MULTIDISCIPLINARY APPROACHES TO EVALUATING THE EARTH’S OLDEST EXPOSED SHEAR ZONE: THE ISUA SUPRACRUSTAL BELT OF SW GREENLAND

The ~35-km-long and ~1-3-km-thick Isua supracrustal belt is the Earth’s largest exposure of sheared Eoarchean rocks. The nature of deformation there is debated, with interpretations ranging from partitioning into hundreds of brittle faults, to strain concentrated along a few ~10-m-scale ductile shear zones, to distributed ductile deformation across the entire belt. Contrasting models for the assembly of the belt and subsequent deformation include plate tectonic collisional models and a heat-pipe tectonic model (i.e., a form of “vertical” tectonics). Testing these models is important for understanding Earth’s tectonic evolution, particularly given the ~15 years of diverse studies that advocate a ~3 Ga onset of plate tectonics. We use many approaches (e.g., field mapping, petrography, EBSD, phase equilibria modelling, geochemical analysis and modelling, geochronology) to test model P-T-t-d predictions. Results show that the belt preserves 1) strain records showing quasi-uniform strain intensity and two opposing shear senses that are randomly distributed in space (consistent with a-type folding), and no evidence of substantial strain localization; 2) metamorphic minerals, microstructures and cross-cutting relationships spanning the entire belt indicating syn-tectonic amphibolite facies metamorphism at ca 550-600 °C and 0.5-0.7 GPa within ca 3.66-3.5 Ga; and 3) ultramafic rocks with spinel and whole-rock geochemical characteristics of crustal cumulates. Platinum-group element (PGE) signatures of other Isua-area ultramafic rocks guided prior interpretations that these rocks were ancient depleted mantle wedge materials (i.e., representative of plate tectonics). However, our data from similar rocks of the ca 3.5-3.2 Ga East Pilbara Terrane (Australia), which is famously non-plate tectonic, shows similar PGE patterns. Thus, these patterns are not unique to plate tectonics. Overall, our findings are only consistent with the heat-pipe tectonic model. Therefore, we propose that the belt was deposited in two main phases at ~3.8 Ga and ~3.7 Ga, and then deformed during a portion of the 3.66-3.5 Ga period within a km-scale ductile shear zone that developed at cold mid-crustal conditions. This ductile shearing may represent either a plate breaking event, or a contractional event predicted via heat-pipe tectonics.