Tectonic uplift

Tectonic uplift is the portion of the total geologic uplift of the mean Earth surface that is not attributable to an isostatic response to unloading. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening (such as mountain building events), changes in the density distribution of the crust and underlying mantle, and flexural support due to the bending of rigid lithosphere.

One should also take into consideration the effects of denudation (processes that wear away the earth's surface). Within the scope of this topic, uplift relates to denudation in that denudation brings buried rocks closer to the surface. This process can redistribute large loads from an elevated region to a topographically lower area as well – thus promoting an isostatic response in the region of denudation (which can cause local bedrock uplift). The timing, magnitude, and rate of denudation can be estimated by geologists using pressure-temperature studies.

Kupe's Sail-20070331
Kupe's Sail at Palliser Bay in New Zealand

Crustal thickening

Crustal thickening has an upward component of motion and often occurs when continental crust is thrust onto continental crust. Basically nappes (thrust sheets) from each plate collide and begin to stack one on top of the other; evidence of this process can be seen in preserved ophiolitic nappes (preserved in the Himalaya), and in rocks with an inverted metamorphic gradient. The preserved inverted metamorphic gradient indicates that nappes were actually stacked on top of each other so quickly, that hot rocks did not have time to equilibrate before being thrust on top of cool rocks. The process of nappe stacking can only continue for so long, as gravity will eventually disallow further vertical growth (there is an upper limit to vertical mountain growth).

Density distribution of the crust and underlying mantle

Although the raised surfaces of mountain ranges mainly result from crustal thickening, there are other forces at play that are responsible for the tectonic activity. All tectonic processes are driven by gravitational force when density differences are present. A good example of this would be the large-scale circulation of the Earth's mantle. Lateral density variations near the surface (such as the creation, cooling, and subduction of oceanic plates) also drive plate motion.

The dynamics of mountain ranges are governed by differences in the gravitational potential energy of entire columns of the lithosphere (see isostasy). If a change in surface height represents an isostatically compensated change in crustal thickness, the rate of change of potential energy per unit surface area is proportional to the rate of increase of average surface height. The highest rates of working against gravity are required when the thickness of the crust (not the lithosphere) changes.[1]

Lithospheric flexure

Lithosphere on the oceanward side of an oceanic trench at a subduction zone will curve upwards due to the elastic properties of the Earth's crust.

Orogenic uplift

Orogenic uplift is the result of tectonic-plate collisions and results in mountain ranges or a more modest uplift over a large region. Perhaps the most extreme form of orogenic uplift is a continental-continental crustal collision. In this process, two continents are sutured together and large mountain ranges are produced. The collision of the Indian and Eurasian plates is a good example of the extent to which orogenic uplift can reach. Heavy thrust faulting (of the Indian plate beneath the Eurasian plate) and folding are responsible for the suturing together of the two plates.[2] The collision of the Indian and Eurasian plates not only produced the Himalaya but is also responsible for crustal thickening north into Siberia.[3] The Pamir Mountains, Tian Shan, Altai, Hindu Kush, and other mountain belts are all examples of mountain ranges formed in response to the collision of the Indian with the Eurasian plate. Deformation of continental lithosphere can take place in several possible modes.

The Ozark Plateau is a broad uplifted area which resulted from the Permian Ouachita Orogeny to the south in the states of Arkansas, Oklahoma and Texas. Another related uplift is the Llano Uplift in Texas, a geographical location named after its uplift features.

The Colorado Plateau which includes the Grand Canyon is also the result of broad tectonic uplift followed by river erosion.[4]

Isostatic uplift

The removal of mass from a region will be isostatically compensated by crustal rebound. If we take into consideration typical crustal and mantle densities, erosion of an average 100 meters of rock across a broad, uniform surface will cause the crust to isostatically rebound about 85 meters and will cause only a 15-meter loss of mean surface elevation.[5] An example of isostatic uplift would be post-glacial rebound following the melting of continental glaciers and ice sheets. The Hudson Bay region of Canada, the Great Lakes of Canada and the United States, and Fennoscandia are currently undergoing gradual rebound as a result of the melting of ice sheets 10,000 years ago.

