Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures

摘要

Archean geological records are increasingly interpreted to indicate a = 3.2 Ga initiation of plate tectonics on Earth. This hypothesis contrasts with dominant plate tectonic interpretations for the Eoarchean (ca. 4.0-3.6 Ga) Isua supracrustal belt (southwest Greenland). Alternatively, recent work shows the belt could have formed via heat-pipe tectonics. Predicted strain distributions across the belt vary between models. Plate tectonic models predict a dominant unidirectional shear sense, corresponding to subduction vergence, and strain localization within similar to 10-m-scale shear zones. In contrast, the proposed heat-pipe model predicts two opposing shear senses, corresponding to opposite limbs of 0.1-m to km-scale a-type folds (i.e., sheath and curtain folds), with relatively equal strain distributed across the belt. Here, we present the first microstructure study using thin-section petrography and electron backscatter diffraction analysis on quartz of oriented samples from throughout the Isua supracrustal belt. Key findings are: (1) the Eoarchean Isua supracrustal belt was deformed at similar to 500 degrees C-650 degrees C, with potential postdeformational recovery at similar or lower temperatures, (2) the spatial distribution of the two opposing shear senses which dominate the belt (top-to-southeast and top-to-northwest) appears to be random, and (3) the strain intensity across the belt appears to be quasiuniform as evidenced by the uniformly low (mostly 0.1) M-indexes of quartz fabrics, such that no = 100 -m-scale shear zones can be detected. Our findings are only consistent with the predictions of the heat-pipe model and do not require plate tectonics, so the geology of the belt is compatible with a = 3.2 Ga initiation of plate tectonics.