Deep crustal metamorphism during continental extension: modern and ancient examples

Abstract

Granulite facies metamorphism in the lower levels of continental crust which is undergoing extension is indicated by unusually high heat flow in modern-day extensional regimes. For certain geometries of extension, particularly those involving crustal-penetrative detachment zones, this metamorphism may occur on a regional scale. The predicted pressure-temperature-time ( P-T-t) paths for such metamorphism involve heating into the granulite facies at constant or decreasing pressure during extension, followed by cooling at constant or increasing pressure after extension stops, and thus they differ considerably from P-T-t paths of metamorphic terrains formed by continental convergence. Many granulite terrains from both the Precambrian and Phanerozoic record preserve P-T-t paths which involve substantial, essentially isobaric, cooling. In such terrains granulite facies metamorphism is typically associated with recumbent structures characterised by subhorizontal stretching lineations which are attributed to intense non-coaxial deformation. Such deformation may be expected for the deep crustal expression of detachment zones. These terrains may provide ancient examples of deep crustal metamorphism during extensional tectonics.