QUATERNARY TECTONIC MOVEMENT OF MID-LOWLAND IN NORTHEAST JAPAN ARC

In the northeast Japan arc and many other arcs, distinct lowlands, called the "mid-lowlands" in this study, run along the volcanic fronts. The Tanabu Lowland, northern part of the Shimokita Peninsula, occupies north end of the mid-lowland of the northeast Japan arc, and is accompanied both by the marine terraces, namely the indicator of the Quaternary uplift movements, and by 50 m thick marine Pleistocene Tanabu Formation, the indicator of the subsidence. Using the tephras as the key beds, the authors however show that the Tanabu Formation is a mid-Pleistocene group of three depositional sequences constructing the marine terraces, and had thus been sharply influenced by the glacio-eustatic sea-level cycles. Particularly, the thickness of each sequence is 40 to 90 m, but ranges within the glacial eustasy. Thus, the Tanabu Formation originated thickly without the subsidence because each sequence was accumulated one after another by glacial sea-level fluctuation. Moreover, all the terraces are cumulative in elevation and much higher than present sea surface like the series of the terraces in other uplifted regions. The above characteristics indicate that the Tanabu Lowland, namely the mid-lowland of the northeast Japan arc, has been uplifting during the Pleistocene. The mid-lowlands may have been caused by relative subsidence, whose uplift rates are smaller than the ones of the surrounding mountains at the Quaternary.