Eoarchean formation of the Isua supracrustal belt

A major shift in Earth’s crustal generation processes at ~3.2 to 2.5 Ga has been inferred from mineralogical, geological, and geochemical records, particularly those recorded by fine-grained sediments and zircon crystals. The most common hypothesis to explain this shift is the onset of plate tectonic recycling following some form of hot stagnant lid geodynamics. However, all prior detailed geologic studies of our best-preserved Eoarchean terrane, the ~3.85 - 3.60 Ga Isua supracrustal belt of SW Greenland, interpret this site to record terrane collision within the context of plate tectonics. This represents a significant counterweight to the assumption underpinning the ~3 Ga tectonic-mode-change models, i.e., the idea that early Earth’s record is broadly representative. The Isua belt is divided into ~3.8 and ~3.7 Ga halves, and these have been interpreted as plate fragments which collided by ~3.6 Ga. Here, we examine the evidence used to support plate tectonic interpretations, focusing on 1) reanalysis of prior geochronological results and associated cross-cutting relationships which have previously been interpreted to record as many as eight tectonic events, and 2) new field observations leading to reinterpretation of basic structural relationships. Simpler interpretations of the geochronological and deformation data are viable: the belt may have experienced nearly homogeneous metamorphic conditions and strain during a single deformation event prior to intrusion of ~3.5 Ga mafic dikes. Curtain and sheath folds occur at multiple scales throughout the belt, with the entire belt potentially representing Earth’s largest a-type fold. We propose a new model: two cycles of volcanic burial and resultant melting and TTG intrusion produced first the ~3.8 Ga rocks and then the ~3.7 Ga rocks above, after which the whole belt was deformed and thinned in a shear zone, producing the multi-scale a-type folding patterns. The Eoarchean assembly of the Isua supracrustal belt is therefore most simply explained by vertical-stacking volcanic and instrusive processes followed by a single shearing event. In combination with well-preserved Paleoarchean terranes, these rocks record the waning downward advection of lithosphere inherent in volcanism-dominated heat-pipe tectonic models for early Earth. These interpretations are consistent with recent findings that early crust-mantle dynamics are remarkably similar across the solar system’s terrestrial bodies.