DEFINITION OF ROCKY MOUNTAINS

Definition of Rocky Mountains

Extending 3,000 miles (4,800 km) from central New Mexico to northwest Alaska in the easternmost

Cordillera, the Rocky Mountains are one of the largest

mountain belts in North America. The mountains are situated

between the Great Plains on the east and a series of

plateaus and broad basins on the west. Mount Elbert in Colorado

is the highest mountain in the range, reaching 14,431

feet (4,399 m). The continental divide is located along the

rim of the Rockies, separating waters that flow to the Pacific

and the Atlantic Oceans. The Rocky Mountains are divided

into the Southern, Central, and Northern Rockies in the United

States, Canadian Rockies in Canada, and the Brooks

Range in Alaska. Several national parks are located in the

system, including Rocky Mountain, Yellowstone, Grand

Teton, and Glacier Bay National Parks in the United States,

and Banff, Glacier, Yoho, Kootenay, and Mount Revelstoke

in Canada. The mountains were a major obstacle to traveling

west during the expansion of the United States, but western

regions opened up when the Oregon trail crossed the ranges

through South Pass in Wyoming.

In New Mexico, Colorado, and southern Wyoming the

Southern Rockies consist of two north-south ranges of folded

mountains that have been eroded to expose Precambrian

cores with overlying sequences of layered sedimentary rocks.

Three basins are located between these ranges, known as the

North, South, and Middle parks. The Southern Rockies are

the highest section of the whole range including many peaks

more than 14,000 feet (4,250 m).

The Middle Rockies in northeastern Utah and western

Wyoming are lower and more discontinuous than the southern

Rockies. Most are eroded down to their Precambrian

cores, surrounded by Paleozoic-Mesozoic sedimentary rocks.

Garnet Peak in the Wind River Range (13,785 feet; 4,202 m)

and Grand Teton in the Teton Range (13,766 feet; 4,196 m)

are the highest peaks in the Central Rockies.

The Northern Rockies in northeastern Washington,

Idaho, and western Wyoming extend from Yellowstone

National Park to the Canadian border. This section is dominated

by north-south trending ranges separated by narrow

valleys, including the Rocky Mountain trench, an especially

deep and long valley that extends north from Flathead Lake.

The highest peaks in the Northern Rockies include Borah

Peak (12,655 feet; 3,857 m) and Leatherman Peak (12,230

feet; 3,728 m) in the Lost River Range.

The Canadian Rockies stretch along the British

Columbia–Alberta border and reach their highest point in

Canada on Mount Robson (12,972 feet; 3,954 m). The

Rocky Mountain trench continues 800 miles (1,290 km)

north-northwest from Montana, becomes more pronounced

in Canada, and is joined by the Purcell trench in Alberta. In

the Northwest Territories (Nunavet) the Rockies expand

northeastward in the Mackenzie and Franklin mountains,

and near the Beaufort Sea pick up as the Richardson Mountains

that gain elevation westward into the Brooks Range of

Alaska. Mount Chamberlin (9,020 feet; 2,749 m) is the highest

peak in the Brooks Range.

The Rocky Mountains are rich in mineral deposits,

including gold, silver, lead, zinc, copper, and molybdenum.

Principal mining areas include the Butte-Anaconda district of

Montana, Leadville and Cripple Creek in Colorado, Coeur

d’Alene in Idaho, and the Kootenay Trail region of British

Columbia. Lumbering is an active industry in the mountains

but is threatened by growing environmental concerns and

tourism in the National Park Systems.

Mesozoic–Early Cenozoic contractional events produced

the Rockies during uplift associated with the Cordilleran

orogeny. Evidence for older events and uplifts are commonly

referred to as belonging to the Ancestral Rocky Mountain

System. The Rocky Mountains are part of the larger

Cordilleran orogenic belt that stretches from South America

through Canada to Alaska, and it is best to understand the

evolution of the Rockies through a wider discussion of events

in this mountain belt. The Cordillera is presently active and

has been active for the past 350 million years, making it one

of the longest-lived orogenic belts on Earth. In the Cordillera,

many of the structures are not controlled by continent/continent

collisions as they are in many other mountain belts, since

the Pacific Ocean is still open. In this orogen structures are

controlled by the subduction/accretion process, collision of

arcs, islands, oceanic plateaus, and strike-slip motions parallel

to the mountain belt. Present-day motions and deformation

are controlled by complex plate boundaries between the

North American, Pacific, Gorda, Cocos, and some completely

subducted plates such as the Farallon. In this active tectonic

setting the style, orientation, and intensity of deformation and

magmatism depend largely on the relative convergence–strikeslip

vectors of motion between different plates.

The geologic history of the North American Cordillera

begins with rifting of the present western margin of North

America at 750–800 million years ago, which is roughly the

same age as rifting along the east coast in the Appalachian

orogen. These rifting events reflect the breakup of the supercontinent

of Rodinia at the end of the Proterozoic, and they

left North America floating freely from the majority of the

continental landmass on Earth. Rifting, and the subsequent

thermal subsidence of the rifted margin, led to the deposition

of Precambrian clastic rocks of the Windemere Supergroup,

and carbonates of the Belt and Purcell Supergroups, in belts

stretching from Southern California and Mexico to Canada.

These are overlain by Cambrian-Devonian carbonates, Carboniferous

clastic wedges, and Carboniferous-Permian carbonates,

then finally Mesozoic clastic rocks.

