Litho-structural framework of the Eoarchean Tussaap supracrustal belt, Itsaq gneiss complex, southwestern Greenland

The Isua and Tussaap supracrustal belts of the Itsaq gneiss complex, southwestern Greenland, form the largest and best-preserved exposure of Eoarchean supracrustal materials on Earth. Previous studies have almost exclusively focused on the ∼35-km-long, arc-shaped Isua supracrustal belt and adjacent ca. 3.8–3.7 Ga meta-tonalite bodies, which are the basis for competing Archean tectonic regime interpretations (i.e., plate versus heat-pipe tectonics). In this study, we performed geologic field mapping of the seldom-explored Tussaap supracrustal belt, located ~11 km south of the Isua supracrustal belt, to better constrain its litho-structural framework and test the predictions of existing Eoarchean tectonic models. Observations from this study and previous works show that the Tussaap supracrustal belt consists of a east-northeast-striking, ~12-km-long and <1-km-wide, mostly continuous belt of greenstone rocks flanked to the north and south by ca. 3.8 Ga meta-tonalite. Lithologies of the Tussaap supracrustal belt consist of interlayered garnet ± staurolite ± sillimanite paragneiss, felsic schist, garnet mafic schist, amphibole-rich garbenschiefer, and minor pegmatite bodies and meta-ultramafic rocks. The northern and southern contacts between the Tussaap supracrustal belt and meta-tonalite are ~100-m-wide transitional zones featuring interlayered and folded meta-tonalite and greenstone rocks that increase in abundance towards each lithologic unit. Both the Tussaap supracrustal belt and adjacent meta-tonalite feature well-developed, southeast-dipping foliation and southeast-plunging stretching lineation (average 162° trend, 40° plunge). Macroscopic sheath and often rootless, disharmonic folds with hinges parallel to stretching lineation occur throughout the study area. In contrast with previous interpretations, no discrete tectonic discontinuities (i.e., brittle faults and ductile shear zones) were observed within the Tussaap supracrustal belt and meta-tonalite. Similarly, no apparent metamorphic field gradient was observed in the study area. This litho-structural framework is consistent with that of the Isua supracrustal belt and meta-tonalite bodies to the north, indicative of spatially-uniform strain and metamorphism. Based on our preliminary observations, the Archean development of the region can be explained by uniform subvertical shearing and folding of an interlayered volcanic-intrusive sequence (i.e., heat-pipe tectonics). Additional structural, geochronologic, and geochemical analyses of the Tussaap supracrustal belt and meta-tonalite are required to further elucidate their emplacement and metamorphic histories and differentiate end-member models of Archean tectonics.