The North American Cordillera
The circum-Pacific orogenic belt continues eastwards from the Alaska Peninsula to join the mountain ranges of Northern Alaska. These lie at the northern end of the North American Cordillera, a series of mountain ranges that occupy the western side of North America, stretching from Alaska in the north to Mexico in the south, a total distance of around 7800km (Figs. 11.1, 11.2).
The Alaska–Canada sector is up to 1000km wide and consists of several separate mountain ranges including the Brooks and Alaska Ranges in northern Alaska, the Mackenzie Mountains in the Yukon, the Coast Mountains in British Columbia, and the Rocky Mountains on the eastern side of the Cordillera, which extend from British Columbia into Alberta (see Figure 11.2 for province and state boundaries). The highest summits are in the coastal sector; Denali (formerly Mt. McKinley) in the Alaska Range, at 6190m, is the highest peak in North America (Fig. 11.3). The eastern ranges are less high; the peaks of the Canadian Rockies are generally around 3000m.
Further south, through the USA, the Cordillera broadens to over 1500km, and is more clearly divided into the Cascade Range and Sierra Nevada on the western side, and the Rocky Mountains of Wyoming and Colorado on the eastern side. Between these ranges is the vast volcanic region of the Columbia Plateau and, further south, the Basin and Range Province, characterised by a series of mountain ranges with intervening valleys created by extensional faulted blocks. This whole region of the western USA is elevated above 1500m. The highest peak in the coterminous United States is Mount Whitney, at 4421m, in the Sierra Nevada.
The Cordillera continues southwards through Mexico, narrowing to about 200km in width towards the border with Guatemala. The main mountain range here is the Sierra Madre Occidental.
Figure 11.1 Mountain ranges of the North American Cordillera. McK Mts, McKenzie Mountains. Shutterstock©Arid Ocean.
Figure 11.2 Principal tectonic features of the North American Cordillera. AP, Alaska Peninsula; AT, Aleutian Trench; BC, Baja California; CoP, Cocos Plate; CP, Columbia Plateau; CV, Cascade volcanic belt; CZ, Cascade subduction zone; DF, Denali Fault; GC, Gulf of California; JFP, Juan de Fuca Plate; MW, Mt Whitney; QCF, Queen Charlotte Fault; SV, Stikine volcanic belt; VI, Vancouver Island. After Moores & Twiss, 1995.
Plate-tectonic overview
This sector of the circum-Pacific orogenic belt system (Fig. 11.2) is superficially rather simpler than the western Pacific sector. The American Plate forms the eastern foreland, and the Pacific Plate, together with several smaller oceanic plates, lies on its western side. Between, however, is a complex region consisting of a large number of separate terranes and tectonic provinces.
The plate boundary in the northern, Alaska, sector is the Aleutian Trench, which sweeps round from an east–west trend at the eastern end of the Aleutian Arc to north–south, parallel to the coast, at which point it becomes the Queen Charlotte transform fault. Near the northern end of Vancouver Island, the plate boundary bends into a NW–SE orientation and forms part of the boundary of the Juan de Fuca Plate (the surviving part of the original Farallon Plate). The eastwards subduction of this oceanic plate along the Cascadia Subduction Zone, which lies off the coasts of Washington and Oregon, is responsible for the many active volcanoes of the Cascades Range.
Figure 11.3 Denali (Mount McKinley) in the Alaska Range. At 6190m, this is the highest mountain in North America. © Shutterstock, by bcambell65.
At its southern end, the Cascadia subduction zone meets the famous San Andreas transform fault, which runs for 2000km through Western California and into the Gulf of California, where it meets the ridge forming the western boundary of the Cocos Plate. The northern end of this ridge is chopped into small segments by a series of short transform faults. West of the San Andreas Fault is a long, thin sliver of continental crust belonging to the Pacific Plate and encompassing the coastal region of Western California and the Baja California Peninsula of Mexico. The eastern boundary of the Cocos Plate is formed by another subduction zone, along which this oceanic plate is being subducted beneath Central America. This part of the Circum-Pacific belt is the subject of the next chapter.
The Alaska–Canada Sector
This part of the circum-Pacific orogenic belt was where the terrane concept was first developed by Coney and his co-workers in 1980. Since then a large number of separate terranes have been proposed, some of which are regarded as ‘super-terranes’, themselves consisting of more than one individual terrane. Many of the terranes are regarded as ‘displaced’ or even ‘exotic’ because they display evidence of having travelled some distance from their original site of formation. The system of nomenclature and interpretation of the terrane complex is to some extent in a state of flux because the region is still comparatively poorly known and difficult of access.