Crustal thickening, which for example is currently occurring in the Himalaya due to the continental collision between the Indian and the Eurasian plates, can also lead to surface uplift; but due to the isostatic sinking of thickened crust, the magnitude of surface uplift will only be about one-sixth of the amount of crustal thickening. Therefore, in most convergent settings isostatic uplift plays a relatively small role and high peak formation can be more attributed to tectonic processes.[6] Direct measures of the elevation change of the land surface can only be used to estimate erosion or bedrock uplift rates when other controls (such as changes in mean surface elevation, volume of eroded material, timescales and lags of isostatic response, variations in crustal density) are known.

Coral islands

In a few cases, tectonic uplift can be seen in the cases of coral islands. This is evidenced by the presence of various oceanic islands composed entirely of coral, which otherwise appear to be high islands (i.e., islands of volcanic origin). Examples of such islands are found in the Pacific, notably the three phosphate islets, Nauru, Makatea, and Banaba as well as Maré and Lifou in New-Caledonia, Fatu Huku in the Marquesas Islands and Henderson Island in the Pitcairn Islands. The uplift of these islands is the result of the movement of oceanic tectonic plates. Sunken islands or guyots with their coral reefs are the result of crustal subsidence as the oceanic plate carries the islands to deeper or lower oceanic crust areas.

Uplift vs. exhumation

The word "uplift" refers to displacement contrary to the direction of the gravity vector, and displacement is only defined when the object being displaced and the frame of reference is specified. Molnar and England,[1] identify three kinds of displacement to which the term “uplift” is applied:

  1. Displacement of the Earth's surface with respect to the geoid. This is what we refer to as "surface uplift"; and surface uplift can be defined by averaging elevation and changes in elevation over surface areas of a specified size.
  2. The "uplift of rocks" refers to the displacement of rocks with respect to the geoid.
  3. The displacement of rocks with respect to the surface is called exhumation.

This simple equation relates the three kinds of displacement:

Surface uplift = uplift of rock - exhumation

The term geoid is used above to mean mean sea level, and makes a good frame of reference. A given displacement within this frame of reference allows one to quantify the amount of work being done against gravity.

Measuring uplift and exhumation can be tricky. Measuring the uplift of a point requires measuring its elevation change – usually geoscientists are not trying to determine the uplift of a singular point, but rather the uplift over a specified area. Accordingly, the change in elevation of all points on the surface of that area must be measured, and the rate of erosion must be zero or minimal. Also, sequences of rocks deposited during that uplift must be preserved. Needless to say, in mountain ranges where elevations are far above sea level these criteria are not always easily met. Paleoclimatic restorations though can be very valuable; these studies involve inferring changes in climate in an area of interest from changes with time of flora/fauna that is known to be sensitive to temperature and rainfall.[7] The magnitude of the exhumation a rock has been subjected to may be inferred from geobarometry (measuring previous pressure and temperature history of a rock or assemblage). Knowing the pressure and temperature history of a region can yield an estimate of the ambient geothermal gradient and bounds on the exhumation process; however, geobarometric/geothermometric studies do not produce a rate of exhumation (or any other information on time). One can infer exhumation rates from fission tracks and from radiometric ages as long as one has an estimated thermal profile.

References

  1. ^ a b England and Molnar, 1990, Surface uplift, uplift of rocks, and exhumation of rocks, Geology, v. 18 no. 12 p. 1173-1177 Abstract
  2. ^ Le Fort, Patrick. "Evolution of the Himalaya." (n.d.): 95-109. Print.
  3. ^ Molnar, P., and P. Tapponnier. "Cenozoic Tectonics of Asia: Effects of a Continental Collision: Features of Recent Continental Tectonics in Asia Can Be Interpreted as Results of the India-Eurasia Collision." Science 189.4201 (1975): 419-26. Print.
  4. ^ Karlstrom, K.E., et al., 2012, Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis, Lithosphere, v. 4, p. 3–22 abstract
  5. ^ Burbank, Douglas W., and Anderson, Robert S. Tectonic Geomorphology. Chichester, West Sussex: J. Wiley & Sons, 2011. Print.
  6. ^ Gilchrist, A. R., M. A. Summerfield, and H. A. P. Cockburn. "Landscape Dissection, Isostatic Uplift, and the Morphologic Development of Orogens." Geology 22.11 (1994): 963-966. Print.
  7. ^ Burbank, Douglas West., and Robert S. Anderson. Tectonic Geomorphology. Malden, MA: Blackwell Science, 2000. ISBN 978-0632043866

