Comparing the Geology of the Pilbara and Isua Archean Terranes: Viability of Vertical Tectonics in the Eoarchean

Abstract

The global tectonic setting(s) of the early Archean is a long-debated topic in geology. There are two major groups of models: uniformitarian models that largely mirror modern plate tectonics and non-uniformitarian models which involve various ‘vertical’ tectonics concepts. These models have been proposed and examined during the last decades based primarily on study of exposed Archean terranes. Isua and Eastern Pilbara are two of the best studied Archean terranes because of their old ages (>3.8-3.6 Ga and >3.5-3.2 Ga), large exposures and relatively good preservation of early Archean lithologies and structures. The uniformitarian model has been widely applied to Isua whereas the vertical tectonics model is largely drawn from the geology of the Eastern Pilbara. This difference in interpretation suggests a few possible scenarios: early Earth had diverse geological systems, or one or both interpretations may be flawed. Plate tectonics appears strongly excluded by the distinctive dome-and-keel structural geometry of the Eastern Pilbara, but it appears possible to question whether Isua may have developed via vertical tectonic processes. To address this question, this project investigates the lithology, geochemistry, geochronology, metamorphism and structural geology of Isua and Eastern Pilbara. Data are compiled from literature sources, new field mapping in Eastern Pilbara, and new whole rock geochemistry of Pilbara ultramafic rocks. Results from the two terranes show that the geometry, lithology, geochemistry, metamorphism styles, and structural patterns are largely similar, with minor differences that can be interpreted as results of local deformation variations. Processes of subduction, arc magmatism, terrane juxtaposition, and formation of core complexes provide most predictions for uniformitarian models, whereas extensive volcanism, TTG doming, and sinking of thick mafic crust are the main processes supplying predictions for non-uniformitarian vertical tectonics models. Although predictions of uniformitarian models can fit much of the assembled data, there are always some key features that cannot be explained. For examples: (1) ultramafic rocks from Pilbara and Isua are chemically consistent with intrusive and extrusive crustal igneous rocks and not consistent with mantle peridotite chemistry, as predicted by plate tectonic models of these terranes. Likewise, (2) the radial dome-and-keel structural geometry of Pilbara cannot be explained via formation of metamorphic core complexes, as predicted by some plate tectonic models, and Isua may represent a similar structural pattern that has undergone later deformation. In contrast, these records are consistent with the predictions of non-uniformitarian vertical tectonics models. Therefore, it appears that a non-uniformitarian vertical tectonics model may