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A brief review of secular trends of Proterozoic sedimentation, volcanism, K20/Na20 ratio of supracrustal rocks, massive sulfide mineral deposits, isotopic Sr ratio in seawater, geochronology, and paleomagnetism clearly shows the episodic nature of orogenic activity. Brief episodes of deformation and generation of continental crust, around -1800 Ma and around -1000 Ma were separated by long (800 Ma) periods of limited continental growth. Rapid thickening of continental lithosphere and growing instability of oceanic lithosphere led to the start of subduction around -800 Ma. Subduction-controlled plate tectonics has dominated orogenic evolution ever since. A theory of unsteady mantle convection limited to the upper mantle (Tozer, 1965) can explain Proterozoic orogenies as the consequence of convective over-turns separated by “warm-up” periods. Present plate motions would control the distribution and motions of mantle currents rather than vice versa. Proterozoic plate motions of limited lateral amplitude led to formation of ensialic orogens. Paleomagnetic polar wander paths suggest that local crustal extension preceded phases of compression responsible for orogenies. The change-over from the Archean to the Proterozoic would correspond to a change in convection regime of the upper mantle from steady to unsteady convection.

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