreading ridge that separated the Farallon plate from thePacific plate (Pacific-Farallon Ridge) approached North America.[9] In the MiddleMiocene (15.97-11.63 million years ago), the Pacific-Farallon Ridge was subducted beneath North America endingsubduction along this part of the Pacific margin; however, the Farallon plate continued to subduct into themantle.[9] The movement at this boundary divided the Pacific-Farallon Ridge and spawned theSan Andreastransform fault, generating an obliquestrike-slip component.[10] Today, the Pacific plate moves north-westward relative to North America, a configuration which has given rise to increased shearing along thecontinental margin.[9]
The tectonic activity responsible for the extension in the Basin and Range is a complex and controversial issue among the geoscience community. The most accepted hypothesis suggests that crustalshearing associated with theSan Andreas Fault caused spontaneous extensional faulting similar to that seen in the Great Basin.[11] However, plate movement alone does not account for the high elevation of the Basin and Range region.[11] The western United States is a region of highheat flow which lowers the density of the lithosphere and stimulatesisostaticuplift as a consequence.[12] Lithospheric regions characterized by elevated heat flow are weak and extensional deformation can occur over a broad region. Basin and Range extension is therefore thought to be unrelated to the kind of extension produced bymantle upwelling which may cause narrow rift zones, such as those of theAfar triple junction.[13] Geologic processes that elevate heat flow are varied, however some researchers suggest that heat generated at a subduction zone is transferred to the overriding plate as subduction proceeds. Fluids along fault zones then transfer heat vertically through the crust.[14] This model has led to increasing interest ingeothermal systems in the Basin and Range, and requires consideration of the continued influence of the fully subducted Farallon plate in the extension responsible for the Basin and Range Province.
In some localities in the Basin and Range, metamorphic basement is visible at the surface. Some of these aremetamorphic core complexes (MCC), an idea that was first developed based on studies in this province. A metamorphic core complex occurs when lower crust is brought to the surface as a result of extension. MCCs in the Basin and Range were not interpreted as being related to crustal extension until after the 1960s. Since then, similar deformational patterns have been identified in MCCs in the Basin and Range and has led geologists to examine them as a group of related geologic features formed by crustal extension during theCenozoic era (66.0 million years ago to present). The study of metamorphic core complexes has provided valuable insight into the extensional processes driving Basin and Range formation.[15]
Prior to the Eocene Epoch (55.8 ±0.2 to 33.9 ±0.1 Ma) the convergence rate of the Farallon and North American plates was fast, the angle of subduction was shallow, and the slab width was huge. During theEocene theFarallon platesubduction-associated compressive forces of theLaramide,Sevier andNevada orogenies ended, plate interactions changed from orthogonal compression tooblique strike-slip, and volcanism in the Basin and Range Province flared up (Mid-Tertiary ignimbrite flare-up). It is suggested that this plate continued to be underthrust until about 19 Ma, at which time it was completely consumed and volcanic activity ceased, in part.Olivine basalt from theoceanic ridge erupted around 17 Ma andextension began.[16][17][18][19]
In addition to small amounts ofNevadapetroleum, the Basin and Range Province supplies nearly all thecopper and most of thegold,silver, andbarite mined in the United States.[citation needed]
Media related toBasin and Range Province at Wikimedia CommonsAuthority control databases: National |
|---|
