Orogeny

Geologic provinces of the world (USGS)

  Shield

  Platform

  Orogen

  Basin

  Large igneous province

  Extended crust

Oceanic crust:

  0–20 Ma

  20–65 Ma

  >65 Ma

Orogeny is the primary mechanism by which mountains are built on continents. An orogeny is an event that takes place at convergent plate margins when plate motion compresses the margin. This leads to both structural deformation and compositional differentiation of the Earth's lithosphere (crust and uppermost mantle). An orogen or orogenic belt develops as the compressed plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis. A synorogenic process or event is one that occurs during an orogeny.

The word "orogeny" (/ɒrˈɔːdʒəni/) comes from Ancient Greek (ὄρος, óros, lit. ''mountain'' + γένεσις, génesis, lit. ''creation, origin''). Although it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain-building as distinguished from epeirogeny.

Physiography

See also: Subduction, Plate tectonics, and Continental collision

Two processes that can contribute to the formation of orogens. Top: delamination of orogenic roots into the asthenosphere; Bottom: Subduction of lithospheric plate to mantle depths. The two processes lead to differently located metamorphic rocks (bubbles in diagram), providing evidence as to which process actually occurred at convergent plate margins.

Subduction of an oceanic plate beneath a continental plate to form an accretionary orogen. (example: the Andes)

Continental collision of two continental plates to form a collisional orogen. Typically, continental crust is subducted to lithospheric depths for blueschist to eclogite facies metamorphism, and then exhumed along the same subduction channel. (example: the Himalayas)

Orogeny takes place on the convergent margins of continents. These may take the form of subduction (where a continent rides forcefully over an oceanic plate to form a noncollisional orogeny) or continental collision (convergence of two or more continents to form a collisional orogeny).

Orogeny typically produces orogenic belts or orogens, which are elongated regions of deformation bordering continental cratons. Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes. Older orogenic belts are typically deeply eroded to expose displaced and deformed strata. These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths.

Orogenic belts are associated with subduction zones, which consume crust, thicken lithosphere, and produce earthquakes and volcanoes. Not all subduction zones produce orogenic belts; mountain building takes place only when the subduction produces compression in the overriding plate. Whether subduction produces compression depends on such factors as the rate of plate convergence and the degree of coupling between the two plates, while the degree of coupling may in turn rely on such factors as the angle of subduction and rate of sedimentation in the oceanic trench associated with the subduction zone. The Andes Mountains are an example of a noncollisional orogenic belt, and such belts are sometimes called Andean-type orogens.

As subduction continues, island arcs, continental fragments, and oceanic material may gradually accrete onto the continental margin. This is one of the main mechanisms by which continents have grown. An orogen built of crustal fragments (terranes) accreted over a long period of time, without any indication of a major continent-continent collision, is called an accretionary orogen. The North American Cordillera and the Lachlan Orogen of southeast Australia are examples of accretionary orogens.

The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogen into a Himalayan-type collisional orogen. The collisional orogeny may produce extremely high mountains, as has been taking place in the Himalayas for the last 65 million years.

The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins. Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia (the ancient heart of North America) over the course of 200 million years in the Paleoproterozoic. The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny, lasting at least 600 million years. A similar sequence of orogenies has taken place on the west coast of North America, beginning in the late Devonian (about 380 million years ago) with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny. The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago.

Orogens

The Foreland Basin System

Orogens show a great range of characteristics, but they may broadly divided into collisional orogens and noncollisional orogens (Andean-type orogens). Collisional orogens can be further divided by whether the collision is with a second continent or a continental fragment or island arc. Repeated collisions of the later type, with no evidence of collision with a major continent or closure of an ocean basin, result in an accretionary orogen. Exmaples of orogens arising from collision of an island arc with a continent include Taiwan and the collision of Australia with the Banda arc. Origens arising from continent-continent collisions can be divided into those involving ocean closure (Himalayan-type orogens) and those involving glancing collisions with no ocean basin closure (as is taking place today in the Southern Alps of New Zealand).