The Antler orogeny is a Late Devonian–Early Carboniferous

(350–400-million-year-old) tectonic event formed during

an arc-continent collision, in which deepwater clastic

rocks of the Robert’s Mountain allochthon in Nevada were

thrust from west to east over the North American carbonate

bank, forming a foreland basin that migrated onto the craton.

This orogenic event, similar to the Taconic orogeny in

the Appalachian mountains, marks the end of passive margin

sedimentation in the Cordillera, and the beginning of

Cordilleran tectonism.

In the Late Carboniferous (about 300 million years ago),

the zone of active deformation shifted to the east with a zone

of strike-slip faulting, thrusts, and normal faults near Denver.

Belts of deformation formed what is known as the ancestral

Rocky Mountains, including the Front Ranges in Colorado

and the Uncompahgre uplift of western Colorado, Utah, and

New Mexico. These uplifts are only parts of a larger system of

strike-slip faults and related structures that cut through the

entire North American craton in the Late Carboniferous,

probably in response to compressional deformation that was

simultaneously going on along three margins of the continent.

The Late Permian–Early Triassic Sonoma orogeny

(260–240 million years ago) refers to events that led to the

thrusting of deepwater Paleozoic rocks of the Golconda

allochthon eastward over autochthonous shallow water sediments

just outboard (oceanward) of the Robert’s Mountain

allochthon. The Golconda allochthon in western Nevada

includes deepwater oceanic pelagic rocks, an island arc

sequence, and a carbonate shelf sequence, and it is interpreted

to represent an arc/continent collision.

In the Late Jurassic (about 150 million years ago) a new,

northwest-striking continental margin was established by

crosscutting the old northeast striking continental margin.

This event is known as the early Mesozoic truncation event

and reflects the start of continental margin volcanic and plutonic

activity that continues to the present day. There is considerable

uncertainty about what happened to the former

extension of the old continental margin—it may have rifted

and drifted away or may have moved along the margin along

large strike-slip faults.

Pacific margin magmatism has been active intermittently

from the Late Triassic (220 million years ago) through the Late

Cenozoic and in places continues to the present. This magmatism

and deformation is a direct result of active subduction

and arc magmatism. Since the Late Jurassic, there have been

three main periods of especially prolific magmatism including

the Late Jurassic/Early Cretaceous Nevadan orogeny (150–130

million years ago), the Late Cretaceous Sevier orogeny (80–70

million years ago), and the Late Cretaceous/Early Cenozoic

Laramide orogeny (66–50 million years ago).

Cretaceous events in the Cordillera resulted in the formation

of a number of tectonic belts that are still relatively

easy to discern. The Sierra Nevada ranges of California and

Nevada represent the arc batholith and contain high-temperature,

low-pressure metamorphic rocks characteristic of arcs.

The Sierra Nevada is separated from the Coast Ranges by

flat-lying generally unmetamorphosed sedimentary rocks of

the Great Valley, deposited over ophiolitic basement in a forearc

basin. The Coast Ranges include high-pressure, low-temperature

metamorphic rocks, including blueschists in the

Franciscan complex. Together, the high-pressure, low-temperature

metamorphism in the Franciscan complex, with the

high-temperature, low-pressure metamorphism in the Sierra

Nevada, represents a paired metamorphic belt, diagnostic of

a subduction zone setting.

Several Cretaceous foreland fold-thrust belts are preserved

east of the magmatic belt in the Cordillera, stretching

from Alaska to Central America. These belts include the

Sevier fold-thrust belt in the United States, the Canadian

Rockies fold-thrust belt, and the Mexican fold-thrust belt.

They are all characterized by imbricate-style thrust faulting,

with fault-related folds dominating the topographic expression

of deformation.

The Late Cretaceous–Early Tertiary Laramide orogeny

(about 70–60 million years ago) is surprisingly poorly understood

but generally interpreted as a period of plate reorganization

that produced a series of basement uplifts from

Montana to Mexico. Some models suggest that the Laramide

orogeny resulted from the subduction of a slab of oceanic

lithosphere at an unusually shallow angle, perhaps related to

its young age and thermal buoyancy.

The late Mesozoic-Cenozoic tectonics of the Cordillera

saw prolific strike-slip faulting, with relative northward displacements

of terranes along the western margin of North

America. The San Andreas Fault system is one of the major

transform faults formed in this interval as a consequence of

the subduction of the Farallon plate. Previous convergence

between the Farallon and North American plates stopped

when the Farallon was subducted, and new relative strike-slip

motions between the Pacific and North American plates

resulted in the formation of the San Andreas system. Remnants

of the Farallon plate are still preserved as the Gorda

and Cocos plates.

Approximately 15 million years ago the Basin and Range

Province and the Colorado Plateau began uplifting and

extending through the formation of rifts and normal faults.

Much of the Colorado Plateau stands at more than a mile

(1.5–2.0 km) above sea level but has a normal crustal thickness.

The cause of the uplift is controversial but may be related

to heating from below. The extension is related to the

height of the mountains being too great for the strength of

the rocks at depth to support it, so gravitational forces are

able to cause high parts of the crust to extend through the

formation of normal faults and rift basins.

See also BROOKS RANGE; CONVERGENT PLATE MARGIN

PROCESSES; JAPAN’S PAIRED METAMORPHIC BELT.

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