Microstructures from the Isua supracrustal belt reveal distributed strain in the absence of major fault structures

Abstract:

Isotopic, geochemical, and geochronological records are increasingly interpreted to indicate a ≤3.2 Ga onset of plate tectonics. In contrast, the Eoarchean (4.0-3.6 Ga) Isua supracrustal belt of SW Greenland is widely interpreted to have formed within a plate tectonic geodynamic framework. Alternatively, recent work shows that the Isua supracrustal belt could have formed via heat-pipe tectonics. These different tectonic scenarios predict contrasting strain distributions across the belt. Proffered plate tectonic models predict a dominant unidirectional shear sense, corresponding to subduction vergence, within ~10-m-scale shear zones. In contrast, the proposed heat-pipe model predicts two opposing shear senses, corresponding to opposite limbs of a-type folds (i.e., sheath and curtain folds) developed at ≥0.1 m scale, with relatively equal strain distributed across a belt-spanning (i.e., km scale) shear zone. The shear zone in this model represents either A) contraction in response to radial subsidence during heat-pipe cooling, or B) contraction marking the end of heat-pipe cooling and the onset of subduction. We present the first microstructure study using thin-section petrography and electron backscatter diffraction (EBSD) on quartz crystals of oriented amphibolitic schist or chert/banded-iron-formation samples from throughout the Isua supracrustal belt. Results show that 1) the Isua supracrustal belt was generally deformed at ~500-650 ºC, with only four out of thirty-three samples potentially showing evidence of deformation at ~300-400 ºC, 2) two opposing shear senses (top-to-SE and top-to-NW) are dominant and their spatial distribution throughout the belt appears to be random, and 3) as evidenced by the uniformly-low quartz fabric strength (most of the samples show <0.1 M-indexes, with only two outliers showing 0.14 or 0.19 M-indexes), the strain intensity across the belt appears to be quasi-uniform, such that no ≤100 m scale shear zones can be detected. These results contradict predictions of the plate tectonic models, and are consistent with the heat-pipe model predictions. Because our results indicate that the Eoarchean Isua supracrustal belt can be interpreted without plate tectonic processes, we find that the geology of the belt is compatible with a ≤3.2 Ga onset of plate tectonics.