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.

Models to describe the nature of the Earth's crustal evolution during the Archean vary substantially, from horizontal lithospheric motions with subduction-like movement (Abbott et al., 1994), to vertical tectonics defined by sub-/intralithospheric diapirism (Collins, 1989; Sizova et al., 2015), extensive volcanism (Moore & Webb, 2013; Turcotte, 1989), and the formation of a single-plate lithosphere. First-order questions include when and how horizontal lithospheric motion (e.g., subduction) became the dominant process by which Earth's interior cools, deforms, and evolves (Lenardic, 2018). A change in geodynamics between ∼3.2-2.5 Ga is proposed based on the global geochemical and zircon record indicating an increased contribution of reworked crust for magma generation while juvenile mantle additions decreased during that time (e.g., Dhuime et al., 2012; Moyen & Laurent, 2018). A recent review of the existing metamorphic record suggests that Earth has experienced spatial and temporal changes of global crustal thermal gradients probably linked to transitions in the tectonic regime, from stagnant-lid tectonics to mobile-lid plate tectonics (Brown & Johnson, 2018). The main changes occurred at ∼2.5 Ga, ∼1.0 Ga, and 0.72 Ga, Ga which are associated with the widespread appearance of paired metamorphic belts, dominance of high T/P gradients (>775°C/GPa), and increasing abundance of low T/P gradients (<375°C/GPa), respectively. The presence of a distinct bimodality of low and high T/P gradients is indeed a feature commonly associated with plate tectonics (Brown, 2010; Miyashiro, 1961, 1973) and its gradual emergence argues for progressive onset and evolution to horizontal-dominated geodynamics since the Neoarchean (Holder et al., 2019). In stark contrast with these interpretations, multiple subduction-driven events are invoked to explain the origin and evolution of the Eoarchean Isua supracrustal belt (ISB) of southwest Greenland (Figure 1), one of the oldest metamorphic terranes known, which some workers have interpreted to reflect the onset of plate tectonics as

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The Cenozoic India-Asia collision generated both the east-trending Himalayan orogen and the north-trending Eastern and Western Flanking Belts located along the margins of the Indian subcontinent. Although the tectonic development of both flanking belts is key to understanding mechanisms of continental deformation during indenter-induced collision, few field-based studies coupled with geochronological and geochemical methods have been applied to these tectonic domains. In this study, we investigate the lateral correlation of lithologic units between the northern Indo-Burma Ranges, the northernmost segment of the Eastern Flanking Belt, and the eastern Himalayan-Tibetan orogen by integrating field observations, U-Pb zircon geochronology, and whole-rock geochemistry. Our findings provide new quantitative constraints to interpretations that the northern Indo-Burma Ranges expose the eastward continuation of several lithologic units of the Himalayan orogen and Lhasa terrane. Our field work documents a stack of thrust-bounded lithologic units present in the study area. The northernmost and structurally highest Lohit Plutonic Complex consists of Mesoproterozoic basement rocks (ca. 1286 Ma) and Late Jurassic-Cretaceous granitoids (ca. 156-69 Ma) with positive ε Nd values and initial 87 Sr/ 86 Sr ratios of ~0.705, which are correlative to the Bomi-Chayu complex and the northern Gangdese batholith, respectively. The structurally lower Tidding-Mayodia mélange complex, composed of basalt, gabbro, ultramafic rocks, and mafic schist of a dismembered ophiolite sequence, is interpreted in this study as the eastward extension of the Indus-Yarlung suture zone. Structurally below the suture zone are the Mayodia gneiss and Lalpani schist, which are interpreted to correlate with the Lesser Himalayan Sequence based on comparable metamorphic lithologies, negative ε Nd values, and similar Mesoproterozoic-Cambrian detrital zircon age spectra. In contrast to the above metamorphic units, the structurally lowest Tezu unit consists of siliciclastic strata that may be correlated with the Miocene-Pliocene Siwalik Group of the Himalayan orogen. Despite the above correlations, notable Himalayan-Tibetan lithologic units are absent in the northern Indo-Burma Ranges, including the Mesozoic-Cenozoic southern Gangdese batholith belt and its cover sequence of the Linzizong volcanic rocks, Xigaze forearc basin, Tethyan Himalayan Sequence, and Greater Himalayan Crystalline Complex of south-central Tibet and the central Himalaya. We interpret the absence of these lithologic units to be a result of a greater magnitude of crustal shortening and/or underthrusting of the Indian cratonal rocks than that across the Himalayan orogen to the west. This interpretation is supported by a southward decrease in the map-view distance between the active range-bounding thrust and the Indus-Yarlung suture zone in the northern Indo-Burma Ranges, from ~200 km in the north near the eastern Himalayan syntaxis to ~5 km in the south across a distance of ~200-300 km.