EARTH EVOLUTION AND SUPERCONTINENTS

The supercontinent cycle is intimately tied to the secular evolution of the Earth sytem and has exerted fundamental control on the rock record since at least the end of the Archean. The continental crust is the archive of Earth history and its record of rock units and events is heterogenous with distinctive peaks and troughs of ages for igneous crystallization, metamorphism, continental margins and mineralization. This temporal distribution is argued to largely reflect the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analyzed.

We recognize 5 stages of Early evolution: 1) Initial accretion and differentiation of Earth’s core/mantle system within the first few 10’s of million of years; 2) Generation of crust in a pre-plate tectonic regime in the period prior to 3.0 Ga; 3) Early plate tectonics involving hot shallow subduction over the period from 3.0-1.7 Ga; 4) Earth’s middle age from 1.7-0.75 Ga, characterized by environmental, evolutionary and lithospheric stability; 5) Modern cold subduction which has existed for last 0.75 Ga. Supercontinents have operated during the last three stages. This evolving tectonic character has likely been controlled by secular changes in mantle temperature and its resultant impact on lithospheric behavour. Crustal volumes, reflecting the interplay of crustal generation and recycling, increased until Earth’s middle age when they plateaued and have likely been decreasing for the last 0.75 Ga.