アラスカ南縁と西南日本外帯の地質対比

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Tectonic frameworks of the southern Alaska margin and the Outer zone of Southwest Japan were correlated on the basis of geologic structures, lithofacies and geologic ages. Both regions have been situated along the convergent margins of the Kula and Pacific plates since early Mesozoic time and are underlain by several geologic units that are distributed zonally, parallel to each other and younging towards the Pacific side (Fig. 4). In the Kodiak Islands-Kenai Peninsula area of the southern Alaska margin, geologic units trend northeasterly and are made up of, from the continental side to the Pacific side, (1) late Triassic to early Jurassic volcaniclastic sequences, (2) early Jurassic Afognak granitic complex and early Jurassic Kodiak-Seldovia schists, (3) early Cretaceous Uyak-McHugh formation consisting mainly of argillite with chert, basalt and sandstone, (4) latest Cretaceous Kodiak-Valdez formation consisting mostly of well-bedded argillaceous turbidites without chert and basalt, (5) Paleocene Ghost rocks formation consisting maingy of argillite and slumped turbidite with minor chert and basalt and (6) Eocene to Miocene shelfslope facies of sandstone and turbidite (Fig. 4, Table 1). The Afognak granitic complex and Kodiak-Seldovia schists are lithologically correlatable to the Cretaceous Ryoke complex and Sambagawa schists respectively, but are not clearly separated by a fault like the Median Tectonic Line that separates the Ryoke from the Sambagawa. The Uyak-McHugh formation is lithologically and structurally correlative with the Permo-Triassic Chichibu group, although the former is bounded on the northwest by the Border Ranges fault and the latter is primarily conformable to the Sambagawa schists. In Southwest Japan the Kurosegawa terrane underlain by middle Paleozoic rocks together with older gneisses and acid igneous rocks occupies a narrow zone on the south of the Chichibu group, whereas no such terrane with continental crust occurs in the southern Alaska margin. In addition, no correlative rocks to the Triassic Sambosan terrane consisting mainly of bedded chert and sandstone with subordinate basalt and limestone are present in the southern Alaska margin. The Kodiak-Valdez formation corresponds to the Cretaceous Shimanto group in respect of structural position. However, lithofacies and deformation are quite different each other the former consists mostly of well-bedded argillaceous turbidites with ubiquitous and remarkable slaty cleavage, whereas the latter consists mostly of sandy turbidites, commonly chaotic, with subordinate chert and basaltic or rhyolitic volcanics. The Ghost rocks formation corresponds to the Eocene to_ Oligocelle Setogawa group in respect of lithofacies deformation and structural position. Major differences of tectonic framework between the southern Alaska margin and the Outer zone of Southwest Japan are ; (1) absence of faultbounded paired metamorphic belts in Alaska, (2) absence of a terrane with thick granitic crust in Alaska, (3) time lags between the correlative units and (4) remarkably shorter depositional durations of individual sedimentary unit in Alaska than those in Southwest Japan (Fig. 4). Comparisons between the southern Alaska margin and the Outer zone of Southwest Japan suggest that sedimentary facies, geologic structures and formation process of tectonic frameworks vary significantly even in similar convergent plate margins. Collision mechanisms of continental and oceanic fragments (e. g. JONES et al., 1972 ; CONEY et al., 1980 ; BENAVRA-HAM et al., 1981); which are valid in the west coast of North America, should be examined in Southwest Japan as well.
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アラスカ南縁と西南日本外帯の地質対比

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