四国北西部肱川流域の段丘地形と地殻変動 Development of Terraces along the Hiji River and Tectonic Movement in Northwestern Shikoku, Japan

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四国北西部肱川流域には4段の河成段丘(H1,H2,M,L)が発達する.H1段丘は,肱川流域でほぼ連続的に分布し,堆積物中にNg-1,加久藤テフラを挾在することから,H1段丘は250~300ka以降に離水したと推定した.<br>H1段丘面と現河床の比高が四国山地側で増大することから,西への傾動運動が認められ,傾動速度は1.8~2.2×10<sup>-8</sup>/yr.である.この傾動は四国山地の緩慢な隆起運動を反映していると考えられる.大洲より下流では,河口側でH1段丘の比高が増大することから南東への傾動運動が認められ,傾動速度は2.0~2.4×10<sup>-8</sup>/yr.である.この傾動は,中央構造線活断層系の横ずれ運動に伴う山地の隆起を反映していると考えられる.平均高度から推定された100万年間の平均隆起速度(大森,1990)と本研究で求めた隆起速度は,肱川中流域においてほぼ一致することから,四国北西部の四国山地では等速な隆起が継続していると考えられる.

In Northwestern Shikoku, the Hiji River flows through the outer zone of Southwest Japan (Figs. 1 and 2). The fluvial terrace surfaces along the Hiji River are classified into four levels: Terrace H1, H2, M, and L in descending order (Fig. 3). Based on the correlation of these terraces, this paper discusses the development of the terraces and the tectonic movement indicated by deformed terrace profiles.<br>The H1 terrace is most extensively distributed along the Hiji River and its tributaries, while the lower terraces are limited to inside the Nomura, Uchiko, and Ozu basins. As the H1 terrace sediment contains an Ng-1 tephra (erupted about 300ka) layer about 5m below the terrace surface at Nomura (Mizuno and Kikkawa, 1991), the age of the H1 terrace is roughly estimated to be 250 to 300ka.<br>H1 and H2 terraces are distributed in the valley from Shimo to Ozu through Uchiko basin to the north of Mt. Kannan; these terraces are not found along Hiji River south of Mt. Kannan. The course of the Hiji River presumably shifted from Shimo to Ozu after the formation of the H2 terrace (Fig. 8).<br>The relative altitude of the H1 terrace surface above the present river bed is 75m at Nomura and 120m at Sakaishi (Fig. 9). This indicates westward tilting by 5.6‰ in the middle reach; and this tilt is cumulative since the formation of the H1 terrace surface in this area. Based on the estimated age of the H1 terrace (250-300ka), the rate of westward tilting is estimated as 1.8-2.2×10<sup>-8</sup>/yr. In the lower reach, the largest relative altitude of the H1 terrace above the present river is 210m at 6km upstream from the river mouth, while it is 170m at Ozu (Fig. 9). This southeastward tilting indicated by the longitudinal profile of the H1 terrace is attributed to the regional uplift of the mountains to the south of Median Tectonic Line. Based on the estimated age of the H1 terrace (250-300ka), the rate of westward tilting in the lower reach is estimated as 2.0-2.4×10<sup>-8</sup>/yr.<br>Because these uplift rates are comparable to the average rate of uplift over a million years (Ohmori, 1990), it appears that mountains have been uplifted at a constant rate in northwestern Shikoku during the latter part of the Quaternary.

収録刊行物

  • 第四紀研究  

    第四紀研究 37(5), 397-409, 1998-12-01 

    Japan Association for Quaternary Research

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各種コード

  • NII論文ID(NAID)
    10002510803
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    105879
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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