Mechanisms to generate ultrahigh-temperature metamorphism

Ultrahigh-temperature (UHT; T-max >= 900 degrees 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. Ultrahigh-temperature (UHT) metamorphism represents the most extreme thermal conditions that the continental crust is capable of sustaining at 15-55 km depth. This Review assesses the characteristics of UHT metamorphism, the tectonic mechanisms, and the implications for metamorphism in present-day orogens.

Automated summary

Ultrahigh-temperature (UHT) metamorphism occurs in the continental crust at depths of 15-55 km and requires unusually high thermal conditions. This review explores the tectonic settings of UHT metamorphism, which are commonly associated with convergent plate margins, supercontinent assembly, arc-backarc systems, thinned lithosphere, orogenic plateaus, and orogenic collapse. Geophysical mapping reveals the occurrence of UHT metamorphism in present-day orogens such as the Tibetan Plateau and the North American Cordillera. Further research should focus on improved geodynamic modelling to establish quantitative correlations between heat sources and the spatial extent of UHT metamorphism.