The Sierra Madre Occidental is a major mountain range system of the North American Cordillera, that runs northwest–southeast through northwestern and western Mexico, and along the Gulf of California. The Sierra Madre is part of the American Cordillera, a chain of mountain ranges (cordillera) that consist of an almost continuous sequence that form the western "sounds" of North America, Central America, South America, and West Antarctica.
The Spanish name sierra madre means "mother mountain range" in English, and occidental means "western", these thus being the "Western mother mountain range".[1] To the east, from the Spanish oriental meaning "eastern" in English, the Sierra Madre Oriental range or "Eastern mother mountain range" runs generally parallel to the Sierra Madre Occidental along eastern Mexico and the Gulf of Mexico.
The range extends from northern Sonora, a state near the Mexico–U.S. border at Arizona, southeast towards to the Trans-Mexican Volcanic Belt and Sierra Madre del Sur ranges. The high plateau that is formed by the range is cut by deep river valleys. This plateau is formed from volcanic rock overlying a basement of metamorphic rock.
This uplift has caused changes in weather patterns; increased rainfall occurring in the mountains has provided areas where ecosystems can form in wetter areas than surrounding land. This water source forms watersheds that provide the arid surroundings with water that makes it possible to irrigate and farm crops. The wet ecosystems are islands of biodiversity, differing significantly from what would otherwise be a desert landscape. Oak forests are the predominant plant life, and extend into the lowland deserts.
This forest and canyon land provided a place for a variety of indigenous people to live, until Spanish settlers with associated mestizos came into the area to found towns for the silver mines in the area. The major industries in the area now are agriculture and forestry, which have become contentious because of land degradation and the native population's opposition to these practices.
The range trends from the north to southeast. Canyons cut by the rivers of the wet western slopes exist in addition to those of the northeast slopes, notably the Copper Canyon.
The range runs parallel to the Pacific coast of Mexico, from just south of the Arizona-Sonora border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins with the Sierra Madre del Sur and the Eje Volcánico Transversal (Transverse Volcanic Axis) of central Mexico after crossing 1,250 km (780 mi).[2] The mountains range from 300 km (190 mi) from the Gulf of California in the north, but begin to approach within 50 km (30 mi) of the Pacific in the south.[3] These mountains are generally considered to be part of the much larger American cordillera, the mountains extending from Alaska down to these across western North America.[4][5]
Sierra Tarahumara or Tarahumara is the name for the region of the Sierra Madre beginning at the Durango border and extending north. This name comes from the Tarahumara natives.[6] This is a dramatic landscape of steep mountains formed by a high plateau that has been cut through with canyons including Copper Canyon, larger and, in places, deeper than the Grand Canyon.[7] This plateau has an average elevation of 2,250 m (7,380 ft) with most of the more eroded canyons on the western slope, due to the higher moisture content.[2]
The highest elevations occur in the Tarahumara range.[8] The exact elevations of the highest peaks are not known within accurate enough ranges to determine their relative elevation. The highest point is probably Cerro Mohinora, located at 25°57′N 107°03′W / 25.950°N 107.050°W / 25.950; -107.050. Estimates for the height of the mountain start around 3,040 m (9,970 ft)[9] and go up to 3,300 m (10,800 ft).[10] However, Cerro Barajas, at 26°24′N 106°5′W / 26.400°N 106.083°W / 26.400; -106.083, may be as high as 3,300 m (10,800 ft)[11] although other sources give 3,170 m (10,400 ft) as the elevation.[12] Cerro Gordo, at 23°12′N 104°57′W / 23.200°N 104.950°W / 23.200; -104.950, may have an elevation between 3,350 m (10,990 ft)[13] and 3,340 m (10,960 ft).[14]
The southern end of the mountains may be referred to as the Sierra Huichola.[15] In this area, the Sierra Madre begins to give way to the Basin and Range Province.[16] Subranges of this area include the Sierra de Alica and the Sierra Pajaritos, both in Nayarit,[17] and the Sierra los Huicholes, Sierra de Morones, Sierra Nochistlán, and Sierra Fría extending eastwards into Jalisco, Zacatecas, and Aguascalientes.
The mountains act as a source of water, in an otherwise arid environment from the increased precipitation from the mountain range. As such, rivers that have headwaters in the mountains provide water for irrigation in the surrounding lands. The need for water to irrigate prompted the construction of dams, which has been the source of several environmental concerns in the area.
