geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, a mélange is a large-scale

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

, 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 varied

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lith ...

. Both tectonic and sedimentary processes can form mélange. Mélange occurrences are associated with

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

ed terranes in orogenic belts. A mélange is formed in the

accretionary wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the ...

above a

subduction zone Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

. The

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

sequences which have been obducted onto

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

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 in

active continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margi ...

settings generally consist of altered

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

al material and blocks of

continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margi ...

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s in a sheared

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

. The mixing mechanisms in such settings may include tectonic shearing forces, ductile flow of a water-charged or deformable matrix (such as

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

), sedimentary action (such as slumping, gravity-flow, and olistostromal action), or some combination of these. Some larger blocks of rock may be as much as across. Before the advent of

plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

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

Examples include the

Franciscan Complex The Franciscan Complex or Franciscan Assemblage is a geology, geologic term for a late Mesozoic terrane of heterogeneous rock (geology), rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was n ...

along the Coast Ranges of central and northern California and the

Bay of Islands The Bay of Islands is an area on the east coast of the Far North District of the North Island of New Zealand. It is one of the most popular fishing, sailing and tourist destinations in the country, and has been renowned internationally for ...

ophiolite complex in

Newfoundland Newfoundland and Labrador is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...

. The Gwna Mélange in the UK extends through

Anglesey Anglesey ( ; ) is an island off the north-west coast of Wales. It forms the bulk of the Principal areas of Wales, county known as the Isle of Anglesey, which also includes Holy Island, Anglesey, Holy Island () and some islets and Skerry, sker ...

and the

Llŷn Peninsula The Llŷn Peninsula ( or , ) is a peninsula in Gwynedd, Wales, with an area of about , and a population of at least 20,000. It extends into the Irish Sea, and its southern coast is the northern boundary of the Tremadog Bay inlet of Cardigan Ba ...

onto Bardsey Island in North Wales. The Northern Palawan melange is distributed in the Philippines' , west coast of Inabamalaki Island, west coast of ; 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 of

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

is part of the Neoproterozoic

Arabian-Nubian Shield The Arabian-Nubian Shield (ANS) is an exposure of Precambrian crystalline rocks on the flanks of the Red Sea. The crystalline rocks are mostly Neoproterozoic in age. Geographically – and from north to south – the ANS includes parts of Israel ...

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 and

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

ose

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

matrix. The main exotic blocks are ophiolitic and include metamorphosed

ultramafic rock Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

s, metagabbros, massive and pillowed

metabasalt Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (fla ...

s and

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

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.

Etymology

The term ''mélange'' in English is a

loan word A loanword (also a loan word, loan-word) is a word at least partly assimilated from one language (the donor language) into another language (the recipient or target language), through the process of borrowing (linguistics), borrowing. Borrowing ...

from French, used to mean a ''mixture'' of disparate components. Its derivation, and therefore to some extent its connotation, is similar to ''

mêlée A melee ( or ) is a confused hand-to-hand fight among several people. The English term ''melee'' originated circa 1648 from the French word ' (), derived from the Old French ''mesler'', from which '' medley'' and ''meddle'' were also derived. T ...

''. ''Mélange'' is the modern form of the

Old French Old French (, , ; ) was the language spoken in most of the northern half of France approximately between the late 8th -4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ...

noun ''meslance'', which comes from the infinitive ''mesler'', meaning "to mix".

References

* Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 178, 514), . * 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),

External links

Trenches and mélanges
from the

University of Alberta The University of Alberta (also known as U of A or UAlberta, ) is a public research university located in Edmonton, Alberta, Canada. It was founded in 1908 by Alexander Cameron Rutherford, the first premier of Alberta, and Henry Marshall Tory, t ...

{{DEFAULTSORT:Melange Structural geology Petrology