A non–plate tectonic model for the Eoarchean Isua supracrustal belt

Abstract

The ca. 3.8–3.6-b.y.-old Isua supracrustal belt of SW Greenland is Earth’s only site older than 3.2 Ga that is exclusively interpreted via plate-tectonic theory. The belt is divided into ca. 3.8 Ga and ca. 3.7 Ga halves, and these are interpreted as plate fragments that collided by ca. 3.6 Ga. However, such models are based on idiosyncratic interpretations of field observations and U-Pb zircon data, resulting in intricate, conflicting stratigraphic and structural interpretations. We reanalyzed published geochronological work and associated field constraints previously interpreted to show multiple plate-tectonic events and conducted field-based exploration of metamorphic and structural gradients previously interpreted to show heterogeneities recording plate-tectonic processes. Simpler interpretations are viable, i.e., the belt may have experienced nearly homogeneous metamorphic conditions and strain during a single deformation event prior to intrusion of ca. 3.5 Ga mafic dikes. Curtain and sheath folds occur at multiple scales throughout the belt, with the entire belt potentially representing Earth’s largest a-type fold. Integrating these findings, we present a new model in which two cycles of volcanic burial and resultant melting and tonalite-trondhjemite-granodiorite (TTG) intrusion produced first the ca. 3.8 Ga rocks and then the overlying ca. 3.7 Ga rocks, after which the whole belt was deformed and thinned in a shear zone, producing the multiscale a-type folding patterns. The Eoarchean assembly of the Isua supracrustal belt is therefore most simply explained by vertical stacking of volcanic and intrusive rocks followed by a single shearing event. In combination with well-preserved Paleoarchean terranes, these rocks record the waning downward advection of lithosphere inherent in volcanism-dominated heat-pipe tectonic models for early Earth. These interpretations are consistent with recent findings that early crust-mantle dynamics are remarkably similar across the solar system’s terrestrial bodies.

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

“…(2) penetrative mineral lineation fabrics plunging steeply to the southeast ( Figure S1) (e.g., Bridgwater et al, 1974;Crowley, 2003;Crowley et al, 2002;Hanmer & Greene, 2002;James, 1976;Komiya et al, 1999;Nutman et al, 2002;Webb et al, 2020); and (3) folds that have been interpreted as 0.1 m to km-scale sheath and curtain folds with fold axes subparallel to stretching lineations (e.g., Hanmer & Greene, 2002;Webb et al, 2020), including km-scale sheath folds that have bullseye map patterns (Figures 1 and S1) (also see Alsop & Holdsworth, 2012 for visualization of sheath folds and associated eye patterns). Accordingly, rocks preserved in the belt mostly appear as L-S tectonites (e.g., James, 1976;Webb et al, 2020), with minor occurrences of L-tectonites ( Figure S1f; also see Figure DR2 of Webb et al [2020]). These structures appear to have developed via intensive shearing and stretching, with up to 90% thinning (Fedo, 2000;Furnes et al, 2007;Webb et al, 2020).…”

“…Accordingly, rocks preserved in the belt mostly appear as L-S tectonites (e.g., James, 1976;Webb et al, 2020), with minor occurrences of L-tectonites ( Figure S1f; also see Figure DR2 of Webb et al [2020]). These structures appear to have developed via intensive shearing and stretching, with up to 90% thinning (Fedo, 2000;Furnes et al, 2007;Webb et al, 2020). However, primary structures such as pillow basalts are locally recognizable.…”

“…proposed age distinctions among rocks of either 3.7 or 3.8 Ga age (e.g., Nutman et al, 2020) cannot be confidently recognized with the resolution of existing geochronology data, so faults inferred on the basis of different interpreted ages represent nonunique interpretations (Webb et al, 2020).…”

“…In light of these uncertainties in plate tectonic interpretations, recent work interprets that the Isua supracrustal belt is a fragment of a heat-pipe lithosphere that was deformed during a ca. 3.66-3.61 Ga contractional deformation event ( Figure 2d) (e.g., Webb et al, 2020). Heat-pipe tectonics features planetary cooling dominated by mafic volcanism, which creates a thick, cold, and strong lithosphere as surface volcanic rocks are progressively buried by continuing deposition of volcanic materials (Moore & Webb, 2013) (To clear up a common misconception, heat pipes themselves are not equivalent to plumes: plumes are thought to span the height of the mantle, whereas heat pipes are simply volcanic conduits through the lithosphere).…”

Archaean Plate Tectonics in the North Atlantic Craton of West Greenland Revealed by Well-Exposed Horizontal Crustal Tectonics, Island Arcs and Tonalite-Trondhjemite-Granodiorite Complexes

“…The lithology/structure-based, the chronology/structure-based, and the geochemicallyconstrained model. Each model takes account of field-based structures, albeit in different ways, and they all interpret the Isua belt in terms of modern-style plate tectonics (for a different view see Webb et al, 2020).…”