Orogens have a characteristic structure, though this shows considerable variation. A foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of the mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine (flysch-style) through shallow water to continental (molasse-style) sediments.

Mountain building

An example of thin-skinned deformation (thrust faulting) of the Sevier Orogeny in Montana. Note the white Madison Limestone repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.

Sierra Nevada Mountains (a result of delamination) as seen from the International Space Station.

Mountain formation occurs through a number of mechanisms.

Mountain complexes result from irregular successions of tectonic responses due to sea-floor spreading, shifting lithosphere plates, transform faults, and colliding, coupled and uncoupled continental margins.

— Peter J Coney

Large modern orogens often lie on the margins of present-day continents; the Alleghenian (Appalachian) and Andean orogenies exemplify this in the Americas. Older inactive orogenies, such as the Algoman, Penokean and Antler, are represented by deformed and metamorphosed rocks with sedimentary basins further inland.

The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by the dense underlying mantle. Erosion of overlying strata in orogenic belts, and isostatic adjustment to the removal of this overlying mass of rock, can bring deeply buried strata to the surface. The erosional process is called unroofing and the resulting exposure of formerly deeply buried strata is called exhumation.

Areas that are rifting apart, such as mid-ocean ridges and the East African Rift, have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography. In strike-slip orogens, such as the San Andreas Fault, restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism.

Regions can also experience uplift as a result of delamination of the orogenic lithosphere, in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift. An example is the Sierra Nevada in California. This range of fault-block mountains experienced renewed uplift due to abundant magmatism after a delamination of the orogenic root beneath them.

Finally, uplift and erosion related to epeirogenesis (large-scale vertical motions of portions of continents without much associated folding, metamorphism, or deformation) can create local topographic highs.

Mount Rundle, Banff, Alberta.

Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where the edge of the uplifted layers are exposed.

Orogenic cycle

See also: Wilson cycle

Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including crustal deformation, crustal thickening, crustal thinning and crustal melting as well as magmatism, metamorphism and mineralization. What exactly happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis.

In addition to orogeny, the orogen (once formed) is subject to other processes, such as sedimentation and erosion. The sequence of repeated cycles of sedimentation, deposition and erosion, followed by burial and metamorphism, and then by crustal anatexis to form granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle. For example, the Caledonian Orogeny refers to a series of tectonic events due to the continental collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana in the Early Paleozoic. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle.

Assemblages of deformed and metamorphosed rocks commonly occur in collisional orogenic belts. Such lithological assemblages are part of nappe complexes that have been thrust over a continental margin during orogenesis and closure of an oceanic basin. Notably, slivers of basement rocks, remnants of rift basins, ophiolite and island-arc assemblages are commonly preserved in orogenic belts. Preserved rift basins may include lithological assemblages originally formed at proximal or distal segments of the rifted margin. In the Alps, for example, plate reconstructions based on the distribution and structure of serpentinite-bearing metasedimentary complexes suggested formation in distal ocean-continent transition zones prior to orogenesis and nappe thrusting. Alpine-type metasedimentary complexes have also been studied in the Scandinavian Caledonides and Pyrenees. The occurrence of rift basins in nappe complexes demonstrates that the tectonostratigraphy of orogenic belts is partly inherited from rifted margins following subduction and continental collision.

In summary, an orogeny is an episode of deformation, metamorphism and magmatism at convergent plate margins, during which many geological processes play a role. Every orogeny has its own orogenic cycle, but composite orogenesis is common at convergent plate margins.

Erosion

Erosion represents a subsequent phase of the orogenic cycle. Erosion inevitably removes much of the mountains, exposing the core or mountain roots (metamorphic rocks brought to the surface from a depth of several kilometres). Isostatic movements may help such exhumation by balancing out the buoyancy of the evolving orogen. Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics). Thus, the final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core.

An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults. These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism.

Biology

In the 1950s and 1960s the study of orogeny, coupled with biogeography (the study of the distribution and evolution of flora and fauna), geography and mid ocean ridges, contributed greatly to the theory of plate tectonics. Even at a very early stage, life played a significant role in the continued existence of oceans, by affecting the composition of the atmosphere. The existence of oceans is critical to sea-floor spreading and subduction.

