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A river anticline is a geologic structure that is formed by the focused uplift of rock caused by high
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
rates from large rivers relative to the surrounding areas. An
anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
is a fold that is concave down, whose limbs are dipping away from its axis, and whose oldest units are in the middle of the fold. These features form in a number of structural settings. In the case of river anticlines, they form due to high erosion rates, usually in orogenic settings. In a mountain building setting, like that of the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
or the Andes, erosion rates are high and the river anticline's fold axis will trend parallel to a major river. When river anticlines form, they have a zone of uplift between 50-80 kilometers wide along the rivers that form them.


Cause and effect

The type of geologic feature that will form is caused by stream power and flexural rigidity of the crust. When stream power increases and flexural rigidity decreases, this causes the structure to progress from a transverse anticline to a river anticline, and in extreme cases to a tectonic aneurysm. Transverse anticlines trend in the direction of and form around small rivers with relatively high crustal strength. River anticlines form around large highly erosive rivers where crustal strength is relatively low. Tectonic aneurysms will form when erosion is very high and the crust is very weak, to form a structural welt. The way that river anticlines form via deep river incisions and the associated crustal rebound, cause rocks deep in the crust to be preferentially exhumed along large rivers like the Arun,
Indus The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmi ...
,
Sutlej The Sutlej or Satluj River () is the longest of the five rivers that flow through the historic crossroads region of Punjab in northern India and Pakistan. The Sutlej River is also known as ''Satadru''. It is the easternmost tributary of the Ind ...
, and
Yarlung Zangbo River The Yarlung Tsangpo, also called Yarlung Zangbo () is the upper stream of the Brahmaputra River located in the Tibet Autonomous Region, China. It is the longest river of Tibet and the fifth longest in China. The upper section is also called Dan ...
. Isolated exhumation causes high pressure and ultra high pressure metamorphic sample to be brought to the surface at sustained rates of up to 5mm per year. Analysis and
radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
of these high pressure and ultra high pressure
metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
can help reconstruct the tectonic evolution of the orogenic belt that formed them.


Evidence

In the Himalaya, the Indian continental plate is crashing into the Eurasian continental plate with nearly north-south motion. Therefore, the compression of the rocks in the Himalaya is in the north-south direction. So, folding should occur trending east-west, as is observed. However, it has also been noted that folding occurs in the north-south direction. It was noted that these folds follow the traces of the major rivers, such as the Arun and the Indus. Originally these folds were explained by assuming that the rivers did not form these anticlines, instead the river's course was by coincidence on top of these geologic features, forming by differential erosion. The idea of
isostatic rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...
was suggested as the best fit mechanism for these north-south trending folds and is now widely accepted.


Formation processes

The formation of a river anticline by isostatic rebound is illustrated in the figure to the right in idealized steps. The principle of
Isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its ...
says that if the
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years o ...
is free to move vertically, then it will float at an appropriate depth in the
asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...
based on the thickness and
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of the lithosphere. River anticlines form when huge amounts of material are removed by river
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
in an area with low crustal rigidity. The crust rebounds up specifically along the river, while the rest of the area remains relatively constant. This will bend the crust forming an anticline, which can take up to ten thousand years. As a river flows through the area, it erodes away large amounts of the overlying rock, which causes a decrease in the lithosphere's mass, leading to an isostatic response. With no overlying rock, the underlying material rebounds up, like removing a weight from a raft. As the river progresses the erosion continues and therefore the rebounding continues, which will form a low wide antiformal structure. For this rebound to occur the erosion from the river must exceed the average erosion rate for the area and exceed the uplift of the orogen. The average erosion rates for the Himalaya are about 1 mm per year, while the erosion rate for the Arun River of the eastern Himalaya is up to 8 mm per year, so it makes sense that we would see river anticlines along the Arun River.


