User:Ntcbrand/metamorphiccorecomplexrevised

Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They are typically domal shaped uplifts of metamorphic rocks and plutonic rocks bounded by shear zones that separate them from unmetamorphosed cover rocks. Because of their domal shape, often times the complexes are referred to as "turtle-backs". They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface. This process takes place when the hanging-wall strata is thinned so much that it results in an isostatic uplift in a dome like form. Interpretations of how these features form are varied and controversal, and include models invovling extension on low-angle normal faults. Most interpretations include domes forming from low-density regions surrounded by high-density crust, allowing for exhumation. The South Liaodong Peninsula metamorphic complex, located in China, is said to be formed from lithosphere convective removal (Faure, Monie, Scharer, Panis, 2008). During the process of extension, high-grade metamorphic rocks (eclogite-, granulite- to amphibolite- facies) are exposed below  low-angle detachment faults (mylonite shear zones) that show ductile deformation on the lower side  (footwall) with amphibolite- to greenschist-facies  syndeformational metamorphism, and  ductile-brittle to brittle deformation on the upper-side (hanging-wall). The low-angle faults/ mylonite shear zones are overlain by strongly brecciated and broken, unmetamorphosed, upper space of plate rocks (hanging wall). The upper plate rocks are cut by many listric (curved faults that get less steep with depth) normal faults and are commonly overlain by syntectonic sedimentary deposits (sediments that are the result of thrusting). With metamorphic core complexes, strain is occurring on the rocks to deform them. This is directly related to the thinning of the upper crust where metamorphic core complexes first originate. The principal axes of strain for the initial plane that is being extended, is usually at prescribed orientations relative to to the fault plane itself. Extension (strain) happens in three main directions, e'1, e'3, and e'2. Typically when people refer to extension, they are speaking in terms of maximum extension being in the horizontal direction, and minimal extension in the vertical direction. Maximum extension could technically occur in the vertical direction, and this would be a result of crustal thickening. e'1 and e'3 are located at 45° to the fault plane, 90° from each other, and lie on the movement plane of the extension. e'1 is classified as maximum extension and e'3 as minimal extension. e'2 lies in the fault plane and is perpendicular to slip direction. Unless a shear sense for the particular fault being extended is established, we cannot determine which direction of extension is e'1 (maximum extension) or e'3 (minimum extension).

As mentioned before, interpretations of metamorphic core complexes are often misunderstood and many questions still remain unanswered. Studies about core complexes have only been active since the 1970's when the idea was discovered in the cordillera belt extending from Canada to Mexico. Extensive research has been conducted on many different complexes throughout the world but because the research has been short spanned, this is the reason for confusion. Questions that often arise about metamorphic core complexes and their components include items such as, whether the rising of complex or plume in the ground was faster than the erosional rate? Often times this is the case, interpretating the structures to be formed in a relatively fast uprising of lower crustal plumes. Another question is about whether the low angle normal faults that are observed happen when they are shallow, or if they form at steeper dipping fault angles and are altered to make them appear shallow. Often times when individuals look at structures of metamorphic core complexes they are confused about the direction of the fault that has been cut by the plume that has risen. On one side of the dome it looks like the normal fault that onced existed, but the rising of the complex has made the opposite side look like a thrust fault. Many make the mistake of calling the faults reversed or thrust when they were actually once a simple, low-angle, normal fault.

Descriptions
are characterized by a generally heterogeneous, older metamorphic-plutonic basement terrane overprinted by low-dipping lineated and foliated mylonitic and gneissic fabrics. An unmetamorphosed cover terrane is typically attenuated and sliced by numerous subhorizontal younger-on-older faults. Between the basement and cover terranes is a decollement and/or steep metamorphic gradient with much brecciation and kinematic structural relationships indicating sliding or detachment.
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The decollement is also called a detachment fault, or a place that separates rocks of different origin.

Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. Lister and Davis (1989) also described dome like complexes as having intense shear zone fabrics that form on the dome flanks, an apparent jump in metamorphic grade downward across the shear zones, and correlation of time between the formation of the ductile shear zones and low-angle faults forming that are subparallel or at a low angle to the complex dome surface.

Location
The core complex model was first developed in the cordillera of western North America, with northern cordilleran core complexes being Eocene in age, while those in Arizona to the south are younger. Some attribute the formation of these complexes to the Sevier Orogeny and the Laramide Orogeny which caused crustal thickening. After the thickening occurred from both orogenic events, the North American Plate started to rift westward, thinning the crust, causing extension, resulting in exhumation of complexes. This caused numerous complexes to form or be exhumed through an eastern extension of Late Cretaceous deformation. We can attribute the extension to interactions between the Laramide overthrust belt and Laramide basement uplifts, overprinted by Tertiary Basin and Range faulting (O'Neill, Lonn, Lageson, Kunk, 2004). Some examples of complexes located in the western United States include, the Bitter Root core complex (Montana/Idaho), the Anaconda core complex (south-west Montana), the Harcuvar core complex (Arizona), and the Ruby Mountain core complex (Nevada). Other core complexes are found in the Aegean, Anatolia, Iran, Tibet, north China, Slovakia and New Zealand. The youngest core complex is found in eastern New Guinea.

New Zealand is a location of high tectonic activity. Recent studies show that aside from the Alpine Fault, most structures there have been attributed to the subduction of the Pacific Plate. The metamorphic core complexes there are from the "rollback" of the subducting plate and association with rifting between Austrailia and Antartica, causing extension (Forster and Lister, 2003). Lister and Davis (2003) estimated the oldest shear zone in New Zealand to be formed at about 122 and 118 million years ago. Some structures are over 200 kilometers in length.

Core complexes on other planets
A feature at the center of Artemis Corona on Venus has been suggested as a metamorphic core complex. This could be the largest metamorphic core complex in the solar system.