Plate Tectonics at 3.8–3.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland

Abstract

A 1&rcolon;5000 scale mapping was performed in the Isukasia area of the ca. 3.8-Ga Isua supracrustal belt, southern West Greenland. The mapped area is divided into three units bounded by low-angle thrusts: the Northern, Middle, and Southern Units. The Southern Unit, the best exposed, is composed of 14 subunits (horses) with similar lithostratigraphy, bound by layer-parallel thrusts. Duplex structures are widespread in the Isua belt and vary in scale from a few meters to kilometers. Duplexing proceeded from south to north and is well documented in the relationship between link- and roof-thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, of greenstone with low-K tholeiitic composition with or without pillow lava structures, chert/banded iron-formation, and turbidites. The cherts and underlying low-K tholeiites do not contain continent- or arc-derived material. The lithostratigraphy is quite similar to Phanerozoic "oceanic plate stratigraphy," except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present-day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100 degrees C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half-space cooling at given parameters of surface and mantle temperatures of 100 degrees and 1450 degrees C, respectively, suggesting that the Archean oceanic lithosphere was rigid. These conclusions-rigidity and lateral plate movement-support the idea that the modern style of plate tectonics was in operation only 0.7-0.8 G.yr. after the formation of the Earth.

Accretionary orogens through Earth history

“…The biostratigraphy, structure, and geochemistry of this offscraped sequence has been studied in, for example, Phanerozoic circum-Pacific orogens in Japan Matsuda & Isozaki 1991;Kimura & Hori 1993;Kato et al 2002), California (Cowan & Page 1975;Sedlock & Isozaki 1990;Isozaki & Blake 1994), Alaska (Kusky & Bradley 1999;Kusky & Young 1999), Eastern Australia (Cawood 1982(Cawood , 1984Fergusson 1985) and New Zealand (Coombs et al 1976;Mortimer 2004). Imbricated ocean plate stratigraphy is also increasingly recognized in Precambrian orogens; for example, in the 600 Ma Mona Complex of Anglesey, North Wales (Kawai et al 2006(Kawai et al , 2007Maruyama et al in press), the 2.7 Ga Point Lake greenstone belt (Kusky 1991), and possibly the 3.5 Ga chertclastic sequence in the Archaean Pilbara craton (Kato et al 1998;Kato & Nakamura 2003) and the 3.8 Ga Isua greenstone belt, West Greenland (Komiya et al 1999). However, the validity of the Pilbara successions as an imbricated ocean-plate has been questioned by Van Kranendonk et al (2007), who favoured a plume-related intracontinental setting.…”

“…The question of when plate tectonics began, what criteria can be used to recognize it in the rock record and, once established, whether it was continuous, episodic and/or alternated with some alternative process are much debated (Sleep 2000;Hamilton 2003;Stern 2005;O'Neill et al 2007;Condie & Kröner 2008). The consensus of opinion, however, is that convergent plate interaction and the recycling and subduction of material from the Earth's surface into the mantle has been active since at least 3.2-3.0 Ga (Cawood et al 2006;Dewey 2007;Condie & Kröner 2008;Windley & Garde 2009) and possibly considerably earlier (Komiya et al 1999;Harrison et al 2005;Nemchin et al 2008;Nutman et al 2009;Polat et al 2009). Well-constrained palaeomagnetic data demonstrate differential horizontal movements of continents in both Palaeoproterozoic and Archaean times, consistent with the lateral motion of lithospheric plates at divergent and convergent plate boundaries (Cawood et al 2006).…”

Evolution of the Earth as an andesite planet: water, plate tectonics, and delamination of anti-continent

“…Hopkins et al (2010) considered that zircon dating back to >4.2 Gya may indicate the operation of plate tectonics in the Hadean. On the other hand, there is mounting evidence that both ocean formation and plate tectonics operation took place in the early Archean (3.6-3.9 Ga) (Nutman et al, 2002;Komiya et al, 1999;Shirey et al, 2008). Once plate tectonics started operating, magmatism occurred in two different tectonic settings: at divergent plate boundaries such as mid-ocean ridges and continental rifts, and at convergent plate boundaries such as arc-trench systems and collision zones.…”

∼3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex

“…The largest body of basic, ultramafic and metasedimentary rocks in the Complex is the c. 35 km long Isua supracrustal belt (e.g. Moorbath et al 1973, Bridgwater and McGregor 1974and Allaart 1976 for the first studies and Nutman et al 1997b, Maruyama et al 1992Komiya et al 1999, Rosing 1999and Polat et al 2002 for examples of new work).…”