Orogenesis

Previous chapters have outlined the principles underpinning the understanding and analysis of orogenic belts in the modern world, with examples and applications to the Alps, the Himalaya, the Andes and western North America. We have also used examples from earlier times in Earth history to illuminate the issues surrounding, for example, the processes of thrust tectonics, metamorphism at extreme conditions, variability in exhumation rates, and contrasts in the amounts of magmatism in collisional and accretionary orogens. In doing this we have presented evidence from Palaeozoic orogens, such as the Variscan/Hercynian and Caledonian, Neoproterozoic orogens including the ‘Pan African’ belts of former Gondwanan fragments in India, Africa and Antarctica, and Mesoproterozoic orogens such as the Grenville of northeastern America. This begs the simple question – can we look back in time and see similarities in orogeny and the making of mountains throughout Earth history, or has mountain building changed in subtle or even dramatic ways since the earliest records of continental crust, deformation and metamorphism? In other words, has there been secular change in orogeny? Within the context of the recognised diversity in Phanerozoic orogenies, were Archaean and Proterozoic orogenies broadly the same, or different?

The question of secular change in mountain building has been the subject of considerable debate and a burgeoning literature. It forms part of the broader issue of the extent to which uniformitarianism can be applied far back in time – whether ‘the present is the key to the past’. Recent major volumes on the subject include Ancient Orogens and Modern Analogues (Murphy et al., 2009) and When did Plate Tectonics Begin on Planet Earth? (Condie & Pease, 2008). These build on an extensive literature in which the temporal evolution of the Earth's tectonic regime has been assessed or speculated upon using thermal and geodynamic modelling, geological observations from ancient and modern terrains, and geochemical and isotopic constraints on rates of continental growth through time (e.g. Hawkesworth & Kemp, 2006a, b; Kemp et al., 2007) (see Fig. 12.5 later).