THE METAMORPHIC ARCHITECTURE OF SUBDUCTION ZONES: INFLUENTIAL DESIGNS BY W. GARY ERNST

Our current understanding of subduction zones has been profoundly shaped by Gary Ernst's integration of metamorphism and plate tectonics in the early 1970s. During the plate tectonic revolution, Ernst demonstrated that (i) high-pressure, low-temperature blueschist and eclogite terranes mark the sites of former convergent plate boundaries and (ii) within high-pressure terranes, such as those present in California, the Alps, and Japan, metamorphic grade increases in the direction of former subduction. In his highly cited 1970 J.G.R. paper, Ernst published his first plate tectonic design; it was not to be his last. This 1970 cross section depicted the tectonic setting of the Franciscan mélange within an east-dipping Mesozoic Benioff zone, complete with isotherms and extending to 150 km depth. Over the years, this tectonic design has evolved to include arc magmatism (1973), metamorphic facies (1976), pterodactyls, and transformation to cylindrical "coffee cup" coordinates.

Recent high resolution seismological and thermal modeling studies confirm the distribution of metamorphic facies in subduction zones predicted by Gary Ernst. Regional seismic tomographic studies have revealed the blueschist to eclogite transition within subducting oceanic crust at a depth of 45 km beneath Oregon and 120-140 km beneath southern Alaska. Dipping low-seismic-velocity wave guides observed in Pacific subduction zones suggest eclogite forms at 100 to 250 km depth. Thermal-petrologic models constructed for specific subduction zones combined with accurate hypocenter locations suggest that intermediate-depth (40-300 km depth) earthquakes are linked to metamorphic dehydration reactions in the subducting slab, particularly those associated with the blueschist-to-eclogite transition. More than thirty years after its original unveiling, Gary Ernst's metamorphic architecture of subduction zones continues to stand the test of time.