PAIRED METAMORPHISM AND THE EVOLUTION OF PLATE TECTONICS

At present, Earth’s mantle convection, which facilitates planetary heat loss, is expressed at the surface as plate tectonics. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and provocative questions outstanding in Earth science. Metamorphic rocks record periods of burial/exhumation and heating/cooling that reflect the tectonic environments in which they formed. Therefore, changes in the global distribution of metamorphic P–T conditions in the continental crust through time offer insights into the secular evolution of Earth’s tectonic processes. Phanerozoic convergent plate margins are characterized by metamorphic rocks that record a bimodal distribution of geothermal gradients (change in temperature with depth, parameterized as the thermobaric ratio: T/P), in the form of paired metamorphic belts, corresponding to metamorphism near to (low T/P) and away from (high T/P) subduction zones. In this talk, we evaluate the emergence of paired metamorphism as a proxy for secular change in the style of (plate) tectonics using a statistical evaluation of the distributions of metamorphic T/P through time. We find that Proterozoic orogenic belts are also characterized by paired metamorphism, but with less distinction between the high- and low-T/P modes of metamorphism than in Phanerozoic orogenic belts. We do not find evidence for global paired metamorphism in the Archean Eon. Viewed together, these observations indicate an increase in the diversity of metamorphic environments through time. We hypothesize that Earth’s modern plate tectonic regime developed progressively since the Archean Eon in conjunction with secular cooling of the mantle and associated changes in the thickness, buoyancy and rheology of oceanic and continental lithosphere, resulting in an evolution in the styles of subduction and collisional orogenesis.