River anticline
A river anticline is a geologic structure that is formed by the focused uplift of rock caused by high erosion rates from large rivers relative to the surrounding areas. An anticline 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 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, Sutlej, and Yarlung Zangbo River. 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 of these high pressure and ultra high pressure metamorphic rocks 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 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 says that if the lithosphere is free to move vertically, then it will float at an appropriate depth in the asthenosphere based on the thickness and density of the lithosphere. River anticlines form when huge amounts of material are removed by river erosion 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 there are two tectonic aneurysms, each on one of the two syntaxis of the orogenic belt: Nanga Parbat in the west and Namche Barwa 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 and the Yarlung Tsangpo River. The syntaxis on either side of the Himalaya are dominated by a strike slip fault zone, instead of a compressional thrust faulting, 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 aneurysms 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. 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 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 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, 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 Nanga Parbat–Haramosh Massif and Namche Barwa–Gyala Peri which occur on the Eastern and Western edges respectively. The Indus River is the mechanism responsible for crustal removal in the Nanga Parbat region, and the Tsangpo River is active in the Namche Barwa region.Proposed tectonic aneurysms are located in the Saint Elias region of Alaska, the Kongur Shan and Muztagh Ata in China, and the Lepontine Dome in the Swiss Alps. 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.