Contraintes PTt et simulations thermomécaniques des processus géodynamiques paléoarchéens (craton de Pilbara, Australie, et ceinture de roches vertes de Barberton, Afrique du Sud)

Mid- to high-pressure metamorphic rocks are commonly interpreted in terms of plate tectonic processes including subduction. In the Archean, these rocks could have been also produced during the sagduction of greenstone covers into their crustal basement. In order to better understand how distinguish these processes, we conducted a comparative study (fieldwork, metamorphic and geochronological studies and numerical simulations), from two Archean (~3.5-3.2 Ga) key localities: i) the East Pilbara Granite-Greenstone Terrane (EPGGT, Australia), considered as a reference model for sagduction, and ii) the Barberton Greenstone Belt (BGB, South Africa) either described in terms of subduction or sagduction. Interestingly, these two terranes display dome-and-keel structure in which narrow belts of greenstone (metabasalts) and overlying sedimentary rocks occur in association with ovoid TTG (Tonalite-Trondhjemite-Granodiorite). The study of the TTG dome emplacement allows us to understand Archean geodynamic processes. Indeed, the main differences between these two areas seem to be the size and composition of the domes. In the EPGGT, the domes are large (> 50 km) and composed of several phases. Our multidisciplinary study supports fast, gravity-driven sagduction tectonics, where surface sedimentary rocks were buried to lower crustal conditions, metamorphosed at ~3.31 Ga and exhumed back to the surface inside the dome in a few million years. A second age, more ancient and probably metamorphic is recorded at ~3.44 Ga. In the BGB, the domes are smaller (5-10 km) and homogeneous. These observations allow us to propose a probably polymetamorphic history (~3.22 Ga, 3.45 Ga and 3.53 Ga) and complex process combining subduction, diapiric ascent and metamorphic core complex. Based on a comparative study of these two terranes, we emphasize that there is no incompatibility between sagduction and subduction, and both processes may have co-existed in the Archean. Sagduction and subduction present similarities as the record of apparent geothermal gradient (i.e. 10-45°C/km), including low values that are similar to those proposed for Archean and modern subduction.