The recrystallization of pre-existing rocks in response to simultaneous changes of temperature, lithostatic pressure, and in many cases shear stress, occurring in orogenic belts where lithospheric plates are converging. The broad areas covered by orogenic belts cause the associated metamorphism to be developed on a regional scale, hence the name attached to this type of metamorphism. Regional metamorphism can be pre-, syn-, or post-tectonic, depending whether the metamorphic event (or events) is (or are) before, synchronous with, or after the orogenic deformation event (or events). Typical rock fabrics produced during regional metamorphism are, in order of increasing grain size (reflecting increasing metamorphic grade), slaty, phyllitic, schistose, and gneissose fabrics. Increase of metamorphic grade in regional terrains typically produces a prehnite–pumpellyite–greenschist–amphibolite–granulite facies series. However, each regional metamorphic terrain is characterized by a unique mineral zonal sequence reflecting a particular pressure–temperature gradient during metamorphism.
Subjects: Earth Sciences and Geography.