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26-555: This area is part of the basin and range geologic region, characterized by linear mountain ranges which are separated by broad, flat basins. The region was impacted by relatively recent volcanic activity, leaving volcanic plugs and cinder cones visible throughout the San Bernardino Valley (Arizona) . Earthquakes have further altered the region and helped allow the flow of many springs and seeps. All of these dynamic geological events have played major roles in shaping

52-638: A half-graben geometry. Other common geometries include metamorphic core complexes and tilted blocks . Examples of active continental rifts are the Baikal Rift Zone and the East African Rift . Divergent plate boundaries are zones of active extension as the crust newly formed at the mid-ocean ridge system becomes involved in the opening process. Zones of thickened crust, such as those formed during continent-continent collision tend to spread laterally; this spreading occurs even when

78-419: A dip to remain active and a new set of faults may be generated. Large displacements may juxtapose syntectonic sediments against metamorphic rocks of the mid to lower crust and such structures are called detachment faults . In some cases the detachments are folded such that the metamorphic rocks are exposed within antiformal closures and these are known as metamorphic core complexes . Passive margins above

104-407: A huge diversity of fish and wildlife, but also a center of human activity for centuries. With expanding settlement beginning in the late 19th century came farming , mining , and livestock production, all of which competed for the same water. While the extensive wetlands here once provided historic habitat for eight different kinds of native fish, the lowering water table led to severe changes in

130-503: A migration corridor for the jaguar . It will likely stop the migration of jaguars and other animals between Mexico and the US. From late 2019 onwards, massive amounts of water have been pumped from the aquifer for the wall's construction, causing likely permanent damage to the natural water flow in the area. Declassified emails released in mid 2020, dating back to the beginning of the wall's construction in fall 2019, found that water extraction for

156-400: A series of normal faults which occur in groups, form in close proximity and dipping in opposite directions. As the crust extends it fractures in series of fault planes , some blocks sink down due to gravity, creating long linear valleys or basins also known as grabens , while the blocks remaining up or uplifted produce mountains or ranges, also known as horsts . Fault scarps are exposed on

182-413: A series of long parallel normal faults . This results in block faulting , where the blocks of rock between the normal faults either subside , uplift , or tilt. The movement of these blocks results in the alternating valleys and mountains. As the crust thins, it also allows heat from the mantle to more easily melt rock and form magma, resulting in increased volcanic activity. With crustal extension ,

208-805: A weak layer develop a specific set of extensional structures. Large listric regional faults dipping towards the ocean develop with rollover anticlines and related crestal collapse grabens . On some margins, such as the Niger Delta , large counter-regional faults are observed, dipping back towards the continent, forming large grabenal mini-basins with antithetic regional faults. Areas of extensional tectonics are typically associated with: Rifts are linear zones of localized crustal extension. They range in width from somewhat less than 100 km up to several hundred km, consisting of one or more normal faults and related fault blocks. In individual rift segments, one polarity (i.e. dip direction) normally dominates, giving

234-476: Is offset along strike such as to create a gap e.g. a left-stepping bend on a sinistral fault, a zone of extension or transtension is generated. Such bends are known as releasing bends or extensional stepovers and often form pull-apart basins or rhombochasms . Examples of active pull-apart basins include the Dead Sea , formed at a left-stepping offset of the sinistral sense Dead Sea Transform system, and

260-461: Is the most well known example of basin and range topography. Clarence Dutton compared the many narrow parallel mountain ranges that distinguish the unique topography of the Basin and Range to an "army of caterpillars crawling northward." The physiography of the province is the result of tectonic extension that began around 17 million years ago in the early Miocene epoch. Opinions vary regarding

286-659: The Hellenic Subduction Zone to the south, causing extensive normal faulting and the formation of horsts and grabens on the seafloor. Many of the islands are the result of peaks reaching above sea level. One of the most studied basin and range topographies is the Basin and Range Province in the western United States, located between the Sierra Nevada and the Rocky Mountains . The extension of

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312-490: The Sea of Marmara , formed at a right-stepping offset on the dextral sense North Anatolian Fault system. Back-arc basins form behind many subduction zones due to the effects of oceanic trench roll-back which leads to a zone of extension parallel to the island arc . A passive margin built out over a weaker layer, such as an overpressured mudstone or salt , tends to spread laterally under its own weight. The inboard part of

338-522: The Sierra Nevada microplate . Though the Aegean Sea Plate is more difficult to study because it is underwater, efforts have been made to conduct GPS surveys of the seafloor and surrounding area. Some studies show regions of extension within the plate, while others suggest a four-microplate model to represent the motion. The plate's deformation is thought to be a result of crustal collapse (beginning c. 14 Ma) combined with slab rollback on