The northern end of the range is more arid; hence, the Yaqui River, which drains that area, is an important source of irrigation in the state of Sonora.[18] The Yaqui drains into the Gulf of California, as do the Fuerte River in Sinaloa and the Humaya River further south.[19] The Río Grande de Santiago drains 100,000 km2 (39,000 sq mi) from the southern slopes of the Sierra Madre.[20] Along the more arid eastern slopes of the mountains, the Nazas River and Aguanaval River drain the mountains into a closed basin.[21] These mountains supply 90% of the water used for irrigation within the watershed.[22] North of this system is the Conchos River which drains into the Rio Grande.[19]
Along many rivers the arid conditions have caused courses to be dammed to provide water for irrigation. These dams have caused concerns along with those caused by other activities. The Yaqui has been dammed with three large reservoirs along its course.[19] The Río Grande de Santiago has also been dammed, including the Aguamilpa Dam begun in 1991[23] and the El Cajón Dam upstream from it.[24] To provide irrigation water for farming the dry basin, the Nazas river was dammed in the 1930s and 1940s.[25] This has led to the former drainage lakes in the area drying up, and soil depletion from the elimination of the river as a source of soil. Due to the continued logging in the area, erosion on the slopes of hillsides has increased.[26]
Numerous small populations live in Copper Canyon, some of them around old missions.[27] Many settlements consist of single families or small family groups near agricultural fields.[28] There are also a few towns of note, including Creel, Chihuahua, with 5,000 inhabitants; it serves as a stop on the train line.[29] South of Copper Canyon is the town of El Salto, Durango, situated along highway 40.[30] To the west is Copala, Sinaloa.[31]
To the south live the indigenous Cora people in the towns of Jesús María and La Mesa del Nayar.[32] In addition, 1,000 people live in the village of Huajimic in Nayarit.[33] The Bolaños River valley of Jalisco is populated with mining communities, such as San Martín de Bolaños, which has a population of around 3,000.[34]
Few paved roads cross the mountains, meaning that travel times can be long, even between cities that are relatively close.[35] In the north, Mexico Federal Highway 16 connects Hermosillo, Sonora and La Junta, Chihuahua.[36] In the mountains, the La Entrada al Pacifico Corridor is an incomplete road intended to connect Los Mochis, Sinaloa and Chihuahua.[37] Durango and Mazatlan are connected by highway 40,[38] around 24 degrees north latitude.[35]
The Sierra Madre Occidental is a high plateau of volcanic rock that is eroded in areas to reveal a basement composed of plutonic and sedimentary rocks underlying the two groups of extrusive volcanics.[39] The lower of these groups is a series of volcanic rocks formed by lava flows. The surface of the plateau is made up of a second series of volcanic ignimbrites, covered mostly by clay rich organic soils. Some intrusions in the area have deposited ore veins that are commercially viable.
The landscape was shaped after rock placement. Rivers cut valleys into the plateau, following folds in the rocks. Rocks have broken off of the slopes, forming rocky sides. The bedrock is covered by the clay rich soils, except in eroded areas, where water carries away deposits leaving a stony surface.
The basement is thought to be Older North American basement, especially in the North,[40] and a number of different accreted terranes, which are mostly covered by Cenozoic volcanism.[41] The northern extent of the Sierra Madre is the Cortés Terrane, although the northern extreme of this contains a small portion of the Carborca Terrane.[41] Some of the northern end may also be underlain by a Precambrian basement placed around the same time as the Grenville Orogeny.[42] Some of this area has a series of Paleozoic sedimentary sequences from a deep marine environment underlying it, considered by some to be part of the Cotes Terrane and by others to be transported.[43] In some areas, distinct sedimentary rocks can be identified by shallow-water Silurian and Mississippian sedimentary rocks underlying deep-water Pennsylvanian and Permian, which are divided by Mississippian rhyolite. The Permian clastics contain some detrital rock that sit alongside low-grade metamorphosed sedimentary and volcanic rocks with serpentinite.[44]
South of the Cortes and Carborca Terranes is the San José de García Terrane, which is a combination of Cretaceous arc volcanics and volcaniclastics, which may be thrusting over the Cortes terrane.[45] The southern part of the range is made up of the Guerrero Composite Terrane, a large body, which is actually a complex of five different subterranes, not all of them within the main body of the Sierra Madre.[41] However, the terrane is mostly covered by volcanics and sedimentary deposits, so it is only visible where erosion has revealed it.[46] The main subterranes of the Guerrero Composite Terrane that are within the Sierra Madre are the Tahue and Zihuatanejo terranes.[41] Dividing the Guerrero Terrane from the rest of the Sierra Madre terranes is a boundary that is thought to represent the Early Cretaceous Arperos Basin,[47] a marine basin which separated the island arc that came to form the Guerrero terrane from the accretions that came to form the Sierra Madre terrane.[48] It contains a lower formation made up of pillow basalts with pillow breccias, tuffs, and shales underlying a group of pelagic limestones, oozes, and turbidites.[48]
