Metamorphic consequences of crustal eclogite production in overthrust orogenic zones

Continental collision can produce crust of much greater than normal thickness in whose lower part eclogite facies conditions may pertain. The resulting density increase of ca. 0.3 Mg m^?3 greatly decreases the buoyancy forces normally regarded as inhibiting the subduction of continental material. The high-density phase may persist for tens of millions of years giving anomalously low surface elevation of the thickened continent and inhibiting erosion.Temperature relaxation in the overthrust pile produces regional metamorphism. The conditions of metamorphism are analysed in terms of varying initial thermal gradient, erosion rate constant and the presence or absence of the eclogite transition. The suite of recorded metamorphic P-T conditions, called the piezothermic array has a slope which is insensitive to the variable conditions. High-temperature metamorphic rocks can be produced either by high initial thermal gradient or by relatively slow erosion of eclogite-weighted crust: to distinguish these causes, details of the chronology of metamorphic recrystallization are required.