GeologyRocks

A melange is formed in the accretionary wedge as sediment and oceanic crust is scraped off the descending plate in a subduction zone. The melange comprises a strongly brecciated unit with large blocks of pre-existing rocks in a deformed fine grained matrix. Olistostromes are similar units but have a gravity sliding origin rather than a tectonic one. For a melange to be described as such it must be of mapable size, contain 'exotic' clasts (i.e. not derived from immediately adjacent units) and be matrix supported. A melange may also form from vertical (gravity-driven) movements within a sediment pile (diapirism). The term melange is derived from French, meaning mixture.

Image Courtesy of Marli Miller, University of Oregon
Caption: Photo of melange. A melange is a mappable body of rock that includes fragments and blocks of all sizes, embedded in a generally sheared matrix.