Rapid heating (<2 Ma) to ultrahigh-temperature metamorphism via asthenospheric upwelling.

ABSTRACT

Ultrahigh-temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900°C. The duration and heat source of UHT metamorphism are critical constraints on the tectonic evolution of orogenic systems. Here, we report the discovery of a sapphirine-bearing granulite from the east-central Himalaya which preserves UHT evidence. The reconstructed pressure-temperature-time path indicates that the temperature increased by almost 200°C within ~2 Ma which is consistent with rapid asthenospheric heat input. Numerical simulations illustrate potential mechanisms for such heating juxtaposition of the deep crustal accretionary system with the upwelling asthenospheric mantle through newly developed apertures generated by slab break-off and/or associated vertical tearing of down-going Indian lithosphere. Spatial-temporal consistencies among the UHT metamorphic phases, postcollisional magmatism, geophysical constraints, and crustal deformation indicate that slab break-off or tearing controls broad swaths of Himalayan tectonics. The consequent upwelling asthenosphere may have been a significant heat source for the Miocene Himalaya and for similar ancient collisional orogenic systems.