External links

African superswell

The African superswell is a region including the Southern and Eastern African plateaus and the Southeastern Atlantic basin where exceptional tectonic uplift has occurred, resulting in terrain much higher than its surroundings. The average elevation of cratons is about 400–500 meters above sea level. Southern Africa exceeds these elevations by more than 500 m, and stands at over 1 km above sea level. The Southern and Eastern African plateaus show similar uplift histories, allowing them to be considered as one topographic unit. When considered this way, the swell is one of the largest topographic anomalies observed on any continent, and spans an area of over 10 million km2. Uplift extends beyond the continents into the Atlantic Ocean, where extremely shallow ocean depths are visible through bathymetric survey. The region can indeed be considered as one large swell because the bathymetric anomaly to the southwest of Africa is on the same order as the topographic anomaly of the plateaus (approximately 500 m).The superswell is a relatively recent phenomenon, probably beginning between 5 and 30 million years ago.

Antecedent drainage stream

An antecedent stream is a stream that maintains its original course and pattern despite the changes in underlying rock topography. A stream with a dendritic drainage pattern, for example, can be subject to slow tectonic uplift. However, as the uplift occurs, the stream erodes through the rising ridge to form a steep-walled gorge. The stream thus keeps its dendritic pattern even though it flows over a landscape that will normally produce a trellis drainage pattern.A superposed stream is a stream that forms over horizontal beds that overlie folded and faulted rock with varying resistance. Having cut down through the horizontal beds, the stream retains its course and pattern as it proceeds to erode the underlying rocks despite their different character. The stream erodes a gorge in the resistant bed and continues its flow as before.

Cerro de la Muerte

Cerro de la Muerte is the highest point in the Costa Rican section of the Inter-American Highway. Its name means "Mountain of Death" or "Summit of Death," since in the past crossing the mountains from the Valle Central meant a three- or four-day journey, on foot or on horseback, and many ill-prepared travelers succumbed to the cold and rain. However, the peak is now easily accessible since the highway runs close by.

A drivable track from the highway (Kilometer 89) leads to a peak, with its cluster of telecommunications aerials. A short hike is also available from the highway to another peak marked with a barrel. A sign marks the high point of the highway (Costa Rica 2) at 3,335 meters (10,942 feet), from where the vehicle track and hiking trail begin. At this altitude, overnight temperatures can dip below freezing, but the sun soon raises the temperatures in the morning, with a high risk of sunburn in the thin clear air. Record temperatures reach below -6°C.

This mountain is in Talamanca range, which extends from eastern Costa Rica into neighbouring western Panama. This range was a volcanic island in the geological past, it raised result of tectonic uplift and its separation from other mountain ranges means that it has developed many endemic species of animals and plants, often with affinities to Andean forms.

The higher areas are páramo habitat, with stunted shrubs, dwarf bamboo, and tree ferns, and smaller plants like blueberry, gooseberry and lady's slipper. Below this zone, the natural vegetation is oak forest with bamboo understory, an excellent place to see the charismatic resplendent quetzal.Nearly 50% of the bird species recorded from Cerro de la Muerte are endemic to the Talamanca range. These include fiery-throated hummingbird, timberline wren, sooty robin, black-billed nightingale-thrush, peg-billed finch and volcano junco.

