Metamorphic core complex
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Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagamatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface. During this process, 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).
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[edit] Descriptions
- Coney (1980) stated that they:
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.
The decollement is also called a detachment fault.
- Lister & Davis (1989):
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.
[edit] 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. Core complexes are found in the Aegean, Anatolia, Iran, Tibet, north China, and New Zealand. The youngest core complex is found in eastern New Guinea. Core complexes have been described from the Mid Atlantic and SW Indian Ridge, cf. recent articles on megamullions by Fujimoto et al. (1999) and Tucholke et al. (1998). Giant megamullions are reported from the Parece Vela back-arc basin in the western Pacific (Ohara, 2001).
[edit] Core complexes on other planets
A feature at the center of Artemis Corona on Venus has been suggested as a metamorphic core complex (Spencer 2001). This could be the largest metamorphic core complex in the solar system.
[edit] References
- Coney, P. (1980) Geol. Soc. Mem. 153.
- Fujimoto et al. (1999) InterRidge News 8(1) 22-4.
- Lister, G., Davis, G. (1989) J. Struct. Geol. 11, 65-94.
- Ohara, Y. (2001) Marine Geophysical Researches 22(1), 47-61.
- Spencer, J. E. (2001) Geological Society of America Bulletin 113(3), 333-345.
- Tucholke et al. (1998) J. Geophys. Res. 103:B5, 9857-9866.