Tectonic aneurysms

A tectonic aneurysm is an isolated zone of extreme uplift and exhumation rates. This forms when uplift from local tectonics are combined with very weak crust and uplift from a river anticline. When a major river flows over an area of tectonic uplift, the erosion from the river will erode the uplifted material. This will cause extremely rapid exhumation along the major rivers, of up to 10 mm per year. Within the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
there are two tectonic aneurysms, each on one of the two
syntaxis Syntaxis is a style in writing or in rhetoric that favors complex syntax, in contrast to the simple sentence structures of parataxis. For example, 19th-century German academic prose, and John Milton's ''Paradise Lost'' poetry in English are no ...
of the
orogenic belt An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collecti ...
:
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
in the west and
Namche Barwa Namcha Barwa or Namchabarwa (; Chinese: 南迦巴瓦峰, Pinyin: ''Nánjiābāwǎ Fēng'') is a mountain peak lying in Tibet in the region of Pemako. The traditional definition of the Himalaya extending from the Indus River to the Brahmapu ...
in the east. These tectonic aneurysms form in similar ways to river anticlines, but with extreme erosion rates and very weak and ductile crust. The syntaxis mark the end of the Himalayan orogen on either side and define the location of two large rivers, the
Indus The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmi ...
and the
Yarlung Tsangpo River The Yarlung Tsangpo, also called Yarlung Zangbo () is the upper stream of the Brahmaputra River located in the Tibet Autonomous Region, China. It is the longest river of Tibet and the fifth longest in China. The upper section is also called Dan ...
. The syntaxis on either side of the Himalaya are dominated by a
strike slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
zone, instead of a compressional
thrust faulting A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
, as in the rest of the orogen. In the west the Indus River flows through the Nanga Parbat and in the east the Yarlung Tsangpo River flows through the Namche Barwa. The very high erosion rates of these two rivers is coupled with weak, hot, thin, dry, crust to form areas of extreme uplift and exhumation.


Deformation mechanism

Deformation caused by tectonic aneurysms are similar to
aneurysm An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus ...
s in blood vessels in which weakening of the confining force allows for localized growth or uplift. However, in the geological setting, deformation occurs over millions of years with significant sustained erosion power ranging from tens of hundreds of kiloWatts per meter.Zeitler, P., Hallet, B., & Koons, P. (2013). Tectonic aneurysms and mountain building Incision or crustal thinning of an area on the surface relative to the background crust thickness causes two things to occur that allow for aneurysm formation. Firstly, due to the brittle nature of crustal rocks and their pressure dependent strength, the decrease in overlying material depresses the crustal strength when compared to surrounding areas. This occurs because the removal of crust decreases the overburden and thus the pressure which influences the strength. Secondly, the
geothermal gradient Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate b ...
increases vertically. Localized deep valleys create weakest areas that focus strain and thereby the movement of deep ductile material. By weakening the crust in a localized area, a preferential region of strain can develop concentrating the flow of material. Ductile rocks deeper in the crust will be able to move towards the potential gradient, whereas brittle rocks near the surface will fracture when subject to increased strain. The transition between
brittle deformation In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tect ...
and ductile deformation is determined by the temperature which is generally controlled by depth as well as rheology. Weak hot minerals, below the ductile transition, with significant partial melt move into the area underlying the thinned crust as a result of the pressure gradient being decreased in the thin area. At a certain point, the pressure will decrease substantially moving from convergent basement rock into thinned crust. This causes rapid decompression at relatively stable and raised isotherms. Decompression melting occurs, which increases the proportion of partial melt within the material and causes rapid heat advection towards the surface. Continued convergent plate movement focuses the flow of material into the syntaxial areas with the localized weakness permitting upward escape as an accommodation mechanism. This process solves the fundamental problem of material being forced into a confined space by creating an outlet. The result of which creates a positive feedback with erosion focusing uplift which transports more weak rock vertically enhancing erosive capabilities. Areas of consistent elevation in river valleys and mountains with relief can be maintained by high exhumation rates of relatively young weak rocks. The ages of minerals in the area will be younger than the surrounding crust due to cooling occurring in an area with a steeper thermal gradient at shallower depths. Mature tectonic aneurysm systems, such as the
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
, can have very high local reliefs of young rocks due to consistent erosion maintaining the elevation in the erosive area and vertical strain forcing material up along the proximal edges.


