LATE ARCHEAN TO EARLY PALEOPROTEROZOIC GLOBAL TECTONICS AND ENVIRONMENTAL CHANGE

Analysis of the tectonostratigraphic records of Late Archean to Early Paleoproterozoic terranes worldwide indicates linkage between global tectonic cyclicity, changing sea levels and environmental conditions that is remarkably similar in style, relative timing of events, and duration to Phanerozoic cycles of supercontinent breakup and aggregation. A Late Archean tectonic cycle started at ~2.78 Ga and involved the breakup of a pre-existing continent (Vaalbara) and one of the most prodigious periods of generation and preservation of juvenile continental crust in Earth history during a period of plume breakout (~2.72 to 2.65 Ga) accompanied by high sea levels. During this period, cratons formed by accretion of granitoid-greenstone terranes at convergent margins started to aggregate to form larger continents (e.g. Kenoraland). Lower sea levels between ~2.65 and 2.55 Ga were followed by a second (~2.51 to 2.45 Ga) period of plume breakout and high sea levels resulting in a global peak in magmatism and deposition of banded iron formations on the trailing margins of the Pilbara and Kaapval cratons. Cratons in South Australia, Antarctica, India and China record cycles of convergent margin magmatism and high-grade metamorphism between 2.56 and 2.42 Ga. Aggregation of continental fragments may have formed a supercontinent by ~2.4 Ga with a return to low sea levels and a period of tectonic quiescence, before the supercontinent started to breakup from ~2.32 Ga. Plume breakout (2.72 to 2.65 Ga and 2.51 to 2.45 Ga) would have enhanced the reduced conditions typical of the Archean biosphere as well as the CH4, and CO2 contents of the atmosphere necessary to counter lower solar luminosity and maintain temperate conditions. The sequence of events between 2.5 and 2.32 Ga including the availability of important crustal oxygen sinks during plume breakout, supercontinent formation, and subsequent tectonic quiescence, led initially to the loss of the Archean CH4 greenhouse soon after 2.45 Ga followed by episodic glaciation with irreversible oxidation of the atmosphere when the global flux of reduced gasses fell below that of photosynthetic oxygen by ~2.32 Ga.