In many important ways, Archean and Paleoproterozoic ('older') orogens differ structurally from contemporary examples. This essay examines the premise that contrasts between older orogens and contemporary orogens reflect long-term changes in the temperature of the continental crust, in the density of supracrustal sections, and in exhumation rates. For example, if continental crust were warmer and exhumation rates faster, earlier in Earth history, then higher grade rocks would occur closer to the surface of older orogens, and the orogens would be lower and wider. This situation might contribute to the formation of wide belts of high-grade gneiss found in ancient crust. If the high-strength layer of the crust were thinner and supracrustal sequences denser, earlier in Earth history, then regional extensional tectonism might lead to crustal-scale boudinage and diapirism. This situation might explain formation of the extensive dome-and-keel provinces found in ancient crust. Testing such speculations, through the application of structural analysis coupled with petrologic studies, dating, and rheological modelling, will constrain models of Earth's long-term physical evolution.
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