Origin of Regional Barrovian Metamorphism in Hot Backarcs Prior to Orogeny Deformation

Abstract

AbstractThis article presents evidence that the high temperatures responsible for much regional metamorphism, the Barrovian sequence, occurs in subduction zone backarcs prior to and not, as commonly inferred, as a consequence of heating during continental collision orogeny. This conclusion follows from the recent recognition that most current subduction zones have 200‐ to 1,000‐km wide backarcs that are uniformly hot. They have Barrovian metamorphism vertical temperature gradients that extrapolate to 800–850 °C at a 35‐km Moho, in contrast to ~450 °C for normal stable crust. This explanation overcomes several difficulties of previous explanations. Collisional crustal shortening and thickening as in Himalaya‐Tibet should reduce the vertical gradient by up to a factor of 2, so the high metamorphic temperatures, if associated with crustal shortening, require an increase in gradient of up to a factor of 4 starting from a stable cool continent. Mechanisms previously suggested for the heat, 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 evidence that some regional metamorphism is syntectonic or predates deformation and that a cool lithosphere is too strong to deform from plate boundary forces. I suggest that the high temperatures of regional metamorphism initiate in the hot backarc prior to deformation events. Multiple phases of metamorphism also may result from uplift during complex ongoing hot backarc deformation. In this interpretation, orogenic deformation is responsible for bringing the previously existing high‐temperature rocks to the surface, not for the metamorphism itself.