- How are Orogenies and Plate Tectonics Related?
- How are orogenies recorded in the geology of a region?
- What Can Rock Types Tell Us about Orogenies?
- What are Fold-And-Thrust Belts and Foreland Basins?
- How is Terrane Accretion Related to Orogeny?
- Glossary Terms
Orogeny means the creation of mountains. Prior to 1960, geologists could describe the uplift and erosion of mountains, based on the sequences of rock types and geologic structures that are evidence of previous orogenies. What was lacking, however, was an understanding of what caused pieces of the crust (and upper mantle) to come together and build up mountain ranges. Plate tectonic theory has provided that understanding and given us a clearer view of the relationship between geologic processes we can observe today and mountain-building events in the past.
An orogeny includes the following processes, which affect a large region or belt of crust, usually at or near the margin of a continent:
- igneous intrusion
Other geologic processes that are typically part of orogeny include uplift of rocks from the ocean floor into continental mountain ranges and accretion of exotic terranes.
Continents have grown through orogenies. Each orogeny has occurred along some part of a continental margin and added large volumes of new rock to the continent in the form of accreted terranes, igneous intrusions, and volcanic eruptions.
Orogenies are given names based on the area where they occur, the interval of time over which it took place, and the styles of crustal deformation and geologic structures that are most prominent. The Cordilleran Orogeny is the name given to all the mountain-building processes that have been occurring in western North America for the last 200 million years or so, since the mid-Triassic. The Cordilleran Orogeny can be broken down into more specific orogenies, which by definition are confined to smaller areas, or are limited to shorter intervals of time, or involve more specific styles of crustal deformation and geologic structures. The currently ongoing Cascadia orogeny, for example, involves the entire Cascade Range, the area just east of the Cascade Range that is undergoing uplift, folding and volcanism, and all the area west of the Cascade Range, including the Coast Ranges. The Cascadia orogeny began approximately 100 million years ago.
Plate tectonic theory and observations of orogenies in the geologic record indicate that most orogenies occur at convergent plate boundaries. Ocean-continent convergence leads to compression of the crust, thrusting, folding, faulting, volcanism on the surface and igneous intrusions within the crust, regional metamorphism of the crust, addition of oceanic crust to the orogenic belt, and accretion of exotic terranes.
Erosion attacks any land uplifted above sea level-the greater the uplift, the more energetic the erosion. The existence of a mountain range shows that the rate of uplift has exceeded the rate of erosion. If the mountain range is particularly high and jagged, it suggests that uplift may still be dominant over erosion in that area. After mountain-building forces subside, the mountains will gradually erode away. The high, jagged profile will become lower and more rounded. Eventually the mountains will be worn down into a low plain.
After an orogeny has ended, the length of time it takes for the mountains to disappear varies greatly, from a few million to more than 100 million years. Eventually, only geologic clues pointing to the former existence of mountains remain. Folds and thrust faults that formed during the orogeny create an enduring record of the event. Granites and other intrusive rocks associated with the cores of many mountain ranges will last longer than the mountains themselves.
The sedimentary rocks that formed on the periphery of the orogen (mountain-building zone) also preserve evidence of the orogeny. Sediments that are deposited close to the orogen are usually more coarse-grained than sediments transported farther away. On the mountains themselves few sediments are likely to form or be preserved, because the sediments tend to be removed from uplifted places and preserved in low places.
After the mountains have eroded away, the landscape is likely to be covered by younger sediments. This covering of the old eroded landscape creates a nonconformity--granites and/or metamorphic rocks covered by layers of younger sedimentary rock. When geologists see a large expanse of granites and metamorphic rocks beneath a nonconformity they realize they are looking at the remnants of an orogeny.
The region in which the nonconformity occurs is likely to be surrounded by geologic basins in which sediments eroded from the mountains are preserved. By determining the relative ages of the rocks and mapping the locations of the nonconformity and basins, geologists can identify when and where the mountains formed.
In addition to the stratigraphy and geologic structures, much of what we know of past orogenies is based on the rocks. They record the types of volcanism, intrusion, metamorphism, and sediment deposition that are characteristic of orogenies.
Plutonic and volcanic rocks are created during orogenies. The plutonic rocks will include the whole range of igneous compositions, from gabbro to granite, but will be predominantly in the intermediate-to-felsic range, with granodiorite and granite the most abundant. The presence of a group of plutons that intruded a large area of crust, forming a batholith, is a signature of an orogeny. Similarly for the volcanic rocks-every common volcanic rock from basalt to felsic tuff will be erupted during the course of an orogeny, but andesite will tend to be the most abundant rock type erupted during an orogeny.
Regional metamorphism will be widespread within the core of the mountains that develop during an orogeny, and rocks adjacent to the plutons that intrude the middle and upper levels of the crust will undergo contact metamorphism. Deep in the orogenic belt, the stress and high heat flow that are concentrated there will form schist and gneiss, high-grade metamorphic rocks.