History of the concept

Before the development of geologic concepts during the 19th century, the presence of marine fossils in mountains was explained in Christian contexts as a result of the Biblical Deluge. This was an extension of Neoplatonic thought, which influenced early Christian writers.

The 13th-century Dominican scholar Albert the Great posited that, as erosion was known to occur, there must be some process whereby new mountains and other land-forms were thrust up, or else there would eventually be no land; he suggested that marine fossils in mountainsides must once have been at the sea-floor. Orogeny was used by Amanz Gressly (1840) and Jules Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes. Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures created by and contained in orogenic belts. His theory essentially held that mountains were created by the squeezing of certain rocks. Eduard Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted James Dwight Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust (similar to tephrotectonics), or convection within the asthenosphere or mantle.

Gustav Steinmann (1906) recognised different classes of orogenic belts, including the Alpine type orogenic belt, typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure.

In terms of recognising orogeny as an event, Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which is still in use today, though commonly investigated by geochronology using radiometric dating.

Based on available observations from the metamorphic differences in orogenic belts of Europe and North America, H. J. Zwart (1967) proposed three types of orogens in relationship to tectonic setting and style: Cordillerotype, Alpinotype, and Hercynotype. His proposal was revised by W. S. Pitcher in 1979 in terms of the relationship to granite occurrences. Cawood et al. (2009) categorized orogenic belts into three types: accretionary, collisional, and intracratonic. Notice that both accretionary and collisional orogens developed in converging plate margins. In contrast, Hercynotype orogens generally show similar features to intracratonic, intracontinental, extensional, and ultrahot orogens, all of which developed in continental detachment systems at converged plate margins.

1. Accretionary orogens, which were produced by subduction of one oceanic plate beneath one continental plate for arc volcanism. They are dominated by calc-alkaline igneous rocks and high-T/low-P metamorphic facies series at high thermal gradients of >30 °C/km. There is a general lack of ophiolites, migmatites and abyssal sediments. Typical examples are all circum-Pacific orogens containing continental arcs.
2. Collisional orogens, which were produced by subduction of one continental block beneath the other continental block with the absence of arc volcanism. They are typified by the occurrence of blueschist to eclogite facies metamorphic zones, indicating high-P/low-T metamorphism at low thermal gradients of <10 °C/km. Orogenic peridotites are present but volumetrically minor, and syn-collisional granites and migmatites are also rare or of only minor extent. Typical examples are the Alps-Himalaya orogens in the southern margin of Eurasian continent and the Dabie-Sulu orogens in east-central China.

See also

• Biogeography – Study of the distribution of species and ecosystems in geographic space and through geological time
• Fault mechanics – Field of study that investigates the behavior of geologic faults
• Fold mountains – Mountains formed by compressive crumpling of the layers of rock
• Guyot – Isolated, flat-topped underwater volcano mountain
• List of orogenies – Known mountain building events of the Earth's history
• Mantle convection – Gradual movement of the planet's mantle
• Tectonic uplift – Geologic uplift of Earth's surface that is attributed to plate tectonics
• Epeirogenic movement – Upheavals or depressions of land exhibiting long wavelengths and little folding

Literature

• The Broken Earth trilogy In this science fantasy series, "orogeny" is the fantastical ability to manipulate the ground, including the power to cause or prevent earthquakes. "Orogenes" are people with this ability.

References

PA Allen (1997). "Isostasy in zones of convergence". Earth Surface Processes. Wiley-Blackwell. pp. 36 ff. ISBN 978-0-632-03507-6.

Gerard V. Middleton; Peter R. Wilcock (1994). "§5.5 Isostasy". Mechanics in the Earth and Environmental Sciences (2nd ed.). Cambridge University Press. p. 170. ISBN 978-0-521-44669-3.

Sagripanti, Lucía; Bottesi, Germán; Kietzmann, Diego; Folguera, Andrés; Ramos, Víctor A. (May 2012). "Mountain building processes at the orogenic front. A study of the unroofing in Neogene foreland sequence (37ºS)". Andean Geology. 39 (2): 201–219. doi:10.5027/andgeoV39n2-a01.

