mélange

A metamorphic rock formation created from materials scraped off the top of a downward moving tectonic plate in a subduction zone. Mélanges occur where plates of oceanic crust subduct beneath plates of continental crust, as along the western coast of South America. They consist of intensely deformed marine sediments and ocean-floor basalts and are characterized by the lack of regular strata, the inclusion of fragments and blocks of various rock types, and the presence of minerals that form only under high pressure and low temperature conditions.