Erosion and tectonics

The interaction between erosion and tectonics has been a topic of debate since the early 1990s. While the tectonic effects on surface processes such as erosion have long been recognized (for example, river formation as a result of tectonic uplift), the opposite (erosional effects on tectonic activity) has only recently been addressed. The primary questions surrounding this topic are what types of interactions exist between erosion and tectonics and what are the implications of these interactions. While this is still a matter of debate, one thing is clear, the Earth's landscape is a product of two factors: tectonics, which can create topography and maintain relief through surface and rock uplift, and climate, which mediates the erosional processes that wear away upland areas over time. The interaction of these processes can form, modify, or destroy geomorphic features on the Earth's surface.

Granite dome

Granite domes are domical hills composed of granite with bare rock exposed over most of the surface. Generally, domical features such as these are known as bornhardts. Bornhardts can form in any type of plutonic rock but are typically composed of granite and granitic gneiss. As granitic plutons cool kilometers below the Earth's surface, minerals in the rock crystallize under uniform confining pressure. Erosion brings the rock closer to Earth's surface and the pressure from above the rock decreases; as a result the rock fractures. These fractures are known as exfoliation joints, or sheet fractures, and form in onionlike patterns that are parallel to the land surface. These sheets of rock peel off the exposed surface and in certain conditions develop domical structures. Additional theories on the origin of granite domes involve scarp-retreat and tectonic uplift.

Hawke Bay

Hawke Bay (often incorrectly called by its former name of Hawke's Bay) is a large bay on the east coast of the North Island of New Zealand. It stretches from Mahia Peninsula in the northeast to Cape Kidnappers in the southwest, a distance of some 100 kilometres (62 mi).

Captain James Cook, sailing in HMS Bark Endeavour, sailed into the bay on 12 October 1769. After exploring it, he named it for Sir Edward Hawke, First Lord of the Admiralty, on 15 October 1769, describing it as some 13 leagues (about 40 miles (64 km)) across. Hawke had decisively defeated the French at the Battle of Quiberon Bay in 1759.

This part of the New Zealand coast is subject to tectonic uplift, with the land being raised out of the sea. For this reason, the coastal land in this area has significant marine deposits, with both marine and land dinosaur fossils having been found inland. The Napier earthquake of 3 February 1931 resulted in several parts of the seabed close to the city of Napier being raised above sea level.

Because the central mountain ranges come close to the coast at the north end of the bay, much of the bay's northerly coastline has deeply eroded tablelands that end in steep seaside cliffs which descend to narrow beaches.

The town of Wairoa lies to the north end of the bay, at the mouth of the Wairoa River and its flood plain, while the port city of Napier lies on the coast, and near the southern end of the bay sits the city of Hastings, on the edge of another flat river flood plain. The main port in the bay is the Port of Napier.

The Hawke's Bay region, as distinct from the bay itself, lies on the coastal land around the bay and also in the hinterland to the south. The bay is named Hawke Bay, whereas the region bears the bay's former name, Hawke's Bay.

Hațeg Island

Hațeg Island was a large offshore island in the Tethys Sea which existed during the Late Cretaceous period, probably from the Cenomanian to the Maastrichtian ages. It was situated in an area corresponding to the region around modern-day Hațeg, Hunedoara County, Romania. Maastrichtian fossils of small-sized dinosaurs have been found in the island's rocks.It was formed mainly by tectonic uplift during the early Alpine orogeny, caused by the collision of the African and Eurasian plates towards the end of the Cretaceous. There is no real present-day analog, but overall, the island of Hainan (off the coast of China) is perhaps closest as regards climate, geology and topography, though still not a particularly good match. The vegetation, for example, was of course entirely distinct from today, as was the fauna.

The Hungarian paleontologist Franz Nopcsa theorized that "limited resources" found on the island commonly have an effect of "reducing the size of animals" over the generations, producing a localized form of dwarfism. Nopcsa's theory of insular dwarfism—also known as the island rule—is today widely accepted.

High (tectonics)

A high in structural geology and tectonics an area where tectonic uplift has taken place relative to its surroundings. Highs are often bounded by normal faults and can be regarded as the opposites of basins. A related word is a massif, an area where relative old rocks layers are found at the surface. A small high can be called a horst.