Lee, C.-T.; Yin, Q; Rudnick, RL; Chesley, JT; Jacobsen, SB (2000). "Osmium Isotopic Evidence for Mesozoic Removal of Lithospheric Mantle Beneath the Sierra Nevada, California" (PDF). Science. 289 (5486): 1912–16. Bibcode:2000Sci...289.1912L. doi:10.1126/science.289.5486.1912. PMID 10988067. Archived from the original (PDF) on 15 June 2011.

John Gerrard (1990). Mountain Environments: An Examination of the Physical Geography of Mountains. MIT Press. p. 9. ISBN 978-0-262-07128-4.

Manley, Curtis R.; Glazner, Allen F.; Farmer, G. Lang (2000). "Timing of Volcanism in the Sierra Nevada of California: Evidence for Pliocene Delamination of the Batholithic Root?". Geology. 28 (9): 811. Bibcode:2000Geo....28..811M. doi:10.1130/0091-7613(2000)28<811:TOVITS>2.0.CO;2.

Arthur Holmes; Doris L. Holmes (2004). Holmes Principles of Physical Geology (4th ed.). Taylor & Francis. p. 92. ISBN 978-0-7487-4381-0.

"The Formation of the Rocky Mountains". Mountains in Nature. n.d. Retrieved 29 January 2014.

David Johnson (2004). "The orogenic cycle". The geology of Australia. Cambridge University Press. pp. 48 ff. ISBN 978-0-521-84121-4.

In other words, orogeny is only a phase in the existence of an orogen. Five characteristics of the orogenic cycle are listed by Robert J. Twiss; Eldridge M. Moores (1992). "Plate tectonic models of orogenic core zones". Structural Geology (2nd ed.). Macmillan. p. 493. ISBN 978-0-7167-2252-6.

However, this orogen was superimposed by rifting orogeny at a later time to result in various extents of reworking. N. H. Woodcock; Robin A. Strachan (2000). "Chapter 12: The Caledonian Orogeny: A Multiple Plate Collision". cited work. pp. 187 ff. ISBN 978-0-632-03656-1.

Reston T.J.; Manatschal G. (2011). "Arc-Continent Collision". In Brown D. & Ryan P.D. (ed.). Building blocks of later collision. Frontiers in Earth Sciences.

Jakob J.; Andersen T.B.; Kjøll H.J. (2019). "A review and revision of the rift-inherited architecture of the south and central Scandinavian Caledonides: a magma-poor to magma-rich transition and the significance of reactivation of rift inheritance during the Caledonian Orogeny". Earth Science Review. doi:10.1016/j.earscirev.2019.01.004. hdl:10852/70340. {{cite journal}}: Cite journal requires |journal= (help)

Manatschal G.; Müntener O. (2009). "A type sequence across an ancient magma-poor ocean–continent transition: the example of the western Alpine Tethys ophiolites". Tectonophysics. 473 (1): 4. Bibcode:2009Tectp.473....4M. doi:10.1016/j.tecto.2008.07.021.

Andersen T.B.; Corfu F.; Labrousse L.; Osmundse P.T. (2012). "Evidence for hyperextension along the pre-Caledonian margin of Baltica". Journal of the Geological Society. 169 (5). Journal of the Geological Society, London: 601. Bibcode:2012JGSoc.169..601A. doi:10.1144/0016-76492012-011. S2CID 73530877.

Clerc C.; Boulvais P.; Lagabrielle Y.; Saint Blanquat .M (2013). "Ophicalcites from the northern Pyrenean belt: a field, petrographic and stable isotope study" (PDF). International Journal of Earth Sciences. 103 (1): 141. Bibcode:2014IJEaS.103..141C. doi:10.1007/s00531-013-0927-z. S2CID 53617225.

