Vol.3 No.1 2010

17/110

Research paper : The advanced geological researches and fundamental national land information (M. Saito)−14−Synthesiology - English edition Vol.3 No.1 (2010) 2.1 Previously known geology of the areaThe Tomochi 1:50,000 sheet includes the mountainous region of Gokanosho in Izumi-mura village (currently Yashiro-shi) said to have been the new home of fleeing warriors of the Heike clan. The sheet is also upstream of the Itsukimura area in the Kumamoto Prefecture of central Kyushu, which is known for the “Itsuki No Komori Uta” (Itsuki Lullaby). It is characterized by a complicated distribution of diverse strata and rocks in an area of steep mountain terrain that makes field survey work difficult (Fig. 1). The sheet area (23.5 km east–west, 18.5 km north–south, surface area approximately 435 km2) contains the following geological elements:1) accretionary complexes in which the rocks of a subducting oceanic plates have been accreted onto continental margins;2) serpentinite including Paleozoic rocks;3) metamorphic rocks that have been subjected to high temperatures or pressures;4) granitic rocks that represent solidified felsic magma rich in SiO2;5) continental shelf deposits;6) pyroclastic flow deposits erupted from volcanoes (Mt. Aso and others).Almost all of the rock types known to exist in the Japanese Archipelago are found in the Tomochi sheet area. Because we expected the geology to be extremely complicated, we used four researchers with specific but different areas of expertise for our field survey, and we aimed at new results for each rock and strata. No previous geological mapping of the area incorporated the principles of plate tectonics, so the creation of a geological map based on plate tectonics (e.g., one that identifies the rocks of the accretionary complexes) was an important objective.2.2 Regional geology and previous geological interpretationThe area that lies on the Pacific Ocean side of the Japanese Archipelago from Kyushu to the Kanto Mountains is known as the Outer Zone of Southwest Japan. Within this zone there is a complicated distribution of accretionary complexes formed by subduction of oceanic crust during the Jurassic (200 million to 145 million years ago), strata of Silurian to Devonian age (440 million to 360 million years ago), Permian to Cretaceous (300 million to 65 million years ago) shallow-marine deposits (and some terrestrial deposits), and serpentinites and other metamorphic and plutonic rocks.2.3 Previous geological interpretationsThe classification of the rock formations of the Jurassic accretionary complex and their relationship with other formations is confused. Individual researchers have developed different classification systems. For example, rock Fig. 1 Location of GMJ 1:50,000 Tomochi sheet (solid rectangle) in relation to the GMJ Yatsushiro and part of the Nomo Zaki 1:200,000 sheet (dashed rectangle)The base map is the Geological Map of Japan 1:1,000,000 3rd Edition[1].Geological Map of Japan1:50,000 “Tomochi”Geological Map of Japan1:200,000 “Yatsushiro and a part of Nomo Zaki”

※このページを正しく表示するにはFlashPlayer9以上が必要です