Orogeny
Orogeny (Greek for "mountain generating") is the process of
mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust and happen within a time frame.
Orogenic events occur solely as a result of the processes of
plate tectonics; the problems which were investigated and resolved by the study of orogenesis contributed greatly to the theory of plate tectonics, coupled with study of flora and fauna,
geography and
mid ocean ridges in the 1950s and 1960s.
The physical manifestations of orogenesis, the process of orogeny, are
orogenic belts or
orogens. An orogen is different from a mountain range in that an orogen may be completely
eroded away, and only recognizable by studying (old) rocks that bear the traces of the orogeny. Orogens are usually long, thin, arcuate tracts of rocks which have a pronounced linear structure resulting in
terranes or blocks of deformed rocks, separated generally by
dipping thrust faults. These thrust faults carry relatively thin plates (which are called
nappes, and are different to
tectonic plates) of rock in from the margins of the compressing orogen to the core, and are intimately associated with
folds and the development of
metamorphism.
The topographic height of orogenic mountains is related to the principle of
isostasy, where the gravitational force of the upthrust mountain range of light, continental crust material is balanced against its buoyancy relative to the dense
mantle.
Erosion inevitably takes its course, removing much of the mountains, leaving the core or
mountain roots, which may be exhumed by further isostasy events balancing out the loss of elevated mass. This is the final form of the majority of old orogenic belts, being a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and dip away from the orogenic core.
Orogeny was used by Gressly (1840) and 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 orogenic belts created and contained. His theory essentially held that mountains were created by the squeezing of certain rocks.
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
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, contested hotly by proponents of vertical movements in the crust (similar to
tephrotectonics), or convection within the
asthenosphere or
mantle (geology).
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.
Zwart (1967) drew attention to the metamorphic differences in orogenic belts, proposing three types, modified by Pitcher (1979);
* Hercynotype (
back-arc basin type);
** Shallow, low-pressure
metamorphism; thin metamorphic zones
** Metamorphism dependent on increase in temperature
** Abundant
granite and
migmatite** Few
ophiolites,
ultramafic rocks virtually absent
** very wide orogen with small and slow uplift
**
nappe structures rare
* Alpinotype (ocean trench style);
** deep, high pressure, thick metamorphic zones
** metamorphism of many facies, dependent on decrease in pressure
** few granites or migmatites
** abundant ophiolites with ultramafic rocks
** Relatively narrow orogen with large and rapid uplift
** Nappe structures predominant
* Cordilleran (arc) type;
** dominated by calc-alkaline igneous rocks,
andesites,
granite batholiths
** general lack of
migmatites, low geothermal gradient
** lack of
ophiolite and abyssal sedimentary rocks (black
shale,
chert, etcetera)
** low-pressure metamorphism, moderate uplift
** lack of
nappes
The advent of plate tectonics has explained the vast majority of orogenic belts and their features. The cooling earth theory (principally advanced by
Descartes) is dispensed with, and tephrotectonic style vertical movements have been explained primarily by the process of
isostasy.
Some oddities exist, where simple collisional tectonics are modified in a transform plate boundary, such as in
New Zealand, or where island arc orogenies, for instance in
New Guinea occur away from a continental backstop. Further complications such as Proterozoic continent-continent collisional orogens, explicitly the
Musgrave Block in Australia, previously inexplicable (see Dennis, 1982) are being brought to light with the advent of seismic imaging techniques which can resolve the deep crust structure of orogenic belts.
The process of orogeny can take tens of millions of years and build mountains from plains or even the ocean floor. Orogeny can occur due to
continental collision or
volcanic activity. Frequently, rock formations that undergo orogeny are severely deformed and undergo
metamorphism. During orogeny, deeply buried rocks may be pushed to the surface. Sea bottom and near shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, the resulting mountains can be very high (see
Himalaya).
Orogeny usually produces long linear structures, known as
orogenic belts. Generally, orogenic belts consist of long parallel strips of
rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with
subduction zones, which consume
crust, produce volcanoes, and build
island arcs. These island arcs may be added to a
continent during an orogenic event.
 |
Taconic orogeny |
*
Caledonian orogeny** the
Taconic phase in the NE U.S. and Canada during the
Ordovician Period.
** the
Acadian phase in the Eastern U.S. during
Silurian and
Devonian Periods.
*
Antler orogeny** Ancestral
Sierra Nevada western
United States.
*
Appalachian orogeny, usually seen as the same as the
Variscan orogeny in Europe.
**
Appalachian Mountains, is a well studied orogenic belt resulting from a late
Paleozoic collision between
North America and
Africa.
*
Grenville orogeny** Worldwide during the late
Proterozoic, 1300-1000 mya. Associated with the assembly of the supercontinent
Rodinia. Formed folded mountains in Eastern North America from
Newfoundland to
North Carolina, 1100-1000 mya.
*
Laramide orogeny**
Rocky Mountains, western North America, 40-70 Myr ago.
*
Nevadan orogeny** developed along western North America during the
Jurassic Period.
