The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. [1] The rift extends from central Colorado in the north to the state of Chihuahua, Mexico, in the south. [2] The rift zone consists of four basins that have an average width of 50 kilometres (31 mi). [1] The rift can be observed on location at Rio Grande National Forest, White Sands National Park, Santa Fe National Forest, and Cibola National Forest, among other locations.
The Rio Grande rift has been an important site for humans for a long time, because it provides a north–south route that follows a major river. The Rio Grande follows the course of the rift from southern Colorado to El Paso, where it turns southeast and flows toward the Gulf of Mexico. Important cities, including Albuquerque, Santa Fe, Taos, Española, Las Cruces, El Paso, and Ciudad Juárez, lie within the rift.
The Rio Grande rift represents the easternmost manifestation of widespread extension in the western U.S. during the past 35 million years. The rift consists of three major basins and many smaller basins, less than 100 square kilometres (39 sq mi). The three major basins (from northernmost to southernmost) are the San Luis, Española, and Albuquerque basins. The rift's northern extent is delineated by the upper Arkansas River basin between Leadville and Salida, Colorado. Further south, the rift is defined by a network of smaller, less topographically distinct alternating basins and ranges. The distinction between these smaller basins and those of the Basin and Range Province becomes blurred in northern Mexico. [3] [4]
Basin size generally decreases to the north in the rift, though the Española covers approximately 120 kilometres (75 mi) north–south and 40 kilometres (25 mi) east–west, and the San Luis is roughly 120 by 80 kilometres (75 by 50 mi). These basins may contain smaller units within them, such as the Alamosa basin within the San Luis, which is bounded by the San Juan and Tusas mountains on the west and the Sangre de Cristo Mountains in the east. [5] The Albuquerque basin is the largest of the three basins, spanning 160 kilometres (99 mi) north–south and 86 kilometres (53 mi) east–west at its widest points. It is the oldest of the three major basins, and contains 7,350 metres (24,110 ft) of Paleogene clastic sediments deposited on Precambrian basement. The southernmost Albuquerque basin contains pre-rift volcanic deposits, while the central and northern portions contain volcanics erupted during rifting. [3]
In cross-section, the geometry of the basins within the rift are asymmetrical half-grabens, with major fault boundaries on one side and a downward hinge on the other. Which side of the basin has the major fault or the hinge alternates along the rift. The alternation between these half-grabens occurs along transfer faults, which trend across the rift to connect the major basin-bounding faults and occur between basins or, in places, within basins. The Precambrian basement changes relief sharply in this area, from 8,700 metres (28,500 ft) below sea level at the bottom of the Albuquerque basin to 3,300 metres (10,800 ft) above sea level in the nearby Sandia Mountains, which flanks the Albuquerque basin to the east. Flanking mountains are generally taller along the east side of the rift (although some of this relief may be Laramide in origin). [1] The thickness of the crust increases to the north beneath the rift, where it may be as much as 5 kilometres (3.1 mi) thicker than it is in the south. The crustal thickness underneath the rift is on average 30–35 kilometres (19–22 mi), thinner by 10–15 kilometres (6.2–9.3 mi) than the Colorado Plateau on the west and the Great Plains to the east. [6]
Formation of the rift began with significant deformation and faulting with offsets of many kilometers starting about 35 Ma. [7] The largest-scale manifestation of rifting involves a pure-shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy. [8] [9] [10] This extension is associated with very low seismic velocities in the upper mantle above approximately 400 kilometres (250 mi) depth associated with relatively hot mantle and low degrees of partial melting. [11] This intrusion of the asthenosphere into the lithosphere and continental crust is thought to be responsible for nearly all of the volcanism associated with the Rio Grande rift.
The sedimentary fill of the basins consists largely of alluvial fan and mafic volcanic flows. The most alkalic lavas erupted outside the rift. [12] The sediments that were deposited during rifting are commonly known as the Santa Fe Group. This group contains sandstones, conglomerates, and volcanics. Aeolian deposits are also present in some basins. [1] [2]
The Rio Grande rift is intersected in northern New Mexico by the NE-SW trending Jemez Lineament which extends well into Arizona. The lineament is defined by aligned volcanic fields and several calderas in the area, including the Valles Caldera National Preserve in the Jemez Mountains. The Jemez Lineament is thought to be a hydrous subduction zone scar, separating Precambrian basement rock of the Yavapai-Mazatzal transition zone from the Mazaztl Province proper. [13] [14] Also on the Colorado Plateau but further north lies the San Juan volcanic field in the San Juan Mountains of Colorado.