The main part of the Alaska–Canada sector is composed of a number of displaced terranes, which have been added to the North American Plate during the Mesozoic. The main collision event took place during the Cretaceous Period. The terranes represent a mixture of crystalline metamorphic complexes, volcanic arc material, arc–trench deposits of various ages, and ophiolite units containing oceanic sediments and ultramafic rocks. Some of these can be shown to have originated far from their present position and are believed to have been transported northwards on the Farallon plate and scraped off the subducting slab.
For the purpose of this account, the region is described in terms of seven terranes or super-terranes, plus several autochthonous tectonic zones, based on the subdivision recognised by the Geological Surveys of the Yukon and British Columbia (Fig. 11.4). The whole complex forms a wide arc oriented roughly east-west in Alaska, and bending round to trend NW–SE in British Columbia. On the north or east side of the belt is the North American foreland and foreland fold-thrust belt.
Figure 11.4 The Alaska–Canada sector of the North American Cordillera. AP, Alaska Peninsula; Ca, Cassiar belt; CB, Colville Basin; CPC, Coast Plutonic Complex; FBRF, Fairweather-Border Ranges Fault; KB, Kootenay Basin; KSZ, Kobak suture zone; Ky, Koyukuk, Nyak, Togiak Basins; QCF, Queen Charlotte Fault; TF, Tintina Fault. Terranes: Ax, Alexander; CC, Cache Creek; CPWY, Chugach–Prince William–Yakutat; Fw, Farewell; Pe, Peninsular; Rb, Ruby–Angayucham; Wr, Wrangellia. Cities: A, Anchorage; Cal, Calgary; Ed, Edmonton; F, Fairbanks; In, Inuvik; V, Vancouver; Vi, Victoria; Wh, Whitehorse. States/Provinces: Al, Alaska; BC, British Columbia; Can, Canada; NWT, Northwest Territories; Yu, Yukon. Mountains: AR, Alaska Range; ChR, Chugach Range; D, Dinali; MR, Mt Robson; MS, Mt. Spurr (volcano). After Geological Surveys of the Yukon and British Columbia, 2011.
The foreland
The North American foreland consists of the ancient Precambrian core of North America, known as Laurentia, or ‘the Canadian Shield’ in Canada, plus material added to it during the Palaeozoic. The Alaskan sector of the foreland lies within the Arctic Alaska Terrane.
The Arctic Alaska Terrane
This 400km-wide zone in Northern Alaska contains rocks ranging in age from Precambrian to Cenozoic, representing the basement and sedimentary cover of the North American foreland. The southern part of the terrane, occupied by the mountains of the Brooks Range, is a foreland fold-thrust complex, while the northern part is known as the Colville Basin. The northern boundary of the zone on land is the north Alaskan coast, but the zone continues offshore onto the continental shelf. This whole region north of the Brooks Range is well known to petroleum geologists as the ‘North Slope’. The southern boundary of the terrane is defined by the Kobuk suture zone, which marks the original margin of the American Plate. This suture zone is a south-dipping thrust along which rocks of the Angayucham Terrane (see below) have been overthrust northwards over the Arctic Alaska Terrane, forming the highest nappe of the fold-thrust belt.
The foreland fold-thrust belt
This belt extends eastwards from the coast of the Chukchi Sea, following the mountains of the Brooks Range in Arctic Alaska across the Alaska–Yukon border towards the boundary with the Northwest Territories, where it is cut off by a strike-slip fault. South of this fault, the belt continues through the Mackenzie Mountains and into British Columbia; here it is represented by the Canadian Rocky Mountains, which extend south-eastwards through Alberta and into the USA. The peaks of the Canadian Rockies are generally around 3000m, the highest being Mt. Robson, at 3954m (Fig. 11.5).
The sedimentary cover in this belt consists of Palaeozoic to Mesozoic platform and continental-slope deposits resting on the Precambrian basement of the Canadian Shield. These strata record the history of the passive margin of the North American continent that existed until subduction of the Farallon plate commenced at its western margin in the Mesozoic. The belt is dominated by folded and thrust units of this sedimentary cover, and provides one of the most spectacular and best documented examples of thrust tectonics.