On the seaward side of an orogeny, or in plate tectonic terms on the trench-facing side of a subduction zone, high-pressure, low-temperature regional metamorphism will take place. This happens to rocks that go part way down a subduction zone before being accreted to the continental margin and uplifted into the accretionary complex. Such high-pressure, low-temperature metamorphic rocks are classified as members of the blueschist group, because if basalt is metamorphosed in those conditions it will develop a blue color from the blue amphiboles that grow as it recrystallizes. The blue amphibole minerals only form in high-pressure, low-temperature conditions.
In sum, all kinds of metamorphic rocks develop during orogeny. Large volumes of schist and gneiss that form at high temperatures will core the main mountain-building belt. The lower mountains that developed on the seaward side of the orogenic belt may have regions of blueschist-high-pressure, low-temperature metamorphic rocks that are created within subduction zones.
During orogeny, the uplifting, eroding mountains shed sediments. These sediments accumulate in basins within or on the margins of the orogenic zone, and provide a record of the orogeny. Where the orogenic belt is primarily a volcanic arc, the sediments will be rich in eroded volcanic rock debris, and clay, which some volcanic rocks quickly become as they are exposed to air and water. Sandstones rich in eroded bits of volcanic rocks and clay are called graywackes. Graywackes are "dirty sandstones," the dirty appearance coming from the dark bits of weathered volcanic rock and the dark gray clay. Graywackes are a sign that a volcanic arc was active in the area at the time.
At a later stage in an orogeny, a volcanic arc undergoes continued uplift and erosion, which exposes the igneous and metamorphic rocks formed in the core of the mountains. The volcanic arc may shift to another location. If the volcanic arc shifts, the sediments being eroded from the mountains will no longer be dominated by volcanic debris. Instead, the eroded sediments will be rich in quartz and feldspar, the main minerals in granitic and gneissic rocks. The sandstones that form will be arkose, which is sandstone that consists mainly of quartz and lots of feldspar. Arkose marks the orogenic stage of uplift and erosion of the crystalline cores of mountain ranges, after the volcanic activity has died down or shifted to a different location.
Another group of sedimentary rocks that are typically involved in orogenies is those that form on the ocean floor. These are likely to include shales with marine fossils in them, marine sandstones (which accumulate in oceanic trenches or the slopes leading to them), limestone, and ribbon chert (layers of chert that accumulate on the deep ocean floor). These sedimentary rocks will become included in an orogenic belt by thrust faulting and accretion at the edge of the continent, which translates them from the floor of the ocean into the orogenic mountain ranges.
Lots of thrusting and folding occurs in the pre-existing continental crust during an orogeny, inland from the subduction zone and volcanic arc. This faulting and folding creates a thickened area of crust called a fold-and-thrust belt. The northen Rocky Mountains, including Glacier National Park, are an example of a fold-and-thrust belt. The area farther inland, beyond the range of folding and thrusting, is known as the foreland area. As a fold-and-thrust mountain range next to the foreland grows, the weight of the thickened crust will tend to depress the bordering foreland area into a basin, where sediments being shed from the mountain range accumulate. As a fold-and-thrust belt develops, it may advance into the foreland, and the previously unfolded rocks and sediments of the foreland basin may become uplifted and incorporated into the fold-and-thrust belt.
Accretion of exotic terranes is a standard part of most orogenies. The accreted terranes were either pieces of oceanic lithosphere from the plate that was subducting beneath the edge of the continent, or thicker sections of crust that were carried on the plate. .
It is also possible for two whole continents to collide as part of an orogeny. This is thought to have occurred during some of the orogenies that built the Appalachian region, with Europe and northwestern Africa colliding with what is now the eastern and southeastern United States. Full-scale continent-continent collisions are not thought to have taken place during the Cordilleran orogenies.
Once an exotic terrane has lodged in its new continental residence, it is likely to undergo many changes. The continuing orogeny is likely to bury it, metamorphose it, intrude it, fold it, and fault it. Large strike-slip faults may break the terrane up into several pieces that become separated from each other
The end result is a complicated jigsaw puzzle, with many of the pieces obscured by intrusion and metamorphism. This is a puzzle that geologists enjoy trying to solve. The next time you make a journey across an mountain range, such as a trip across the North Cascades in Washington state, or a trip across the Rocky Mountains in Montana, Wyoming or Colorado, take time to examine and think about the rocks you encounter on the way. You will agree that it is a puzzle. You may also find it fascinating and intriguing.
Glossary terms that appear on this page: plate tectonics; exotic terrane; accreted terrane; igneous intrusion; ocean-continent convergent plate boundary; compression; regional metamorphism; plutonic rock; volcanic rock; gabbro; granite; intermediate; felsic; granodiorite; batholith; basalt; tuff; andesite; contact metamorphism; stress; schist; gneiss; subduction zone; accretionary complex; blueschist; amphibole; mineral; volcanic arc; clay; sandstone; graywacke; quartz; feldspar; arkose; shale; limestone; chert; lithosphere; strike-slip fault
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes