Thermal and Mechanical Controls on the Evolution of Archean Crustal Deformation: Examples From Western Australia

Dome-and-keel formation in Archean granite–greenstone terrains was uniquely favored by high abundances of heat-producing elements (HPEs) in the crust, and the ubiquity of pre-doming “greenstone-over-granite ” stratification, which consti-tuted large-scale density inversions and facilitated long-term steepening of the geotherm via burial of felsic basement-hosted HPEs. This contribution investi-gates the influence of these factors on the thermo-mechanical stability of the crust, with reference to two contrasting Archean granite–greenstone terrains in Western Australia. In the Neoarchean Eastern Goldfields Province, the rapid (2715–2665 Ma) accumulation of a thin (8-km) greenstone succession atop HPE-poor felsic crust (radiogenic heat flow contribution qc = 45 mW m−2) raised mid-crustal temperatures from 230 °C to 370 °C prior to 2650 Ma dome-and-keel formation. In such cold, strong crust, the development of broad, granite-cored antiforms and narrow, greenstone-cored synforms with little strike variation is likely to directly reflect far-field, horizontal tectonic forces, oriented perpendicu-lar to the regional structural grain. In contrast, the Mesoarchean East Pilbara Granite–Greenstone Terrane underwent slow (3515–3325 Ma) accumulation of tick (14-km) greenstones atop HPE-rich (qc = 70 mW m−2) basement, which raised mid-lower crustal temperatures from 400 °C to 750–800 °C, prior to 3300 Ma dome-and-keel formation. The effective viscosity of this hot, weak crust was further reduced by partial melting of the felsic basement; the resulting “clas-sical ” architecture (featuring granite domes flanked by greenstone keels with a variety of strike orientations) may therefore reflect partial convective overturn and vertical reorganization of the crust during thermo-mechanical stabilization