Basin and Range Province

The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The physiography of the province is the result of tectonic extension that began around 17 million years ago in the early Miocene epoch. The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The physiography of the province is the result of tectonic extension that began around 17 million years ago in the early Miocene 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 the Great Basin. Major ranges include the Snake Range, the Panamint Range, the White Mountains, the Sandia Mountains, and the Tetons. The highest point fully within the province is White Mountain Peak in California, while the lowest point is the Badwater Basin in Death Valley at −282 feet (−86 m). The province's climate is arid, with numerous ecoregions. Most North American deserts are located within it. 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 marching toward Mexico.' The Basin and Range Province should not be confused with The Great Basin, which is a sub-section of the greater Basin and Range physiographic region defined by its unique hydrological characteristics (internal drainage). The Basin and Range Province includes much of western North America. In the United States, it is bordered on the west by the eastern fault scarp of the Sierra Nevada and spans over 500 miles (800 km) to its eastern border marked by the Wasatch Fault, the Colorado Plateau and the Rio Grande Rift. The province extends north to the Columbia Plateau and south as far as the Trans-Mexican Volcanic Belt in Mexico, though the southern boundaries of the Basin and Range are debated. In Mexico, the Basin and Range Province is dominated by and largely synonymous with the Mexican Plateau. Evidence suggests that the less-recognized southern portion of the province is bounded on the east by the Laramide Thrust Front of the Sierra Madre Oriental and on the west by the Gulf of California and Baja Peninsula with notably less faulting apparent in the Sierra Madre Occidental in the center of the southernmost Basin and Range Province. Common geographic features include numerous endorheic basins, ephemeral lakes, plateaus, and valleys alternating with mountains (as described below). The area is mostly arid and sparsely populated, although there are several major metropolitan areas, such as Las Vegas, Phoenix and Tucson. It is generally accepted that basin and range topography is the result of extension and thinning of the lithosphere, which is composed of crust and upper mantle. Extensional environments like the Basin and Range are characterized by listric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing a horst and graben 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 extended continental crust around the world. The crust in conjunction with the upper mantle comprises the lithosphere. The base of the lithosphere beneath the Basin and Range is estimated to be about 60 – 70 km. Opinions vary regarding the total extension of the region; however, the median estimate is about 100% total lateral extension. Total lateral displacement in the Basin and Range varies from 60 – 300 km since the onset of extension in the Early Miocene 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. The tectonic mechanisms responsible for lithospheric extension in the Basin and Range province are controversial, and several competing hypotheses attempt to explain it. Key events preceding Basin and Range extension in the western United States include a long period of compression due to the subduction of the Farallon 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 Neogene time and continues to the present. By Early Miocene time, much of the Farallon Plate had been consumed, and the seafloor spreading ridge that separated the Farallon Plate from the Pacific Plate (East Pacific Rise) approached North America. In the Middle Miocene, the East Pacific Rise was subducted beneath North America ending subduction along this part of the Pacific margin; however, the Farallon Plate continued to subduct into the mantle. The movement at this boundary divided the East Pacific Rise and spawned the San Andreas transform fault, generating an oblique strike-slip component. Today, the Pacific Plate moves north-westward relative to North America, a configuration which has given rise to increased shearing along the continental margin.