The youngest eruptions in the rift region are in the Valley of Fires, New Mexico, and are approximately 5,400 years old. [15] [16] The Socorro, New Mexico, region of the central rift hosts an inflating mid-crustal sill-like magma body at a depth of 19 km that is responsible for anomalously high earthquake activity in the vicinity, including the largest rift-associated earthquakes in historic times (two events of approximately magnitude 5.8) in July and November 1906. [17] [18] [19] Earth and space-based geodetic measurements indicate ongoing surface uplift above the Socorro magma body [20] at approximately 2 mm/year. [21]
The Rio Grande rift's tectonic evolution is fairly complex. The fundamental change in the western margin of the North American plate from one of subduction to a transform boundary occurred during Cenozoic time. The Farallon plate continued to be subducted beneath western North America for at least 100 million years during Late Mesozoic and early Cenozoic time. Compressional and transpressional deformation incurred by the Laramide Orogeny lasted until about 40 Ma in New Mexico. [23] [24] [25] This deformation may have been a result of the coupling between the subducting Farallon plate and the overlying North American plate. Crustal thickening occurred due to Laramide compression. After the Laramide Orogeny and until 20 Ma, a major period of volcanic activity occurred throughout the southwestern United States. Injection of hot magmas weakened the lithosphere and allowed for later extension of the region. [26]
Cenozoic extension started about 30 million years ago (Ma). There are two phases of extension observed: late Oligocene and middle Miocene. [27] The first period of extension produced broad, shallow basins bounded by low-angle faults. The crust may have been extended as much as 50% during this episode. Widespread magmatism in mid-Cenozoic time suggests that the lithosphere was hot, the brittle-ductile transition was relatively shallow. [26] There is evidence that the second period of extension began earlier in the central and northern Rio Grande rift than in the south. [1] A third period of extension may have begun in the early Pliocene. [28]
One theory is that the Colorado Plateau acts as a semi-independent microplate [29] and one way of explaining the creation of the Rio Grande rift is by the simple rotation of the Colorado Plateau 1-1.5° in a clockwise direction relative to the North American craton. [1] Other explanations that have been offered are that the extension is driven by mantle forces, such as large-scale mantle upwelling [30] or small-scale mantle convection at the edge of the stable craton; [31] collapse of over-thickened continental crust; [32] initiation of transform faulting along the western margin of the North American plate; [33] or detachment of a fragment of the Farallon plate beneath the Rio Grande region that enhanced asthenospheric upwelling in the slab window. [34]
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
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.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all, active rift systems.
A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of them result from tensional forces, caused by a process known as oceanic trench rollback, where a subduction zone moves towards the subducting plate. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression. However, in 1970, Dan Karig published a model of back-arc basins consistent with plate tectonics.
The Potrillo volcanic field is a monogenetic volcanic field located on the Rio Grande Rift in southern New Mexico, United States and northern Chihuahua, Mexico. The volcanic field lies 22 miles (35 km) southwest of Las Cruces, and occupies about 4,600 square kilometers (1,800 sq mi) near the U.S. border with Mexico.
The Baikal Rift Zone is a series of continental rifts centered beneath Lake Baikal in southeastern Russia. Current strain in the rifts tends to be extending with some shear movement. A series of basins form along the zone for more than 2,000 kilometres (1,200 mi), creating a rift valley. The rifts form between the Eurasian Plate to the west and the Amur Plate to the east.
The Jemez Lineament is a chain of late Cenozoic volcanic fields, 800 kilometers (500 mi) long, reaching from the Springerville and White Mountains volcanic fields in East-Central Arizona to the Raton-Clayton volcanic field in Northeastern New Mexico.
The Afar Triple Junction is located along a divergent plate boundary dividing the Nubian, Somali, and Arabian plates. This area is considered a present-day example of continental rifting leading to seafloor spreading and producing an oceanic basin. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift. The latter extends a total of 6,500 kilometers (4,000 mi) from the Afar Triangle to Mozambique.
Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of volcanism is normally associated with areas of extensional tectonics, particularly rifts.
Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces produced by faulting brought about subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.
A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.
The geology of New Mexico includes bedrock exposures of four physiographic provinces, with ages ranging from almost 1800 million years (Ma) to nearly the present day. Here the Great Plains, southern Rocky Mountains, Colorado Plateau, and Basin and Range Provinces meet, giving the state great geologic diversity.
The Espinaso Formation is a geologic formation in New Mexico. It has a radiometric age of 34.6 to 26.9 million years, corresponding to the late Eocene through Oligocene epochs.
The Latir volcanic field is a large volcanic field near Questa, New Mexico, that was active during the late Oligocene to early Miocene, 28 to 22 million years ago (Ma). It includes the Questa caldera, in whose deeply eroded interior is located the Molycorp Questa molybdenum mine.
The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.
The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States, in the central part of the Colorado Plateau. The volcanic field consists of over 80 volcanoes and associated intrusions of unusual potassium-rich compositions, with an age range of 26.2 to 24.7 million years (Ma).
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(help) Abstract at: "Annual Meeting". Seismological Research Letters. 66 (2). page 44 of 15–61. 1995. doi:10.1785/gssrl.66.2.15.