Uplift of fluvial terraces by the activity of the Kamidera fault in the northern part of the Biwako-seigan fault zone

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  • 琵琶湖西岸断層帯北部上寺断層の活動による河成段丘の隆起
  • ビワコ セイガン ダンソウタイ ホクブ カミデラ ダンソウ ノ カツドウ ニ ヨル カセイ ダンキュウ ノ リュウキ

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Abstract

The Ohmi basin is located at the northern part of the Kinki Triangle and is bordered by the Ibuki and Suzuka Mountains to the east and by the Nosaka, Hira, and Hiei Mountains to the west. Along the western margin of the Ohmi basin runs the Biwako-seigan fault zone, a 60-km-long and N10°E-trending reverse fault zone. Around the northern part of the Biwako-seigan fault zone (e.g., Aibano, and Kamidera faults), fluvial terrace surfaces are well developed along the Ado and Ishida Rivers, and are important indices in understanding the tectonic history of the area. However, these well-preserved geomorphic features have rarely been used as tectonic markers due to lack of age constraints on the middle and higher terraces.<BR>The purpose of this study is to provide age constraints on the middle and higher terraces and thus to evaluate the activity of the Biwakoseigan fault zone. We first divided fluvial terraces, from the highest to lowest, into Aibano 1, Taisanjino 1∼4, Okuyama, and Nakano surfaces, based on aerial photograph interpretation and field surveys. We carried out a drilling survey on the terraces to sample loamy soil. We then identified widespread volcanic ashes within loamy soil by precise cryptotephra analysis. The main results obtained in this study are as follows : (1) The KikaiAkahoya tephra (K-Ah : 7.3 ka), Aira-Tanzawa tephra (AT : 26-29 ka), and Kikai-Tozurahara tephra (K-Tz : 95 ka) are identified within loamy soil ; (2) Ages of the fluvial terraces are estimated at 160-200 ka (Aibano 1), 105-140 ka (Taisanjino 1), 95-130 ka (Taisanjino 2), 90-100 ka (Taisanjino 3), 60-80 ka (Taisanjino 4), and 25-30 ka (Okuyama) ; (3) Average uplift rates estimated from the ages and vertical displacement of the terraces are : 0.6-1.2 mm/yr (Taisanjino 1), 0.6-1.1 mm/yr (Taisanjino 2), 0.6-0.8 mm/yr (Taisanjino 3), 0.6-0.9 mm/yr (Taisanjino 4), and 0.8-1.2 mm/yr (Okuyama). Thus, since Taizanjino 1 surface was formed, the terraces on the west side of the Kamidera fault seem to have been uplifted at an almost constant rate (0.6-1.2 mm/yr) by the activity of the Kamidera fault.

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