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Ingeology, amélange is 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 faulted terranes inorogenic belts. A mélange is formed in theaccretionary wedge above asubduction zone. Theultramaficophiolite sequences which have beenobducted ontocontinental crust are 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 margin settings generally consist of alteredoceanic crustal material and blocks ofcontinental slopesediments in a shearedmudstonematrix. The mixing 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, andolistostromal action), 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 tectonics in 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 include theFranciscan Complex along theCoast Ranges of central and northern California and theBay of Islands ophiolite complex inNewfoundland. The Gwna Mélange in the UK extends throughAnglesey and theLlŷn Peninsula onto 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 ofEgypt is part of the NeoproterozoicArabian-Nubian Shield and 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 andschistosevolcaniclastic matrix. The main exotic blocks are ophiolitic and include metamorphosedultramafic rocks,metagabbros, massive and pillowedmetabasalts andpelagic sedimentary 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]
The termmélange inEnglish is aloan word fromFrench, used to mean amixture ofdisparate components. Its derivation, and therefore to some extent its connotation, is similar tomêlée.[2]Mélange is the modern form of theOld French nounmeslance, which comes from theinfinitivemesler, meaning "to mix".[3]
