STRAIN DISTRIBUTION ACROSS THE ISUA SUPRACRUSTAL BELT, SW GREENLAND: IMPLICATIONS FOR EARTH’S EARLY TECTONICS

Strain distribution across the Eoarchean Isua supracrustal belt, Earth’s oldest 10s-of-km² supracrustal fragment, is disputed. Resolving whether strain here is concentrated in shear zones or distributed quasi-uniformly offers tests of plate tectonic and non-plate tectonic models, and thus bears on the question: is plate tectonics required to understand the first third of Earth history? To address this, we integrate field-based research (including new fieldwork in July-August 2022) with analytical explorations of the belt’s P-T-t-D-X evolution. The basic setting is largely agreed upon: metavolcanics, quartzites, BIFs, and minor ultramafic lenses, carbonates, and clastic rocks all occur in two parallel sequences, of 3.8 Ga and 3.7 Ga ages, which are both warped, foliated, feature a steep SE-plunging stretching lineation, and flanked by meta-tonalites with ages matching those of the respective adjacent sequence. The belt is now ~35 km long, and up to ~4 km wide. Plate tectonic models envisage that the belt (1) represents an accretionary prism or (2) formed as 3.8 Ga and 3.7 Ga arc sequences collided along an Eoarchean mylonitic suture zone; here, other interpreted mylonitic shear zones record a complex collisional history while in low strain zones primary structures such as basaltic pillows are preserved. With field and EBSD-on-quartz analyses, we have observed quasi-uniform high strain without discrete mylonitic zones at and between proposed shear zones and across preserved primary structures. The lineation is prominent, with ~10–100 m² areas dominated by pencil cleavage. Sheath / curtain folds including minor faults occur at outcrop- to km-scales; these folds are associated with otherwise random reversals of shear sense. Microstructural evidence, uniform peak metamorphic conditions, deformation temperatures, and geochronological data record a single ~2.8 Ga amphibolite facies tectonothermal event, which was followed by a static overprint and later (local) retrogression. These findings indicate that the assembly of Isua protoliths can be explained by deposition of volcanic and sedimentary rocks, as in vertical tectonic models like heat-pipe cooling. In turn, this shows that Earth’s early lithospheric construction could match that of our solar system’s other terrestrial bodies.