Continental growth and the crustal record

The continental crust is the archive of Earth history. The spatial and temporal distribution of the Earth's record of rock units and events is heterogeneous with distinctive peaks and troughs in the distribution of ages of igneous crystallisation, metamorphism, continental margins and mineralisation. This distribution reflects 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 analysed. New models based on the U-Pb,Hf and O isotope ratios of detrital zircons suggest that at least similar to 60-70% of the present volume of the continental crust had been generated by 3 Ga. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at similar to 3 Ga. This appears to have been linked to significant crustal recycling and the onset plate tectonics. The 60-70% of the present volume of the continental crust estimated to have been present at 3 Ga, contrasts markedly with the <10% of crust of that age apparently still preserved and it requires ongoing destruction (recycling) of early formed crust and subcontinental mantle lithosphere back into the mantle through processes such as subduction and delamination. (C) 2013 Elsevier B.V. All rights reserved.