Olivier Merle (1998). "§1.1 Nappes, overthrusts and fold-nappes". Emplacement Mechanisms of Nappes and Thrust Sheets. Petrology and Structural Geology. Vol. 9. Springer. pp. 1 ff. ISBN 978-0-7923-4879-5.

For example, see Patrick L Osborne (2000). Tropical Ecosystems and Ecological Concepts. Cambridge University Press. p. 11. Bibcode:2000teec.book.....O. ISBN 978-0-521-64523-2. Continental drift and plate tectonics help to explain both the similarities and the differences in the distribution of plants and animals over the continents and John C Briggs (1987). Biogeography and Plate Tectonics. Elsevier. p. 131. ISBN 978-0-444-42743-4. It will not be possible to construct a thorough account of the history of the southern hemisphere without the evidence from both the biological and the earth sciences

Paul D. Lowman (2002). "Chapter 7: Geology and biology: the influence of life on terrestrial geology". Exploring Space, Exploring Earth: New Understanding of the Earth from Space Research. Cambridge University Press. pp. 286–87. Bibcode:2002esee.book.....L. ISBN 978-0-521-89062-5.

Seema Sharma (2005). "Atmosphere: origin". Encyclopaedia of Climatology. Anmol Publications PVT. LTD. pp. 30 ff. ISBN 978-81-261-2442-8.

Zwart, HJ (1967). "The duality of orogenic belts". Geol. Mijnbouw. 46: 283–309.

Pitcher, WS (1979). "The nature, ascent and emplacement of granitic magmas". Journal of the Geological Society. 136 (6): 627–62. Bibcode:1979JGSoc.136..627P. doi:10.1144/gsjgs.136.6.0627. S2CID 128935736.

Cawood, PA; Kroner, A; Collins, WJ; Kusky, TM; Mooney, WD; Windley, BF (2009). Accretionary orogens through Earth history. Geological Society. pp. 1–36. Special Publication 318.

Sources

• Élie de Beaumont, JB (1852). Notice sur les Systèmes de Montagnes [Note on Mountain Systems] (in French). Paris: Bertrand. English synopsis in Dennis (1982){{cite book}}: CS1 maint: postscript (link)
• Buch, L. Von (1902). Gesammelte Schriften (in German). Berlin: Roth & Eck.
• Dana, James D. (1873). "On Some Results of the Earth's Contraction From Cooling, Including a Discussion of the Origins of Mountains, and the Nature of the Earth's Interior". American Journal of Science. 5 (30): 423–43. Bibcode:1873AmJS....5..423D. doi:10.2475/ajs.s3-5.30.423. S2CID 131423196.
• Dennis, John G. (1982). Orogeny. Benchmark Papers in Geology. Vol. 62. New York: Hutchinson Ross Publishing Company. ISBN 978-0-87933-394-2.
• Hall, J (1859). "Palaeontology of New York". New York National Survey. 3 (1).
• Suess, Eduard (1875). Die Entstehung Der Alpen [The Origin of the Alps]. Vienna: Braumüller.
• Harms; Brady; Cheney (2006). Exploring the Proterozoic Big Sky Orogeny in Southwest Montana. 19th annual Keck symposium.
• Kevin Jones (2003). Mountain Building in Scotland: Science : A Level 3 Course Series. Open University Worldwide Ltd. ISBN 978-0-7492-5847-4. provides a detailed history of a number of orogens, including the Caledonian Orogeny, which lasted from the late Cambrian to the Devonian, with the main collisional events occurring during Ordovician and Silurian times.
• Tom McCann, ed. (2008). Precambrian and Palaeozoic. The Geology of Central Europe. Vol. 1. Geological Society of London. ISBN 978-1-86239-245-8. is one of a two-volume exposition of the geology of central Europe with a discussion of major orogens.
• Suzanne Mahlburg Kay; Víctor A. Ramos; William R. Dickinson, eds. (2009). Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision; Memoir 204. Geological Society of America. ISBN 978-0-8137-1204-8. Evolution of the Cordilleras of the Americas from a multidisciplinary perspective from a symposium held in Mendoza, Argentina (2006).

External links

• Maps of the Acadian and Taconic orogenies
• Antarctic Geology