Because of the relative uplift the accommodation space for sediments was relatively small and a high will have thinner sedimentary layers deposited on it compared to the surrounding basins. Therefore, highs are not good places to study stratigraphic sequences as the sequence may be less detailed or even absent.

Lepontin dome

The Lepontine dome or Lepontin dome is a region of tectonic uplift in the Swiss part of the Alps. It is located in the Lepontine Alps and Glarus Alps.

The Alps north of the Periadriatic Seam are usually divided into three large nappe complexes. From bottom to top these are the Helvetic, Penninic and Austroalpine nappes. East of the dome all three are found on top of each other. The same counts for the region west of the dome, if the Sesia unit is seen as part of the Austroalpine nappes. The dome itself however only shows Penninic and Helvetic (the boundary between the two is still disputed) rocks. Apparently the uplift of the dome caused the upper Austroalpine material to be totally eroded away.

The creation of the dome was caused by a phase of east-west directed extension in the Miocene that occurred throughout the Eastern and Central Alps. This extensional phase was probably a result of slab detachment in the upper mantle. Similar large extensional structures appear in more places in the Alps, examples are the Hohe Tauern window and the smaller Engadin and Rechnitz windows.

Log Springs Formation

The Log Springs Formation is a geologic formation in the Jemez, Nacimiento, and Sandia Mountains of New Mexico. It is a sequence of continental red beds interpreted as reworked terra rossa soils and sediments from nearby highlands filling karst topography in the underlying Arroyo Penasco Group. Its outcrops are spotty everywhere but near the type section in the southern Jemez Mountains. It probably correlates with the Molas Formation.Though lacking in fossils, the formation is estimated as being Namurian (late Mississippian to early Pennsylvanian) in age based on fossils in underlying and overlying beds. Its clastic beds record the beginnings of tectonic uplift associated with the Ancestral Rocky Mountains.

Molasse basin

The Molasse basin (or North Alpine foreland basin) is a foreland basin north of the Alps which formed during the Oligocene and Miocene epochs. The basin formed as a result of the flexure of the European plate under the weight of the orogenic wedge of the Alps that was forming to the south.

In geology, the name "molasse basin" is sometimes also used in a general sense for a synorogenic (formed contemporaneously with the orogen) foreland basin of the type north of the Alps. The basin is the type locality of molasse, a sedimentary sequence of conglomerates and sandstones, material that was removed from the developing mountain chain by erosion and denudation, that is typical for foreland basins.

Raised shoreline

A raised shoreline is an ancient shoreline exposed above current water level. These landforms are formed by a relative change in sea level due to global sea level rise, isostatic rebound, and/or tectonic uplift. These surfaces are usually exposed above modern sea level when a heavily glaciated area experiences a glacial retreat, causing water levels to rise. This area will then experience post-glacial rebound, effectively

raising the shoreline surface.

Examples of raised shorelines can be found along the coasts of formerly glaciated areas in Ireland and Scotland, as well as in North America. Raised shorelines are exposed at various locations around the Puget Sound of Washington State.

River incision

River incision is the narrow erosion caused by a river or stream that is far from its base level. River incision is common after tectonic uplift of the landscape. Incision by multiple rivers result in a dissected landscape, for example a dissected plateau. River Incision is the natural process by which a river cuts downward into its bed, deepening the active channel. Though it is a natural process, it can be accelerated rapidly by human factors including land use changes such as timber harvest, mining, agriculture, and road and dam construction. The rate of incision is a function of basal shear-stress. Shear stress is increased by factors such as sediment in the water, which increase its density (Lague, et. al. 2005). Shear stress is proportional to water mass, gravity, and Sw, where t= Shear Stress (N/m2), g= Weight Density of Water (N/m3, lb/ft ), D = Average water depth (m, ft), and Sw = Water Surface slope (m/m, ft/ft). Increases in slope, depth, or density of water increase the water’s potential to cause erosion (Lague 2014).