Covering the southern basement are sedimentary and metamorphosed sedimentary rocks containing argillite, limestone,[49] volcanic schist, slate, and greywacke which were exposed in the canyon of the Santiago River.[43] Near these sediments late Oligocene to early Miocene granite and granodiorite intrusive bodies occur. A possible cause of this is that these bodies were roof pendants which were uplifted by plutons.[49] Mesozoic limestone also occurs in northern portion of the mountains, mostly from the Cretaceous.[35]
Towards the end of the Cretaceous, the Laramide orogeny increased the activity of magmatism in the area, forming the first major igneous series in the area.[50] The igneous series are made up of formations of plutonic and volcanic rock, which would later be exposed. Interbedded with these rocks are sedimentary deposit rocks. In the center of the range, some of these rocks have been deformed by tectonic forcing that occurred at the same time. The southern part of the range contains none of the volcanism that is apparent in the northern range. These formations ended in the Paleocene.[43] Eocene volcanism formed a series of andesitic and rhyolitic formations in the area, with spatial and temporal variations throughout. Most of the gold and silver deposits are also in these rocks.[51]
In the Oligocene, ash flows became the predominant deposit of the area, with interbedded lava flows between. These ash flows began the second series of high magmatism formations. The Mid-Tertiary ignimbrite flare-up formed a series of ignimbrite formations, layered one atop another,[50] that are sometimes broken by lava flows.[40] The ignimbrite formations in this area cover the largest area of any known series, with ten calderas identified in the province.[52] Three of these calderas are in Copper Canyon. The lava has formed a series of mafic rocks that comprise the Southern Cordilleran Basaltic Andesite Suite.[53] The tuffs are above a thick formation of lava rock. Around five or six units have been identified, mostly around the Copper Canyon area. These tuffs have allowed wide-ranging correlation with formations in other geographical areas, for example Death Valley.[54]
During the Miocene, three periods of volcanism marked separate events in the Sierras. Each period is marked by different volcanic processes.[55] The first was a continuation of the Oligocene flare-up, which lasted through the early Miocene. This may be a distinct second occurrence of the same process, placing silica-rich lava above Oligocene rocks.[43] Also in the early Miocene more intermediate basaltic andesites occur along faults and in grabens.[55] Beginning in the early Miocene and continuing into the middle, an arc of andesite was placed during the spreading of the Gulf of California.[49] The basin and range-style faulting of the middle-late Miocene took place at the same time as the placement of alkali basalts,[51] In the westernmost slopes mafic dikes formed. These events have also been linked to the subduction of the Farallon Plate.[56] Episodes of volcanism continued into the Quaternary.[51]
The Sierras are believed to be a thick core covered by volcanics and eroded by numerous rivers. Some have suggested that the basement of the mountains have numerous intrusions made by mafic magma.[43] At their surface, the mountains are made up of large-scale ignimbrite sheet that has been incised by rivers flowing from rainfall in the mountains.[57][58] The surface of the plateau is almost exclusively from the second series of flows causing the rock that is most visible to be ignimbrites with lava flow layers.[51] The region has a general strike from just west of north to just south of east.[58]
Faults in the area tend to be younger than the upper volcanics, excluding those associated with calderas,[51] but do occur in distinct periods.[40] The large faults along the eastern slope[51] have caused the slope to be made up of large escarpments that face into the interior basins.[58] These faults are commonly covered by alluvium and basalt, making them difficult to discern, but can be found in certain fault blocks.[51]
Grabens have formed in the north, and some of these are filled with volcanic and sedimentary floors,[49] and long depressions have formed in the southern Sierra Madre.[59] The faulting decreases in the center of the range where the faults are mostly small normal faults, but increases once again on the western slope.[51] In fact, the major normal faults tend to diverge around the center of the core and join at either end.[40] Some of these faults may have allowed ignimbrites to erupt from the surface, and so there may be some association between the ignimbrite occurrence and faults;[59] this can be used to find out information about different kinds of basement rock present.[43]
There are two faults dividing the Sierra Madre from the central Mexican Plateau. The more southern one is the north–south Aguascalientes fault extending 150 kilometres (93 mi) from Altos de Jalisco to near Zacatecas, where it joins the more northern northwest–southeast trending San Luis–Tepehuanes fault extending from Guanajuato to Durango.[60] The southern boundary of the Sierra Madre Occidental with the Jalisco block is a series of folds, thrust, and left lateral faults date from the middle Miocene. This may be evidence for a left transpressional shear zone.[43] The area between this boundary and the Aguascalientes fault is filled with numerous grabens and half-grabens, as well an east–west left shear zone, the El Roble shear zone. At the west end of these grabens and half-grabens are the Pochotitan and the more northerly San Pedro fault systems, which form the western boundary of the faults in the southern mountains.[56] In the north there is an unknown boundar