Locations

Tectonic aneurysms are found in areas with localized high relief of relatively young rocks when compared to their surroundings. Actively observed systems that have been studied the most are located in 2 main regions of the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
, the
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
Haramosh Haramosh Peak (; also known as Haramosh or Peak 58) is a mountain located in the Karakoram range of the Gilgit-Baltistan of Pakistan. The Total population of haramosh Vellay at least 8000 people.according to researchers. Its height is also often ...
Massif and
Namche Barwa Namcha Barwa or Namchabarwa (; Chinese: 南迦巴瓦峰, Pinyin: ''Nánjiābāwǎ Fēng'') is a mountain peak lying in Tibet in the region of Pemako. The traditional definition of the Himalaya extending from the Indus River to the Brahmapu ...
Gyala Peri Gyala Peri ( Chinese: 加拉白垒, Pinyin: ''Jiālābáilěi'') is a peak just beyond the eastern end of the Himalayas at the entrance to Tsangpo gorge. It is part of Nyenchen Tanglha Shan, although it is sometimes included in Namcha Barwa ...
which occur on the Eastern and Western edges respectively. The
Indus River The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmir ...
is the mechanism responsible for crustal removal in the
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
region, and the
Tsangpo River The Yarlung Tsangpo, also called Yarlung Zangbo () is the upper stream of the Brahmaputra River located in the Tibet Autonomous Region, China. It is the longest river of Tibet and the fifth longest in China. The upper section is also called Dan ...
is active in the
Namche Barwa Namcha Barwa or Namchabarwa (; Chinese: 南迦巴瓦峰, Pinyin: ''Nánjiābāwǎ Fēng'') is a mountain peak lying in Tibet in the region of Pemako. The traditional definition of the Himalaya extending from the Indus River to the Brahmapu ...
region. Proposed tectonic aneurysms are located in the Saint Elias region of
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...
, the Kongur Shan and
Muztagh Ata Muztagh Ata or Muztagata (, Музтағ Ата, literally "ice-mountain-father"; ; formerly known as Mount Tagharma or Taghalma and Wi-tagh) is the second highest (7546 metres) of the mountains which form the northern edge of the Tibetan Plate ...
in China, and the Lepontine Dome in the
Swiss Alps The Alpine region of Switzerland, conventionally referred to as the Swiss Alps (german: Schweizer Alpen, french: Alpes suisses, it, Alpi svizzere, rm, Alps svizras), represents a major natural feature of the country and is, along with the Swis ...
. These locations show incipient or similar, less significant characteristics to actively observed systems. Glacial mechanisms of erosion and transport are believed to be responsible in many alpine areas including the Saint Elias system.


Nanga Parbat-Haramosh Tectonic Aneurysm

The
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
-Haramosh is the most studied region in the context of tectonic aneurysms. The region has extreme relief over very short distances with the
Indus River The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmir ...
valley approximately 7 kilometers lower in elevation than the peak of the mountain. Within the study area,
Biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...
cooling ages (280 °C ± 40 °C) are consistently less than 10 million years old indicating rapid exhumation rates in the area. Studies of composition and structure of the rocks in the area suggest exhumation of depths below 20 kilometers. Exhumation rates from the massif and the valley are significantly higher than background rates. Calculations of peak exhumation rates range from 5 to 12 mm per year depending on the location. The mountain top has a lower rate than the bottom of the valley yet both are significantly higher when compared to background rates outside of the syntax. Exposed
granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated w ...
within the central aneurysm area represents low-pressure melting and advection as material moved into areas with decreasing pressure. Up to 20 kilometers of domal unroofing over a very short period of time has been inferred based on the sample ages ranging from 1 to 3 million years.


Namche Barwa-Gyala Peri

The
Namche Barwa Namcha Barwa or Namchabarwa (; Chinese: 南迦巴瓦峰, Pinyin: ''Nánjiābāwǎ Fēng'') is a mountain peak lying in Tibet in the region of Pemako. The traditional definition of the Himalaya extending from the Indus River to the Brahmapu ...
-Gayla Peri tectonic aneurysm is located on the Eastern side of the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
with the active
Tsangpo River The Yarlung Tsangpo, also called Yarlung Zangbo () is the upper stream of the Brahmaputra River located in the Tibet Autonomous Region, China. It is the longest river of Tibet and the fifth longest in China. The upper section is also called Dan ...
flowing down the valley between the mountains. Many researchers conclude the tectonic aneurysm model is the best explanation of the observed structures and tectonic arrangement of the region. The argon-argon biotite ages and zircon fission track ages of rocks from the area are 10 million years old or less, which is young compared to the surrounding rocks. Similar high reliefs seen in the
Nanga Parbat Nanga Parbat ( ur, ) (; ), known locally as Diamer () which means “king of the mountains”, is the ninth-highest mountain on Earth, its summit at above sea level. Lying immediately southeast of the northernmost bend of the Indus River in ...
are also evident with the Namche Barwa region, with approximately 4 kilometers of vertical elevation change over a short horizontal distance. High and low-grade
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
s are found in the region with evidence to suggest a variation of metamorphic activity between regions from the strain center and the edges. The exhumation occurs in a circular area with young, high-grade decompression melts focused in the center. Around the outside of the focus
rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
to
strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
ratios suggest melting with fluid present. The presence of fluid within melt has been modeled to occur as a result of immense precipitation allowing water to penetrate into shallow crustal rocks over long periods of time. Ages and barometric regimes of the rocks were used to calculate the volume of overburden removed, which was used to determine 3 millimeters of annual incision over the last 10 million years.