*
Ouachita orogeny**
Ouachita Mountains of
Arkansas and
Oklahoma is an orogenic belt that dates from the late
Paleozoic Era and is most likely a continuation of the
Appalachian orogeny west across the
Mississippi embayment -
Reelfoot Rift zone.
*
Penokean orogeny**
Wisconsin,
Minnesota, and
Michigan, U. S. A. and southern
Ontario,
Canada, 1900 Myr ago.
*
Sevier orogeny**Rocky Mountains, western North America, 140 - 50 million years ago.
*
Trans-Hudsonian orogeny**Extends from
Hudson Bay west into
Saskatchewan then south through the western Dakotas and Nebraska. Result of the collision of the
Superior craton with the
Hearne craton and the
Wyoming craton during the
Proterozoic.
*The
Caledonian orogeny** Formation of the highlands of west
Norway,
Britain and
Ireland in the
Silurian Period.
*
Uralian orogeny** Formation of the
Ural Mountains,
Eurasia, during the
Permian Period.
*The
Variscan orogeny (also called the Hercynian orogeny)
** Formation of the mountains of western
Iberia, SW
Ireland, SW
England central
France, southern
Germany and
Czechoslovakia during the
Devonian and
Carboniferous Periods.
*The
Alpine orogeny, encompassing:
** the Formation of the
Alps during the
Eocene through
Miocene Periods.
** the
Carpathean orogeny building the
Carpathian Mountains of east Europe during the
Miocene Period.
** the
Hellenic orogeny in
Greece and
Aegean area during
Eocene through
Miocene Period.
* The
Aravalli-Delhi Orogen (
precambrian)
* The
Cimmerian and
Cathayasian orogenies
**Active through
Triassic and
Jurassic Periods along south and southeast
Asia.
*
Alpine orogeny, encompassing:
** The
Himalayan orogeny, forming the
Himalaya Mountains, as a result of the ongoing collision of the
Indian Plate with the
Eurasian Plate.
*
Andean orogeny**
Andes Mountains, 0-200 Myr ago.
*
Pan-African orogeny (
Neoproterozoic)
* Sleaford Orogeny (2440-2420 Ma),
Gawler Craton,
South Australia* Glenburgh Orogeny (c. 2005 - 1920 Ma), Glenburgh Terrane,
Western Australia.
* Kimban Orogeny (c. 1845-1700 Ma),
Gawler Craton, South Australia
* Yapungku Orogeny (c. 1700 Ma), North
Yilgarn craton margin, Western Australia
* Mangaroon Orogeny (c.1680 - 1620 Ma),
Gascoyne Complex, Western Australia.
* Kararan Orogeny (1650- Ma),
Gawler Craton, South Australia
* Barramundi Orogeny (c. 1600 Ma), MacArthur Basin, northern Australia
* Isan Orogeny, c. 1600 Ma,
Mt Isa Block,
Queensland* Olarian Orogeny, Olary Block, South Australia
* Capricorn Orogeny,
Gascoyne Complex, Western Australia
* Musgrave Orogeny (c. 1080 Ma),
Musgrave Block, Central Australia.
* Edmundian Orogeny (c. 920 - 850 Ma),
Gascoyne Complex, Western Australia.
*
Petermann Orogeny (c. 510 Ma
Permian),
Central Australia *
Delamerian Orogeny,
South Australia and
Victoria, Australia,
Ordovician*
Lachlan Orogeny, c. 540 and 440 Ma.,
Victoria and
New South Wales*
Alice Springs orogeny in central
Australia, Early
Carboniferous*
Hunter-Bowen Orogeny, (c. 260 - 225 Ma)
Permian to
Triassic, Queensland and New South Wales
* Napier orogeny (4000 ± 200 Myr ago.)
* Rayner orogeny (~ 3500 Myr ago.)
* Humboldt orogeny (~ 3000 Myr ago.)
* Insel orogeny (2650 ± 150 Myr ago.)
* Early Ruker orogeny (2000 - 1700 Myr ago.)
* Late Ruker / Nimrod orogeny (1000 ± 150 Myr ago.)
* Beardmore orogeny (633 - 620 Myr ago.)
* Ross Orogeny (~ 500 Myr ago.)
*
Continental collision*
Maps of the Acadian and Taconic orogenies*
Antarctic GeologyL. Elie de Beaumont, 1852.
Notice sur les Systèmes de Montagnes lit
Note on Mountain Systems, Bertrand, Paris, 1543 p. (English synopsis in Dennis (1982))
Buch, L. Von, 1902.
Gesammelte Schriften, Roth & Eck, Berlin.
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, pp. 423-443.
Dennis, John G., 1982.
Orogeny, Benchmark Papers in Geology, Volume 62, Hutchinson Ross Pulishing Company, New York ISBN 0-87933-394-4
Hall, J., 1859.
Palaeontology of New York, in New York National Survey No. 3, Part 1, 533 p.
Suess, Eduard, 1875.
Die Entstehung Der Alpen lit.
The Origin Of The Alps, Braumüller, Vienna, 168 p.
