東北地方第四紀Tephrochronology

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The volcanoes of Quaternary age in Northeast Honshu are distributed along two belts which are named the Nasu and Chokai Volcanic Belts. The Nasu Volcanic Belt is situated along the Backbone Ranges of Northeast Honshu and the Chokai in the Dewa Hilly Lands. Northeast Houshu is divided into five sections, each has its own geologic and geomorphic characteristics. There are, from east to west, Kitakami-Abukuma Massif, lowland on the western side of the massif. Backbone Ranges, lowland on the western side of the ranges, and Dewa Hillylands. The Kitakami-Abukuma Massif consists largely of pre-Tertiary and granitic rocks. Along the eastern foot of the Abukuma Massif and in the areas between the Kitakami and Abukuma Massifs, and between the Kitakami Massif and Cape Shiriya, there are hillylands mainly consisting of Cenozoic sediments. The lowlands between the Kitakami-Abukuma Massif and the Backbone Ranges comprise several basins which are distributed along the main valleys of the Kitakami, and other trunk rivers. The Backbone Ranges consist mainly of Tertiary rocks and comprise the central belt of the Tertiary of the region. The lowlands between the Backbone Ranges and the Dewa Hilly Lands comprise several basins. The Dewa Hilly Lands consist largely of Tertiary rocks which rest upon the pre-Tertiary and granitic rocks with unconformity. Kitamura (1959) summarized the Tertiary orogenesis in this region as shown in Figure 2.
Most of the Quaternary sediments and terraces of Northeast Honshu are developed in the mentioned two lowlands and coastal areas and distributed in the north to south direction in belts parallel to the main structural trend of the region. An analogous trend is found in the distribution of the volcanoes which are located near the tectonic lines. They have a northeast trend, obliquely crossing the main structural trend. The tectonic lines with a NW-SE trend were important throughout the pre-Quaternary history of the region, and at least, during the Pleistcene time, they controlled the northeastwards tilting blocks and determined the sitesof the basins within the above cited two lowland areas. For determination of the Quaternary tephrochronology in Notheast Honshu, available are such data as shown by the structural control on the distribution of the volcanoes, the sediments and terraces of the Quaternary age.
In the middle latitudes of Japan, ash is generally spread eastwards from the volcanoes because it was transported by the prevailing westerly wind. In Northeast Honshu, the Quaternary sediments are developed more or less continuously and the terraces with a N-S trend are crossed longitudinally by the distribution of the volcanic ashes. The volcanic ash layers which originated from the Bandai-Azuma Volcanoes are distributed in the area between Koriyama and Fukushima along the Abukuma River, and between Hisanohama and Soma along the Pacific Coast. The ashes from the Zao Volcano are distributed between Watari and Shiogama along the Pacific Coast and are separated from those of the Bandai-Azuma Volcanoes. The ash layers derived from the Kurikoma Volcano are distributed east and northeastward and remote from those of the Iwate Volcano. Except for the most extensively distributed ash layers originated from the Towada-Hakkoda Volcanoes whch extend southward to the northern area of ash distrisbuted from the Iwate Volcano, and covered in part by the eastern part of the ash derived from the Iwaki Volcano, the ash layers from one volcano or group of volcanoes in Northeast Honshu are distributed in areas separated and at some distance from those of the neighbouring volcanoes.
A discontinuity in the distribution of the Quaternary sediments and terraces is also found in this region.