Foreland and intermontane basins
Several large sedimentary basins overlie the margin of the North American Plate. The Colville Foreland Basin, north of the Brooks Range in Arctic Alaska, contains Upper Cretaceous to Cenozoic sedimentary rocks derived from the rising mountains of the Brooks Range. Another, much wider, intermontane basin lies west of the Mackenzie Mountains in Northwest Territories, but narrows southwards through British Columbia where it dies out. This basin is bounded on its west side by a major dextral strike-slip fault, the Tintina Fault. In southern Alberta, this basin reappears as the Kootenay Basin.
Figure 11.5 Mount Robson, Jasper National Park. This 3952m peak is the highest mountain in the Canadian Rocky Mountains. © Shutterstock, by BGSmith.
The central accretionary complex
This zone is about 800km wide in Alaska but narrows to around 600km through the British Columbian sector. It consists of three main fault-bounded blocks that are themselves split into sections by fault splays. Each block forms a northerly convex arc trending from NE–SW in the west to NW–SE in the southeast. Each of these blocks is overlain by Cretaceous–Cenozoic sedimentary basins at their western ends in Alaska. The faults bounding these blocks have all experienced a dextral strike-slip sense of movement, which has resulted in a general anticlockwise rotation of the more southerly blocks relative to the American foreland, and caused an accumulation of terranes at the western end of the complex. The fault blocks themselves do not correspond to terranes, since each block contains more than one terrane, and the boundary faults cut across the terrane boundaries in places, since the main fault-block movements post-date the terrane accumulation.
Seven separate tectonic units are differentiated in Figure 11.4, between the foreland thrust belt and the coast. Some of these are composite ‘super-terranes’ and all of them are regarded as displaced to a greater or lesser extent from their origins. The terranes are, from northeast to southwest: Ruby–Angayucham, Farewell, Yukon–Tanana, Stikine–Quesnellia, Wrangellia, Alexander and the composite Chugach–Prince William–Yakutat unit.
Ruby–Angayucham
This is a composite super-terrane consisting of a Precambrian–Palaeozoic basement (Ruby) of North American foreland type overlain by allochthonous oceanic rocks of Permo–Triassic age (Angayucham) deformed and metamorphosed in the Upper Cretaceous Laramide Orogeny. The unit forms two main outcrops, on the Seward Peninsula and north of the city of Fairbanks. It also forms a narrow band along the southern edge of the Arctic–Alaska Terrane, where it has been overthrust onto the latter terrane within the Brooks Range. The central outcrop is overlain on its northern, western and southern sides by sedimentary basins that received terrigenous sediments ranging from the Upper Cretaceous onwards.
Farewell
This terrane is regarded as an exotic continental block. Its contacts with neighbouring terranes are obscured by the sedimentary basins that surround it on three sides, and it is separated by a fault from the Ruby–Angayucham unit to the north.
Yukon–Tanana
This unit consists of Upper Palaeozoic and older rocks, thought to belong originally to the continental margin of the North American foreland, which adjoins it to the east. The unit has been transported north-westwards by dextral movements along the Tintina Fault. Its southwest boundary is the dextral Denali Fault.
Stikine–Quesnellia
This composite terrane represents a Permian to Jurassic volcanic arc, or arcs, accreted to the North American continent during Jurassic subduction, and includes two large outcrops of ophiolitic rocks, known as the Cache Creek assemblage. The terrane adjoins continental rocks of the Cassiar fold-thrust belt to the east, while the Yukon–Tanana terrane lies on its southwest side in the northern half of the outcrop, and the Alexander and Wrangellia terranes in the southern. The intrusions of the Coast Range Plutonic Complex occupy a 1100km-long strip within the southeastern part of the outcrop.
Wrangellia
This composite super-terrane is an assemblage of island arcs and oceanic plateaux of Carboniferous to Jurassic age considered to have formed in an equatorial region far to the south of its present location, and transported northwards on the Farallon Plate. The terrane forms a large outcrop in Alaska, sandwiched between the Denali and Border Ranges faults and another in southern British Columbia, where it occupies the Queen Charlotte Islands and Vancouver Island. Here, the Queen Charlotte Fault forms the western boundary and the Alexander Terrane the eastern.
