Mélange
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Ingeology,amélangeis a large-scalebreccia,a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of variedlithologies.Both tectonic and sedimentary processes can form mélange.
Mélange occurrences are associated withthrust faultedterranes inorogenicbelts. A mélange is formed in theaccretionary wedgeabove asubduction zone.Theultramaficophiolitesequences which have beenobductedontocontinental crustare typically underlain by a mélange. Smaller-scale localized mélanges may also occur in shear or fault zones, where coherent rock has been disrupted and mixed by shearing forces.
Large-scale melanges formed inactive continental marginsettings generally consist of alteredoceanic crustalmaterial and blocks ofcontinental slopesedimentsin a shearedmudstonematrix.The mi xing mechanisms in such settings may include tectonic shearing forces, ductile flow of a water-charged or deformable matrix (such asserpentinite), sedimentary action (such as slumping,gravity-flow,andolistostromalaction), or some combination of these. Some larger blocks of rock may be as much as 1 kilometre (0.62 mi) across.
Before the advent ofplate tectonicsin the early 1970s, it was difficult to explain mélanges in terms of known geological mechanisms. A particularly troubling paradox was the occurrence of blueschist blocks (low temperature and high pressure metamorphic rocks) in direct contact with graywacke (a coarse sandstone with lithic fragments) that was deposited in a sedimentary environment.
Examples
[edit]Examples include theFranciscan Complexalong theCoast Ranges of central and northern Californiaand theBay of Islandsophiolite complex inNewfoundland.The Gwna Mélange in the UK extends throughAngleseyand theLlŷn Peninsulaonto Bardsey Island in North Wales. The Northern Palawan melange is distributed in the Philippines'Miniloc Island[clarify],west coast of Inabamalaki Island, west coast ofEl Nido[clarify];Cudugman Point on Bacuit Bay, and in the Cuyo Group of Islands. It consists of a jumble of various rock types contained in a matrix of grey-green slaty mudstone and siltstone.
The Eastern Desert ofEgyptis part of the NeoproterozoicArabian-Nubian Shieldand displays different occurrences of Neoproterozoic ophiolitic mélanges. The mélanges contain exotic and native blocks and fragments of variable sizes and types set in a sheared andschistosevolcaniclasticmatrix. The main exotic blocks are ophiolitic and include metamorphosedultramafic rocks,metagabbros,massive and pillowedmetabasaltsandpelagicsedimentary rocks. Based on the mode of occurrences of the ophiolitic components and the processes of mélange formation, the ophiolitic mélanges of the Central Eastern Desert are classified and mapped into tectonic mélange, olistostrome and olistostromal mélange. Both tectonic and sedimentary processes played a major role during mélange formation in a back-arc or inter-arc setting.[1]
Etymology
[edit]The termmélangeinEnglishis aloan wordfromFrench,used to mean amixtureofdisparatecomponents. Its derivation, and therefore to some extent its connotation, is similar tomêlée.[2]Mélangeis the modern form of theOld Frenchnounmeslance,which comes from theinfinitivemesler,meaning "to mix".[3]
References
[edit]- ^Classification and origin of the Neoproterozoic ophiolitic mélanges in the Central Eastern Desert of Egypt; Authors: El Bahariya, Gaafar A. Affiliation: AA (Geology Department, Faculty of Science, Tanta University, Tanta, Egypt; Publication: Tectonophysics, Volume 568, pp. 357–70. 09/2012 Origin: ELSEVIERdoi:10.1016/j.tecto.2012.03.020Bibliographic Code: 2012Tectp.568..357E
- ^"Online Etymology Dictionary".
- ^"Online Etymology Dictionary".
- Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 178, 514),ISBN0-7167-2438-3.
- Hsu, K.J., 1970, Preliminary report and geologic guide to Franciscan melanges of the Morro Bay – San Simeon area, San Luis Obispo County, California: California Geological Survey Special Publication 35.
- Raymond, L.A., 1984, Classification of melanges: Geological Society of America, Special Paper 198, pp. 7–20.
- British Geological Survey:"Geology of the country around Aberdaron," HMSO, London (1993),ISBN0-11-884487-3
External links
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