Archean thermal regime and stabilization of the cratons
Résumé
Archean provinces are presently characterized by low heat flow, with an average of 41 mW m-2, less than the global continental average (56 mW m-2) . The range of regionally averaged heat flow values in Archean Provinces (18-54 mW m-2) is narrower than in younger terranes. At the end of the Archean, when crustal heat production was double the present, surface heat flow varied over a range (≈45-90 mW m-2) as wide as that presently observed in Paleozoic Provinces. For the present-day vertical distribution of radio-elements, high heat production during the Archean is insufficient to account for elevated lower crustal temperatures. High temperature-low pressure metamorphism conditions require additional heat input, for example by emplacement of large volumes of basaltic melts, crustal thickening or larger concentrations of radio-elements in the lower crust. In the Archean, with a crust thicker than 40 km or with the radio-elements uniformly distributed throughout a 40 km thick crust, the lower crust was near melting and, with an effective viscosity ≈1019Pa s, it could not sustain the stress due to crustal thickening. Long-term crustal stability requires the enrichment of the upper crust in radio-elements through melt extraction from the lower crust. After root emplacement, thermal conditions in cratons remained far from equilibrium for 1-2 Gy. Depending on the mechanism of root formation, temperature at 150 km might only be 150 K higher than present, implying that the lithospheric mantle remained sufficiently cold and strong to preserve Archean features. In thick continental lithosphere, temperatures in the crust and deep in the continental root are effectively decoupled for a long time.