Mechanisms to generate ultrahigh-temperature metamorphism

Ultrahigh-temperature (UHT; T max ≥ 900 °C) metamorphism requires unusually high heat in continental crust at depths of 15–55 km, but how such extreme thermal conditions are achieved is enigmatic. In this Review, we investigate UHT metamorphism, based on advances in metamorphic petrology and numerical modelling, to identify the tectonic settings where UHT metamorphism occurs. UHT rocks are spatially related to convergent plate margins and spatially correlate with the assembly of supercontinents, such as the formation of Rodinia (1,350–850 Ma). Commonly, UHT occurrences are linked to arc–backarc systems, thinned lithosphere or orogenic plateaus. Elevated mantle heat in younger arc and backarc systems is related to slab rollback, whereas thinned lithosphere in ancient orogens is related to lithospheric peeling or shallow slab breakoff. By contrast, UHT metamorphism in orogenic plateaus is a result of radiogenic heating during thickening, sometimes with elevated mantle heat during orogenic collapse. Geophysical mapping of Moho temperature and depth beneath present-day orogens reveals the locations where UHT metamorphism is occurring, such as in the Tibetan Plateau and the North American Cordillera. Future research should include improved geodynamic modelling of UHT metamorphism and the respective tectonic settings to establish quantitative correlations between a viable heat source and the spatial extent of UHT metamorphism.