Saint Elias

The proposed four million years old tectonic aneurysm system in the
Saint Elias Mountains The Saint Elias Mountains (french: Chaîne Saint-Élie) are a subgroup of the Pacific Coast Ranges, located in southeastern Alaska in the United States, Southwestern Yukon and the very far northwestern part of British Columbia in Canada. The range ...
in Alaska was formed by glacial erosion on the mountains developed by underthrusting of the Yakutat microplate beneath the North American margin. The aneurysm occurs in the Northern plate corner in which transitions from
dextral Sinistral and dextral, in some scientific fields, are the two types of chirality (" handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...
strike-slip motion to thrust sense motion thereby focusing strain. The interpreted relationship between erosion mountain development has more variations between researchers than Himalayan systems due to the age of the system and constraints regarding field work due to glacier cover. In the St. Elias range collision and underthrusting caused surface uplift forming mountains. The elevation increase climate regime allowed glacier development resulting in extreme glacial erosion potential. Since its inception, glacial erosion transported sediments West into the
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...
and onto the continental margin. After which, approximately two million years ago, the formation of a
décollement Décollement () is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are ass ...
caused the locus of strain to propagate south. The shift in strain focus resulted in mountain development farther South which disrupted the climatic system thereby decreasing precipitation in Northern regions of the Saint Elias Mountains. The erosion and exhumation are now concentrated on the southern portion of the mountain range which produces young cooling ages associated with the current tectonic aneurysm center. Young
detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Steph ...
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the ...
fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiome ...
(240 °C ± 40 °C) and
apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...
fission track and
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly ...
- thorium/
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. It ...
(110 °C ± 10 °C) cooling ages of sediments in glacial catchment areas support the theory of erosive influence on the St. Elias tectonic system. Rates of exhumation were inferred by calculating the difference between detrital zircon and apatite ages in sediments. The smaller the difference between zircon and apatite ages represents a faster movement of material through the isotherms and faster cooling. In the northern corner of contact between the plates, the zircon and apatite ages do not differ significantly, thereby providing evidence of rapid exhumation. The proximity to the depositional environment along the coastal margin and within
fjord In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Germany, Greenland, the Faroe Islands, Icel ...
s preserves a record of sedimentation rate which is used to interpret exhumation rates of 0.3 mm year originally and approximately 1.3 mm/year for the last million years. The sediment age and thickness are used to track the movement of the focus of erosion from the north to the south. The presence of a definitive tectonic aneurysm system in the region is widely disputed with many researchers concluding insufficient focused exhumation is occurring to support the hypothesis. Significant glacial cover limits the number of field samples and geological observations that can be made directly on the surface thereby adding uncertainty to interpretations. Alternative theories argue tectonic transpressional control of exhumation with little erosive influence on the overall system. Younger ages are explained by focused strain areas resulting from faulting.


Field work in proposed regions

By comparing the depth in Earth at which particular minerals crystallize and the elevation at which they were sampled, the age of minerals can be used to determine the rate which the strain zone moved material vertically. Various dating methods on specific
fluid inclusions image:Inclumed.gif, 250px, Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, ...
and minerals were used in order to provide chronological data of the exhumation rate of rocks in the area. The age dates were used to reconstruct the history of exhumation and thermal regimes by comparing them to pressure and temperature crystallization boundaries of the minerals.
Uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly ...
- thorium and uranium-
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. It ...
cooling ages of samples of apatite indicate the timing of 70 °C cooling. Higher
closure temperature In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature is the temperature at which a syste ...
s were dated using
argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...
methods for
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...
samples (300 °C) and
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the ...
fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiome ...
(230 °C - 250 °C) methods. By analyzing the ages of minerals with various
closure temperature In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature is the temperature at which a syste ...
s, researchers can infer the speed at which they moved through the isotherms. When the difference between the age of a mineral that cooled at a high temperature and one that cooled at a low temperature are relatively similar, then exhumation is inferred to be rapid. The
geothermobarometry Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobarometry is a combination of ''geobarometry'', where a pressure of mineral formation is resolved, an ...
is obtained using
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different sp ...
-
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...
plagioclase in order to constrain higher pressure metamorphic regimes. Shallower exhumation rates (low-temperature cooling ages) alone can not realistically be used to describe tectonic aneurysms as deep isothermal gradient changes may not significantly affect shallower depths. Furthermore, shallow low-temperature cooling can be more largely related to erosion dominated exposure rather than tectonic driven uplift. Sample ages from minerals with higher cooling temperatures signify exhumation of deeper material which is the modeled function of a tectonic aneurysm.
Seismic velocity A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. ...
profiles are often used over large study areas in order to identify possible isothermal irregularities. Low-velocity data is indicative of hotter rocks with a higher degree of the partial melt which slows
P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
s when compared to the surroundings. Magnetotelluric sampling is done to test the
resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
of the rocks which is used to infer the amount of fluid in the rocks.


References

{{reflist Anticlines Erosion Mountain geomorphology Tectonics