Alexander
This terrane consists of marine shales, carbonates and red beds of Silurian to Devonian age containing faunas similar to those of North-eastern Siberia. The terrane is considered to have amalgamated with Wrangellia during the Carboniferous and it has been suggested that Wrangellia and Alexander should be considered as a single terrane. The Alexander rocks have been correlated with those of the Farewell Terrane, discussed above.
The Aleutian volcanic arc
This currently active volcanic arc extends through the Alaskan Peninsula and into the Alaska Range, north of Anchorage in southeastern Alaska, and is the result of the northward subduction of the Pacific Plate. There are more than 80 potentially active volcanoes in Alaska, 43 of which have been active historically. Most are on the Aleutian island arc, discussed in the previous chapter. One of the more recently active volcanoes in mainland Alaska is Mount Spurr, 130km west of Anchorage, whose 1992 explosive eruption deposited ash as far as the city itself.
The Coast Range Arc
The Coast Range plutonic complex, often termed the Coast Range Batholith, extends for 1100km from Southern Alaska to Vancouver, parallel to the coast and up to 100km inland. It consists of a variety of late Cretaceous to Eocene plutonic bodies of typically granodioritic composition, intruded into the volcanic rocks of the Stikine Terrane. The arc, produced by the subduction of the oceanic Farallon Plate, is a typical example of an ‘Andean’ type of active continental margin. Once this plate had been consumed beneath the continental margin, the relative motion of the Pacific and American plates was parallel to the coast, and the plate boundary became a transform fault. The volcanic arc continues southwards into the Cascade Range of Washington State in the USA, where it is still active (see below).
The outboard terranes
These consist of three fault blocks, arranged in an arc along the southern coast of Alaska. The largest, and innermost, the Chugach Terrane, extends in an arc from Kodiak Island through the Chugach Mountains and wedges out near the Yukon border. It is bounded to the north by the Border Ranges Fault, on which the terrane is overthrust northwards over the Aleutian volcanic arc. The terrane reappears as a coastal sliver further south, where it is in fault contact with the Alexander Terrane. The two smaller fault blocks, the Prince William and Yakutat terranes, occur on the outer side of the Chugach terrane, along the southern Alaska coast. These terranes collectively represent an accretionary prism developed on the outer side of Wrangellia during the Mesozoic subduction phase. Since subduction ceased during the Cenozoic, the outboard terranes have been moving northwards relative to the North American continent along the Fairweather–Border Ranges Fault at a rate currently estimated at c.45mm/a.
Tectonic history
Throughout the Palaeozoic, the eastern part of the orogenic belt lay along the passive margin of the North American continent. Subduction of the Farallon Plate, part of the ancestral Pacific system, along the western margin of the continent commenced in the Jurassic. Several of the individual terranes, including Wrangellia and Alexander, accreted together first before colliding with the American continent in the mid-Cretaceous. This collision was responsible for the main phase of deformation and metamorphism within the orogen and is known as the Laramide Orogeny. The process of terrane accretion is illustrated schematically in Figure 11.6.
Figure 11.6 Cartoon sections showing the Canadian sector before strike-slip displacement. How the orogen could have been constructed by successive accretions of displaced terranes. Note that the convergence direction was probably highly oblique to the continental margin. Colours as in Figure 11.4.
The later, Cenozoic, history of the orogenic belt was characterised by lateral movements along several major dextral strike-slip faults, parallel to the Queen Charlotte and San Andreas transform faults that form the present Pacific–American plate boundary. These movements slid the various terranes northwards relative to the eastern part of the orogen. Late-orogenic collapse due to gravitational spreading resulted in extensional faulting that has reversed the movement on some of the earlier thrust faults. The northwestern part of the orogen hosts a Cenozoic accretionary prism and parts of the coastal belt are still tectonically active.
The Western United States and Mexico
After crossing into the United States, the Cordilleran belt expands to over 1500km in width, taking in the four States of California, Nevada, Utah and most of Colorado at its widest part. This section of the Cordillera can be divided into three main tectonic zones and/or provinces: the Rocky Mountain belt (including the Colorado Plateau), the Basin and Range Province and the western accretionary zone, together with four igneous provinces: the Columbia Volcanic Province, the Cascade Volcanic Arc, the Sierra Nevada Batholith and the Sierra Madre Occidental Volcanic Province (Fig. 11.7).
