Mélange
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- This article is about the geologial usage, for other uses see: Melange (disambiguation).
In geologic context, mélange is a large scale breccia formed in the accretionary wedge above a subduction zone. The mélange typically consists of a jumble of large blocks of varied lithologies of altered oceanic crustal material and blocks of continental slope sediments in a sheared mudstone matrix.
Mélange occurrences are associated with thrust faulted terranes in orogenic belts. The ultramafic ophiolite sequences which have been obducted onto continental crust are typically underlain by a mélange. Examples include the Franciscan Formation along the Coast Ranges of central and northern California and the Bay of Islands ophiolite complex in Newfoundland. Before the advent of plate tectonics in the early 1970s, melanges were quite perplexing to geologists in California (circa 1920 to 1970). 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. Incompatible rocks occur together in melanges, thanks to insightsfrom plate tectonics and subduction zones. So the paradox of depositional environment was finally explained in light of plate tectonics. Geologists sometimes think of melanges as analogous to garbage dumps: a lot of strange things can occur together.
An example in the United Kingdom is the Gwna Melange; extending through Anglesey, the Llyn Peninsula and the Island of Bardsey in North Wales.It consists of a jumble of various rock types contained in a matrix of grey-green slaty mudstone and siltstone.
A mélange is a mapable 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. Some larger blocks of rock may be as much as 1 km across! Both tectonic and sedimentary processes can form mélange. Olistostromes are mélanges formed by gravitational sliding under water and accumulation of flow as a semi fluid body with no bedding.
[edit] Etymology
The term mélange in English language is a loan word from French, used to mean mixture of disparate components (what would be referred to in the sciences as a heterogeneous mixture). Its derivation, and therefore to some extent its connotation, is similar to Mêlée [1][2]. Mélange is the modern form of the Old French noun meslance, which comes from the infinitive mêler, meaning "to mix".
[edit] References
- Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 178, 514), ISBN 0-7167-2438-3.
- 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, p. 7-20.
- British Geological Survey: "Geology of the country around Aberdaron," HMSO, lONDON (1993), ISBN 011 884487 3
