The recrystallization of rocks under a high geothermal gradient so that at any given temperature the pressure is relatively low. The term originally referred to a belt of metamorphic rocks stretching south-westwards from the Abukama Plateau in Japan, and characterized by the development of andalusite and sillimanite in rocks that were originally shales (pelites). This belt lies parallel to, and on the continental side of, a high-pressure metamorphic belt.