Figure 11.7 Principal tectonic features of the Western USA and Northern Mexico. B&R, Basin and Range; CM, continental margin (pre-orogenic); CSZ, Cascades Subduction Zone; CVA, Cascades Volcanic Arc; CVP, Columbia Volcanic Province; LBF, La Babia Fault; MFTB, Mexican fold-thrust belt; MSF, Mohave–Sonora Fault; OBB, Ouachita Belt boundary; RGR, Rio Grande Rift; SMVP, Sierra Madre Occidental Volcanic Province; SNB, Sierra Nevada Batholith. Geographic names: CM, Cerro Mohinora (mountain); DV, Death Valley; GoC, Gulf of California; GoM, Gulf of Mexico; GP, Gannett Peak; ME, Mt. Elbert; MR, Mt. Rainier; MW, Mt. Whitney; SMO, Sierra Madre Oriental; SN, Sierra Nevada; SnR, Snake Range; SR, Sacramento Range; SwR, Sawatch Range; Wash, Washington; WP, Wheeler Peak; WRR, Wind River Range; Yel, Yellowstone. After Coney et al., 1980; and Fitz-Diaz et al., 2011.
Along the eastern side of the Cordilleran belt, the Rocky Mountains belt continues from Alberta into Montana, where it divides into eastern and western branches, each containing individual ridges over 3000m high, with numerous peaks over 4000m. The eastern branch, which continues through Wyoming and into Colorado, is divided into several distinct ranges. The highest peak in this sector is Mount Elbert (4401m) in the Sawatch Range in Colorado. This belt continues southwards, as the Sacramento Range, through New Mexico, to the Mexican border, after which it continues through northern Mexico as the Sierra Madre Oriental Range. The western branch forms a wide, eastward-facing arc through western Wyoming and Utah into Arizona. Gannett Peak (4202m) in the Wind River Range in Wyoming is the highest in this sector.
As in the Alaska–Canada sector, the Rocky Mountains belt corresponds to the marginal fold-thrust zone developed on the North American foreland. The eastern branch is known as the Laramide Belt, and the western, the Sevier. The Laramide Belt is characterised by uplifted fault-bounded basement blocks of the North American Foreland, together with their platform cover, whereas the Sevier is a typical thin-skinned belt, in which the folds and thrust sheets involve only the sedimentary cover, and the basement is not involved (Fig. 11.8). The eastern branch continues into Mexico, where it occupies the mountains of the Sierra Madre Oriental and is cut off by the thin-skinned Mexican fold-thrust belt.
The Sevier Belt represented the western margin of the North American continent until the Cretaceous, and its cover contains strata ranging from Cambrian to mid-Mesozoic in age. The deformation here began in the Jurassic and continued into the early Cretaceous, whereas the Laramide deformation took over in the late Cretaceous and lasted until the Eocene.
The Colorado Plateau
Between the Laramide and Sevier mountain belts is a region of relatively featureless ground, mostly over 2000m in altitude, including large parts of Colorado, New Mexico, eastern Arizona, and southeastern Utah, known as the Colorado Plateau. This is a largely desert area drained by the Colorado River and its tributaries, which cut through the horizontal Mesozoic strata that cover much of the region and provide the spectacular coloured cliffs seen in famous US national parks such as Glen Canyon and Grand Canyon (Fig. 11.9). To avoid confusion, it should be noted that what is described here, and defined in Figures 11.7 and 11.8, is the geological, or tectonic, province; however, the Colorado Plateau as described geographically extends much further west, and includes the southern part of the Sevier branch of the Rocky Mountain belt. The eastern part of the Plateau is split by the Rio Grande Rift, which is an extensional fault-bounded valley occupied by the Rio Grande river.
The Precambrian basement of the Colorado Plateau is part of the North American Foreland, and the platform cover contains strata dating from Cambrian to Neogene in age. The basement and its Palaeozoic cover are visible in the walls of the Grand Canyon, but much of the scenery elsewhere on the Plateau is formed from brightly coloured Mesozoic sandstones. The stability of the Plateau is the result of its relative strength, which has enabled the orogenic stresses arising from tectonic activity to the west to be transmitted though the region to the Laramide belt on its eastern side.
