PENROSE MEDAL: THE EVOLUTION OF PLATE TECTONICS ON PLANET EARTH: TWO LITHOSPHERIC TRANSITIONS IN THE LAST 4 GYR

In the past 4 Gyr, Earth has undergone two major tectono-thermal transitions as shown by the geologic record. During the First Transition between 3 and 2 Ga, deformation becomes concentrated along globally linked plate boundaries, collisional orogens appear, large plate motions are recorded by APW paths, ophiolites become well established, major zircon age peaks record episodic increased production of felsic magma and probably also in the volume of continental crust, the mantle diverges into two temperature and compositional regimes, a new enriched mantle reservoir appears, and the degree of mantle melting rapidly decreases. The volume of hydrated mantle also probably increased during this transition, and the supercontinent cycle began near the end of the transition. During the Second Transition between 1 and 0.5 Ga, ophiolites and mélange increase in abundance, ultra-high pressure (UHP) metamorphic rocks and blueschists appear, marine carbonates record a rapid rise in seawater ⁸⁷Sr/⁸⁶Sr, and basaltic magma sources in the mantle increase significantly in compositional variability. Interestingly, the 187 and 94 Myr mantle cycles recorded by LIP and zircon age peaks are not affected by either transition. Both transitions reflect thermal thresholds affecting lithosphere behavior. The First Transition may represent a change from stagnant-lid (such as drip, heat pipe or sluggish lid) tectonics into plate tectonics with short-lived bursts of plate tectonics, which did not become well established until ~ 2 Ga. Decreasing mantle temperature and increasing lithosphere strength during the Second Transition are responsible for increased preservation of ophiolites and mélange and for the appearance of UHP and blueschist metamorphism. Increasing lithospheric strength also led to greater relief and erosion and to more sediments recycled into the mantle, thus increasing mantle compositional variability during the Second Transition.