Secular variation of metamorphism and the evolution of plate tectonics

The Department of Earth & Environmental Sciences, jointly with Rensselaer Polytechnic Institute, presents a special edition of the K.D. Nelson Lecture Series featuring guest speaker, Michael Brown.

Dr. Brown will share insights drawn from metamorphic rocks about when plate tectonics first began operating on Earth.

On contemporary Earth, crustal metamorphism is predominantly associated with convergent plate boundaries and subduction-to-collision orogenesis. Metamorphism is classified into three types using the thermobaric ratio (T/P) retrieved from the peak mineral assemblage of dated samples, since this varies both spatially and temporally in orogens. Using T/P, three types of metamorphism have been defined (Fig. (a)), where each is generally associated with a particular tectonic setting (from low to high T/P: subduction, mountain belt and orogenic hinterland). Orogens younger than 850 Ma record bimodal metamorphism (lower and higher T/P), whereas metamorphism at lower T/P is absent from older orogens (Fig. (c) and (d)). A strong case can be made for plate tectonics back to the Neoproterozoic and, probably, back to the early Paleoproterozoic, when breakup of supercratons and reconfiguration of the fragmented continental lithosphere led to the formation of the supercontinent Columbia, in which cratons are sutured by orogenic belts that record bimodal metamorphism and preserve seismic evidence of probable subduction. Whether plate tectonics operated before the Paleoproterozoic requires more than just a regional argument for subduction; it requires evidence of a global network of narrow boundaries within a fragmented lithosphere, which is difficult to demonstrate given the limited area of preserved Archean crust and ambiguity in interpretation of the geology.

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