UNDERSTANDING REGIONAL, BARROVIAN METAMORPHISM

I present evidence for the conclusion that the high temperatures of most regional Barrovian metamorphism occurs in subduction zone backarcs prior to and not as commonly inferred, as a consequence of continental or terrane collision orogeny. This conclusion follows from the recent recognition that most current subduction zones, including the Cordillera from Mexico to Alaska have 200-1,000 km wide backarcs that are uniformly hot, even where there is no evidence for extension. They have Barrovian vertical temperature gradients that extrapolate to 800-850 C at a 35 km Moho, in contrast to ~400-500 C for normal stable crust. This explanation overcomes several difficulties of previous explanations. Collisional crustal shortening and thickening as in the Himalaya-Tibet should stretch the vertical gradient by up to a factor of two, so the high metamorphic temperatures, if associated with crustal shortening, require an increase in temperature of up to a factor of four starting from a stable cool continent. Mechanisms related to orogenic processes previously suggested for the heat, including underthrusting of near-surface radioactive heat generation, ductile/frictional heating, igneous activity, and deformation-induced lithosphere thinning, have difficulty producing such large heating. They also are not consistent with the evidence that some regional metamorphism is syntectonic or predates deformation, and that a cool starting lithosphere is too strong to be deformed by plate tectonic forces. I suggest that the high temperatures of regional metamorphism initiate in the hot subduction zone backarc prior to collision. Later metamorphism events may result from deformation uplift of the already hot backarc. In this interpretation, orogenic deformation is responsible for bringing the previously existing high temperature rocks to the surface, not for the metamorphic high temperatures themselves.