Characteristic thermal regimes of plate tectonics and their metamorphic imprint throughout Earth history: When did Earth fi rst adopt a plate tectonics mode of behavior?

Where plates converge, one-sided subduction generates two contrasting thermal environments in the subduction zone (low dT/dP) and in the arc and subduction zone backarc or orogenic hinterland (high dT/dP). This duality of thermal regimes is the hallmark of modern plate tectonics, which is imprinted in the ancient rock record as penecontemporaneous metamorphic belts of two contrasting types, one characterized by higher-pressure–lower-temperature metamorphism and the other characterized by higher-temperature–lower-pressure metamorphism. Granulite facies ultrahigh-temperature metamorphism (G-UHTM) is documented in the rock record predominantly from the Neoarchean to the Cambrian, although it may be inferred at depth in some younger Phanerozoic orogenic systems. Medium-temperature eclogite–high-pressure granulite metamorphism (E-HPGM) also is fi rst recognized in the Neoarchean, although well-characterized examples are rare in the Neoarchean-to-Paleoproterozoic transition, and occurs at intervals throughout the Proterozoic and Paleozoic rock record. The fi rst appearance of E-HPGM belts in the rock record registers a change in geodynamics that generated sites of lower heat fl ow than previously seen, inferred to be associated with subduction-to-collision orogenesis. The appearance of coeval G-UHTM belts in the rock record registers contemporary sites of high heat fl ow, inferred to be similar to modern arcs, abd backarcs, or orogenic hinterlands, where more extreme temperatures were imposed on crustal rocks than previously recorded. Blueschists fi rst became evident in the Neoproterozoic rock record, and lawsonite blueschists, low-temperature eclogites (high-pressure metamorphism, HPM), and ultrahigh-pressure metamorphism (UHPM) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers low to intermediate apparent thermal gradients typically associated with modern subduction zones and the eduction of deeply subducted lithosphere, including the eduction of continental crust subducted during the early stage of the collision process in subduction-to-collision orogenesis. During the Phanerozoic, most UHPM belts have developed by closure of relatively short-lived ocean basins that opened due to rearrangement of the continental lithosphere within a continent-dominated hemisphere as Eurasia was formed from Rodinian orphans and joined with Gondwana in Pangea,