364-497: The United States Fish and Wildlife Service . Basin and range Basin and range topography is characterized by alternating parallel mountain ranges and valleys. It is a result of crustal extension due to mantle upwelling , gravitational collapse, crustal thickening, or relaxation of confining stresses. The extension results in the thinning and deformation of the upper crust, causing it to fracture and create

390-452: The beta factor , where t 0 is the initial crustal thickness and t 1 is the final crustal thickness. It is also the equivalent of the strain parameter stretch . In areas of relatively low crustal stretching, the dominant structures are high to moderate angle normal faults, with associated half grabens and tilted fault blocks . In areas of high crustal stretching, individual extensional faults may become rotated to too low

416-488: The Hellenic Subduction Zone. Extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body 's crust or lithosphere . The types of structure and the geometries formed depend on the amount of stretching involved. Stretching is generally measured using the parameter β , known as

442-651: The United States and are protected by the Endangered Species Act . Other rare, range-limited species known from the refuge include the Cochise pincushion cactus ( Escobaria robbinsiorum ) Huachuca water umbel ( Lilaeopsis schaffneriana ), San Bernardino springsnail ( Pyrgulopsis bernandina ), and Chiricahua leopard frog ( Lithobates chiricahuensis ). Construction of the Trump wall cut through

468-527: The collisional event is still in progress. After the collision has finished the zone of thickened crust generally undergoes gravitational collapse , often with the formation of very large extensional faults. Large-scale Devonian extension, for example, followed immediately after the end of the Caledonian orogeny particularly in East Greenland and western Norway . When a strike-slip fault

494-523: The habitat and the eventual local extinctions of many species. The San Bernardino National Wildlife Refuge was established in 1982 as part of a novel approach of rescuing these species; manmade artesian wells connected to the aquifer that could host the endangered fish being threatened by the draining of their habitats. The refuge protects several endangered species of fish endemic to the Yaqui River 's watershed that reach their northernmost limits at

520-521: The horst block and expose the footwall of the normal fault. This is a type of block faulting known as grabens and horsts . This basin and range topography is symmetrical having equal slopes on both sides of the valleys and mountain ranges. Tilted block faulting , also known as half-graben or rotational block faulting, can also occur during extension. Large gently dipping normal faults, also known as detachment faults , act as platforms in which normal faulted blocks tilt or slide along. However, instead of

546-490: The province was believed to have begun in the late Cenozoic Era, roughly 20 Ma . Between 1992 and 1998 scientists conducted GPS surveys to map the deformation of the Basin and Range province. In the study, Thatcher et al. discovered that most deformation was happening in the west, adjacent to the Sierra Nevada block, while less deformation was happening in the east. This coincides with the northwestward movement of

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572-609: The refuge. As the Yaqui River primarily flows through Mexico , the refuge is also the only area in the United States where these fish are found. The fish include the Yaqui chub ( Gila purpurea ), beautiful shiner ( Cyprinella formosa ), Yaqui catfish ( Ictalurus pricei ), Yaqui topminnow ( Poeciliopsis sonoriensis), Yaqui longfin dace ( Agosia chrysogaster sp 1), and Mexican stoneroller ( Campostoma ornatum ). The former four species are considered federally endangered within

598-443: The total extension of the region; however, the median estimate is about 100% total lateral extension. The tectonic mechanisms responsible for lithospheric extension in the Basin and Range province are controversial, and several competing hypotheses attempt to explain them. The Aegean Sea Plate consists of thinned continental crust. The northern part of the plate is currently a region of crustal extension caused by slab rollback on

624-422: The valley, catching and storing crucial water, and helping determine the variety of plants and animals present. The San Bernardino Valley once supported permanently flowing creeks, springs, and marshy wetlands. The giant sacaton grassland in the valley has been described as "a luxuriant meadow some eight or ten miles long and a mile wide." The dependable source of water and grass made the area not only invaluable to

650-505: The wall drained several of the refuge's crucial artesian wells , forcing evacuations of the endangered fish in them. Large-capacity water pumps had to be purchased and installed in order to mimic the natural pond outflow lost by the water extraction. Refuge manager Bill Radke has referred to the wall's construction as the “current greatest threat to endangered species in the southwest region ”. [REDACTED]  This article incorporates public domain material from websites or documents of

676-408: The whole block subsiding only one side, the block may slip along the detachment fault, tilting toward the fault plane, again creating mountains (ranges) and valleys (basins), many tilted slightly in one direction at their tops due to the motion of their bottoms along the main detachment fault. This basin and range topography has one steep side and the other is more gradual. The Basin and Range Province

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