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世界の地質図 > グリーンランド > イスア > 説明文3

Geological structure: Duplex structure

The Southern Unit displays very clear geological evidence that the Isua supracrustal belt originated from an accretionary complex. It is composed of ultramafic rocks (Fig. 9ab), a basaltic lava flow sequence of massive and pillow lava (Fig. 8) and hyaloclastite, and a sedimentary sequence of pelagic chert (Fig. 7a) and BIF (Fig. 7b), and terrigenous turbidite and conglomerate (Fig. 12). Cherts and associated sediments form several belts bound by layer-parallel faults, converging in some cases and diverging in others (Fig. 5, 12). Duplex structures are on scales of meters to a few km. At least eight major subunits with horses are defined by the layer-subparallel faults. We reconstructed the original lithostratigraphy within each horse by removing folds and/or thrusts, based on detailed sketches of the outcrops. The reconstructed lithostratigraphies within horses are simple and mutually similar, each consisting of pillow basalt, chert, and turbidite in ascending order (Fig. 15). Major tectonic breaks separating horses on both sides are commonly layer-parallel faults (Fig. 14).

Figure 14: a, Simplified cross section of piling subunits along (A)-(B) on b. Small-scale folds and dikes are removed because of simplification. b, Distribution pattern of duplexes bounded by layer-parallel faults in the Southern Unit. Eight duplexes are defined by both lithostratigraphy within a horse and fault distribution. Duplexes consist of several horses. The layer-parallel boundary thrusts among horses, both inter-duplexes and intra-duplex thrusts, are left-lateral, and relative direction of fault displacement is shown by small arrows. They converge to the south or west. The structures are characterized by convergence and divergence of layer-parallel faults and are duplex structures. Faults at the bottom and top are called floor- and roof-thrusts, respectively. Proterozoic dikes and many Archean basaltic dikes are removed from map for simplification.

Figure 15: Lithostratigraphies of Duplexes I to VIII. Each lithostratigraphic unit is composed of metabasite, chert, muddy chert and mafic sediment, in ascending order. The metabasite consists of a pile of pillowed basalts with massive basaltic flows and hyaloclastites. A reworked hyaloclastite with rough surfaces and numerous metamorphic garnet megaporphyroblasts occurs at the top. Mafic sediment with graded bedding covers the chert at the top. Muddy chert occurs as a transitional sediment between the chert and the mafic sediment. The reconstructed lithologies are comparable to the Phanerozoic oceanic plate stratigraphy in Japan.

The stratigraphic top within each horse always faces southeastward, except in a few strongly folded areas. Petrographic examination of chert indicates that no continental or arc materials occur within the chert beds and underlying pillow lava flows. Detrital materials start to appear in thin laminated chert beds after the end of the major chert deposition, and become common in the turbidite layers. The sequential change of sedimentary lithology, from chert to turbidite, is common to all horses. The reconstructed lithostratigraphy is remarkably similar to oceanic plate stratigraphy of circum-Pacific accretionary complexes. The presence of duplexes indicates the geological structure was formed by layer-parallel shortening, whereas evidence of oceanic plate stratigraphy means lateral movement from pelagic ridge to continental margin. The lines of evidence indicate that the Isua supracrustal belt is the oldest accretionary complex. Komiya and others (1999) demonstrated that in the Southern Unit, and possibly in the whole Isua supracrustal belt, an accretionary complex formed through accretion of oceanic crust to the hangingwall, by duplexing. A sequence of duplexing scenarios can be reconstructed as follows. The structural bottom is Duplex I, and the top is Duplex VIII, as judged by the southward polarity of duplexing. The younger horses develop downward; therefore, the oldest duplex is VIII. Accretion proceeded from Duplex VIII to I with time. Within a given Duplex, the southeastern horse is the oldest. The similar lithostratigraphy within each horse suggests that the oceanic crust segments subducted in the same area and period. The mode of formation of the duplex structure throws light on the direction of the growth of the accretionary prism and the subduction of oceanic lithosphere. The duplex polarity is defined by the direction of convergence of faults, which divide horses, and coincides with plate convergence. The southward convergent morphology of the duplex structure is widespread over the mapped area, indicating that the Isua accretionary prism grew from south to north. A paleogeographic reconstruction is shown in Figure 15 to illustrate the sequential events of the accretionary prisms in the Southern Unit at about 3700-3800 Ma (Komiya et al., 1999). Geologic records preserved within turbidite sediments also provide strong constraints on the type of active plate margin. If they were formed along a large continental margin, they would be dominantly quartzofeldspathic. If they were formed in intra-oceanic environments, such as the present-day western Pacific domains, they would be dominantly mafic in composition. The sedimentary petrology of the turbidite sequences clearly indicates that the Isua accretionary complex was formed in the latter environment, because mafic sediments are dominant in the turbidite sequences. In the Archean, most orogenic belts were formed in intra-oceanic island arcs, and then grew progressively through the collision-amalgamation process.

Figure 16:Travel history of Early Archean oceanic lithosphere in the Isua accretionary belt, West Greenland. This diagram shows paleogeographic reconstruction of accreted oceanic lithosphere, followed by successive underthrusting of oceanic crust to form duplexes from Duplex VIII to I with time.

 
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