Figure 11.8 Schematic sections across the North American Cordillera: A, northern and B, southern sectors to illustrate the types of crustal structure. The northern margin is dominated by the actively subducting Cascade Arc, whereas the southern is bounded by the San Andreas Transform Fault. CA, Cascade Arc; Col Plat, Colorado Plateau; SAF, San Andreas Fault; SNB, Sierra Nevada Batholith. Note that individual structures are illustrative only and not intended to be accurate. After Moores & Twiss, 1995.
Figure 11.9 The Grand Canyon, Arizona, in the southwestern part of the Colorado Plateau. Note the horizontal strata. © Shutterstock, by Jason Patrick Ross.
The Basin and Range Province
This wide region extends from Oregon and Idaho in the north to the Sierra Madre Occidental Volcanic Province in the south, and widens to about 800km between the Sierra Nevada Mountains in the west and the Sevier ranges in the east. The province is characterised by a series of narrow mountain ranges separated by wide flat valleys. The central and northern parts of the province are generally over 2000m in altitude, with the individual mountain ranges rising to over 3000m. Further south, in Arizona and northern Mexico, much of the land is less than 1000m high. The highest mountain is Wheeler Peak (3982m) in the Snake Range, in Nevada. The valleys are mostly hot dry deserts, including the notorious Death Valley near the western margin of the province.
The Basin and Range Province is well known to geologists as a classic example of extensional tectonics. The mountain ranges correspond to uplifted blocks (horsts) bounded by normal faults, and the valleys to the intervening structural depressions (graben). The faults are listric: that is, they level out at depth to enable large extensional displacements to take place along them (Fig. 11.8). Many of the uplifted blocks expose the Precambrian basement and are known as metamorphic core complexes; such complexes are typical of extended regions and were discussed previously in the chapters on the Alpine–Himalayan Belt.
Although the crust is of normal thickness, the province experiences a higher than normal heat flow, and the mantle lithosphere is abnormally thin. This explains the relatively high altitude and structural weakness of the Basin and Range, which contrasts with the strong, cool lithosphere of the Colorado Plateau. The origin of the province as a distinct tectonic entity commenced with the low-angle subduction of the Farallon Plate in the Jurassic, which created an over-thickened crust in the whole Rocky Mountain belt up to the Laramide Front, including both the Basin and Range and the Colorado Plateau. After the mid-Eocene, the subduction regime changed: the convergence direction became oblique, and a volcanic arc formed along the western margin of the province, attributable to a steepening of the slab. This resulted in the initiation of a back-arc extensional regime on the upper plate, confined to the region overlying the subduction zone, but excluding the Colorado Plateau. Estimates of the amount of extension vary from about 80% upwards, but this amount could produce the required amount of thinning from an initial thickness similar to that of the adjacent Colorado Plateau. The extensional phase lasted until around 25Ma ago, when the subduction zone was replaced by the San Andreas Transform Fault.
The Western Accretionary Zone
This zone consists of displaced terranes accreted to the North American continent during the Mesozoic, similar to those described in the Alaska–Canada sector of the orogen (Fig. 11.7). The wide northern part of the zone hosts the Columbia and Cascade Arc volcanic provinces. Further south, in California, the zone narrows; much of it here is occupied by the Sierra Nevada Batholith.
At a point about 150km from the northern Californian border, near the city of Eureka, the plate boundary at the American continental margin changes from subduction zone to transform fault, with an accompanying abrupt change in the topography of the interior. From here southwards, the western side of the Cordillera consists of two parallel mountain ranges: the Coast Range, which hugs the Californian coast for over 900km nearly as far as Los Angeles, and the much more impressive Sierra Nevada, situated around 200km to the east and separated from it by a wide plain.
The San Andreas Fault divides the accretionary zone into two: the eastern side belongs to the North American Plate whereas west of the fault, the zone consists of displaced terranes belonging to the Pacific Plate.
The Columbia Volcanic Province
West of the Rocky Mountains, the northern part of the Cordillera in the USA is markedly different from its counterpart in Canada (Fig. 11.7). The Basin and Range Province extends northwards into Oregon, where much of the area is flat, relatively featureless desert – more basin than range – and eastwards into Idaho. Here, and further north, the Basin and Range Province is overlapped by the geographical region known as the Columbia Plateau, which in tectonic terms is part of the same extensional region as the Basin and Range.
