Two Major Transitions in Earth History: Evidence of Two Lithospheric Strength Thresholds

Past geological and geochemical changes suggest the existence of two transitions in Earth history at 2.5–2 and 1–0.5 Ga. Twenty-one changes during Transition One and eight during Transition Two signify global-scale shifts in terrestrial tectono-thermal systems. Transition-One changes include the onset of major zircon age peaks, increases in average plate speeds, increases in the number of collisional orogens and associated metamorphism, and continental sedimentation. In addition, changes in rock abundances and chemical composition of continents, of detrital sediments, and in seawater composition reflect an increase in volume of felsic component in the continental crust as well as increasing continental emergence. Changes in composition of oceanic basalts during this time reflect mantle cooling, oxidation, and the appearance of an enriched component in mantle magma sources. A rise in large igneous province (LIP) frequency may reflect enhanced mantle-plume activity responding to thermal-compositional disturbance in the deep mantle due to sinking slabs. Transition Two is characterized by the appearance and establishment of blueschists, ultrahigh-pressure (UHP) metamorphism, and greatly enhanced ophiolite preservation beginning at 750–600 Ma, and this transition may signal the first subduction of continental crust. Each transition is proposed to reflect a thermal threshold during which the lithosphere strengthens. Transition One is interpreted to record the onset, propagation, and eventual establishment of plate tectonics, and Transition Two the onset and establishment of subduction of a strong continental lithosphere.