Ophiolite

(petrology) A distinctive assemblage of mafic plus ultramafic rocks, generally considered to be fragments of oceanic lithosphere that have been tectonically emplaced onto continental margins and island arcs.

A distinctive assemblage of mafic plus ultramafic rocks generally considered to be fragments of the oceanic lithosphere that have been tectonically emplaced onto continental margins and island arcs. An ophiolite is a formation made up of an association of typical rocks in a clearly defined sequence. A complete idealized ophiolite sequence from bottom to top includes (1) an ultramafic tectonite complex composed mostly of multilayered, deformed harzburgite, dunite, and minor chromitite; (2) a plutonic complex of layered mafic-ultramafic cumulates at the base, grading upward to massive gabbro, diorite, and possibly plagiogranite; (3) a mafic sheeted-dike complex; (4) an extrusive section of massive and pillow lavas, pillow breccias, and intercalated pelagic sediments; and (5) a top layer of abyssal or bathyal sediments, which may include ribbon chert, red pelagic limestone, metalliferous sediments, volcanic breccias, or pyroclastic deposits. Most ophiolites lack complete sections, and are dismembered and fragmented. Their estimated original thickness is variable, ranging from about 2 km (1.2 mi) to more than 8 km (5 mi). See also Earth crust; Lithosphere.

Ophiolites typically occur in collisional mountain belts or island arcs and define a suture zone marking the boundary where two plates have welded together. The ophiolite complex is interpreted as evidence for a closed marginal ocean or back-arc basin. Throughout the world, ophiolites occur as long narrow belts, up to 10 km (6 mi) wide, that can extend more than 1000 km (600 mi) in length, in two distinct geographic settings. (1) Those in the Alpine-Mediterranean region, Tethyan ophiolite, were formed in small ocean basins that were surrounded by older, attenuated continental crust. (2) Those in western North America and the Circum-Pacific (Cordilleran) region seem to have formed in inter-arc basins. The Cordilleran ophiolites, such as the Trinity ophiolite and the Coast Range ophiolite of California, are generally incomplete, metamorphosed, or dismembered, but they commonly form the basement rocks for many North American continental margin terranes. See also Basin; Continental margin; Structural geology; Geodynamics.

Ophiolites represent new oceanic crust formed in a variety of spreading environments, including oceanic ridge, back-arc basin, and island arcs above a subduction zone, and subsequently emplaced onto the continents. Their occurrence along plate sutures marks the sites of ancient tectonic interaction between oceanic and continental crust. Ophiolites provide the best opportunity for geologists to study the ocean floor on land; they also offer vertical sections in addition to horizontal distributions. Moreover, ophiolite formations record the ages of oceanic fragments that escaped, disappearing into subduction zones.