River terraces (tectonic–climatic interaction)

Terraces can be formed in many ways and in several geologic and environmental settings. By studying the size, shape, and age of terraces, one can determine the geologic processes that formed them. When terraces have the same age and/or shape over a region, it is often indicative that a large-scale geologic or environmental mechanism is responsible. Tectonic uplift and climate change are viewed as dominant mechanisms that can shape the earth’s surface through erosion. River terraces can be influenced by one or both of these forcing mechanisms and therefore can be used to study variation in tectonics, climate, and erosion, and how these processes interact.

Rába

The Rába (German: Raab; Hungarian: Rába; Slovene: Raba [ˈɾáːba]) is a river in southeastern Austria and western Hungary and a right tributary of the Danube. Its source is in Austria, some kilometres east of Bruck an der Mur below Heubodenhöhe Hill. It flows through the Austrian states of Styria and Burgenland, and the Hungarian counties of Vas and Győr-Moson-Sopron. Its is 298.2 km (185.3 mi) long, of which about 100 km in Austria. It flows into a tributary of the Danube (Mosoni-Duna) in northwestern Hungary, in the city of Győr. Towns along the Rába include Gleisdorf, Feldbach (both in Austria), and Szentgotthárd and Körmend (in Hungary). In the early Cenozoic the river used to flow in the opposite direction, but tectonic uplift reversed this flow.

The Rába Slovenes, living in the Rába Valley (Sln. Porabje, Hung. Vendvidék), are the westernmost group of Hungarian Slovenes. The Raba Valley is part of the wider region of Prekmurje.

Stream capture

Stream capture, river capture, river piracy or stream piracy is a geomorphological phenomenon occurring when a stream or river drainage system or watershed is diverted from its own bed, and flows instead down the bed of a neighbouring stream. This can happen for several reasons, including:

Tectonic earth movements, where the slope of the land changes, and the stream is tipped out of its former course

Natural damming, such as by a landslide or ice sheet

Erosion, either

Headward erosion of one stream valley upwards into another, or

Lateral erosion of a meander through the higher ground dividing the adjacent streams.

Within an area of karst topography, where streams may sink, or flow underground (a sinking or losing stream) and then reappear in a nearby stream valley

Glacier retreat

The additional water flowing down the capturing stream may accelerate erosion and encourage the development of a canyon (gorge).

The now-dry valley of the original stream is known as a wind gap.

Tectonic influences on alluvial fans

Tectonic forces have been shown to have major influences on alluvial fans. Tectonic movements such as tectonic uplift are driving factors in determining the development, shape, structure, size, location, and thickness of alluvial fans and influence the formation of segmented fans. By understanding the tectonic influences, the geologic history (the story of how the area was formed) can be determined by taking information from an alluvial fan and determining the tectonic history of the region.

Turakirae Head

Turakirae Head is a promontory on the southern coast of New Zealand's North Island. It is located at the western end of Palliser Bay, 20 kilometres southeast of Wellington, at the southern end of the Remutaka Range. The head is an excellent example of tectonic uplift within the Wellington region. There are a series of raised terraces which show where the beach has been prior to a seismic event. Turakirae Head is also home to a seal colony.

Windmill Hill (Gibraltar)

Windmill Hill or Windmill Hill Flats is one of a pair of plateaux, known collectively as the Southern Plateaux, at the southern end of the British Overseas Territory of Gibraltar. It is located just to the south of the Rock of Gibraltar, which descends steeply to the plateau. Windmill Hill slopes down gently to the south with a height varying from 120 metres (390 ft) at the north end to 90 metres (300 ft) at the south end. It covers an area of about 19 hectares (47 acres), though about 6 hectares (15 acres) at the north end is built over. The plateau is ringed to the south and east with a line of cliffs which descend to the second of the Southern Plateaux, Europa Flats, which is itself ringed by sea cliffs. Both plateaux are the product of marine erosion during the Quaternary period and subsequent tectonic uplift. Windmill Hill was originally on the shoreline and its cliffs were cut by the action of waves, before the ground was uplifted and the shoreline moved further out to the edge of what is now Europa Flats.

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