The Columbia Plateau covers an area of over 500,000km2, covering large parts of the States of Washington, Oregon and Idaho. Much of it is over 1000m elevation, and several peaks reach over 3000m in height. It is bounded on its western side by the mountains of the Cascade Range and in the east by the Rocky Mountains belt. Most of the Plateau is underlain by Neogene to Recent igneous rocks of the Columbia Volcanic Province, including the voluminous Columbia River plateau basalts, which are up to 3km thick and cover an area of around 160,000km2.
The origin of the volcanism, which includes the active hydrothermal activity of Yellowstone National Park, is thought not to be related to the Cascade subduction zone, but has been attributed to the presence of a mantle hotspot currently situated beneath Yellowstone. The ages of the volcanics become younger eastwards, believed to be indicative of the westward movement of the North American Plate over a stationary hotspot. The region of the plateau in general must have experienced considerable extension and thinning in order to accommodate the lava pile.
The Cascade Volcanic Arc
The Cascade Range is a relatively narrow belt of volcanic mountains, situated about 150km from the coast, that extend from southern British Columbia for 1100km through Washington and Oregon into northern California (Fig. 11.7). The range hosts many peaks over 3000m in height, the highest being Mount Rainier, at 4392m, in Washington State. Only two volcanoes have been active in recent times – one being Mt. St. Helens, immediately south of Mt. Rainier, which is notorious for the eruption that produced the catastrophic mud avalanche of 1980. The northern, Canadian, sector is volcanically inactive. The Cascade Volcanic Arc is the result of the subduction of the Juan de Fuca Plate (part of the old Farallon Plate) along the Cascadia Subduction Zone, commencing in the late Eocene, and is all that remains of the active subduction system that followed the whole coast of North America during the Mesozoic (see Fig. 11.2).
The basement of the Cascade Arc consists of displaced terranes accreted to the North American continent during the Mesozoic, similar to those described in the Alaska–Canada sector of the orogen.
The Sierra Nevada Batholith
The Sierra Nevada Range is about 640km in length and 110km across, and includes several peaks over 4000m in height, including Mount Whitney (4421m), the highest peak in the United States south of Alaska (Fig. 11.10). Much of this range is carved out of the Sierra Nevada Batholith, which is an Upper Cretaceous plutonic complex formed at the time when the Farallon Plate was being subducted beneath the west coast, and similar to the Coast Plutonic Complex of Canada.
The Sierra Madre Occidental Volcanic Province
The mountains of the Sierra Madre Occidental form the central spine of Mexico from the US border to near the City of León, and form the western rim of the central Mexican plateau (Fig. 11.7). The range is everywhere over 2000m high, the highest peak being Cerro Mohinova, at 3300m. The volcanic province was initiated during the Eocene, after the Laramide event, due to the subduction of the Farallon Plate beneath the continental margin, at that time west of Baja California. This volcanic activity ceased in the late Eocene after the commencement of rifting in the Gulf of California, to be replaced in the south by the younger Trans-Mexican Volcanic Province, discussed in the next chapter.
Tectonic history
The western edge of the North American continent behaved as a passive margin throughout the Palaeozoic, accumulating large thicknesses of sediment deposited on the continental shelf and slope. Subduction of the Farallon Plate commenced in the early Jurassic, when several volcanic arcs and marginal basins were formed offshore. When the convergence pattern changed later in the Jurassic due to the opening of the Central Atlantic, the earlier-formed terranes were accreted to the continent, to be followed by further accretion during the early part of the Cretaceous. This accretionary zone formed the basement of the western half of the Cordillera.
In the late Cretaceous, a further change in the plate-tectonic regime caused an increase in convergence rate leading to a shallower subduction angle. This in turn gave rise to the thin-skinned Sevier fold-thrust belt, followed in the Eocene by the Laramide event, which resulted in the orogenic belt expanding further into the foreland. This period also saw the emplacement of the Sierra Nevada Batholith in the accretionary zone. During the Miocene, the over-thickened crust in the western part of the Cordillera experienced major extension and thinning, leading to the formation of the Basin and Range Province.
The most recent phase of volcanic activity commenced in the Eocene with the extrusion of the lavas of the Sierra Madre Occidental, followed by the voluminous Columbia River flows and the Cascade Arc. Activity continues at present in the Cascade Arc and at Yellowstone, at the eastern limit of the Columbia system.
Figure 11.10 Mount Whitney: the highest peak in the Sierra Nevada, carved from granite of the Sierra Nevada Batholith. © Shutterstock © Byron W. Moore.
