While studying rocks in the Scottish Highlands in the late 1800s, George Barrow mapped a sequence of mineral zones representing increasingly higher grades of metamorphism at inferred increasing temperature and depth in Earth. Now known to represent the most common type of regional metamorphism, the Barrovian sequence has been widely documented in areas that experienced the elevated temperatures associated with continental collision and other tectonic deformation.
Barrovian metamorphism is distinguished by a high vertical temperature gradient that, when extrapolated, yields temperatures of 800°C to 850°C at the base of 35-kilometer-thick crust—nearly double that of stable continental areas. Previous research has found a number of mechanisms to explain these high temperatures, including frictional heating, magmatism, and underthrusting of crust containing abundant radioactive heat generation. However, none of these mechanisms are entirely consistent with field evidence showing that some regional metamorphism occurs prior to or during deformation—or the fact that only lithosphere that is already warm is weak enough to be deformed by the forces generated at plate boundaries.
Here Hyndman proposes a new theory to overcome the problems of these previous explanations. Namely, the high temperatures responsible for Barrovian metamorphism are not caused by heat generated during and after deformation; instead, these temperatures predate continental collision and other tectonic deformation.
According to the author, the high temperatures have their origin in precollision hot back arcs—broad areas, up to 1,000 kilometers wide, found landward of the subduction zones that must occur on at least one side as continents converge and oceans close. This idea is based on recent observations that most modern subduction zones have uniformly hot back arcs with thin lithospheres and vertical temperature gradients that are remarkably consistent with Barrovian metamorphism. Most collision deformation and regional metamorphism around the world are concentrated in former hot, weak back arcs, which had Barrovian temperature gradients prior to ocean closure and collision.
By concluding that regional metamorphism and deformation can result from back-arc crust that was already heated to high temperatures prior to deformation, this paper offers an innovative vision of the thermal structure of many ancient and modern collision zones. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2018GC007650, 2018)
—Terri Cook, Freelance Writer