Coastal Terraces and their Deformations in Amami-Oshima, Ryukyu Islands, Southern Japan
Coastal Terraces and their Deformations in Amami-Oshima, Ryukyu Islands, Southern Japan
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日本南部琉球群岛奄美大岛的海岸阶地及其变形
DOI：
10.5026/jgeography.86.6_383
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发表时间：
1977
影响因子：
3.1
通讯作者：
Y. Ikeda
中科院分区：
文献类型：
--
作者：
Y. Ikeda
Amami-Oshima位于从九州延伸到台湾的琉球弧线的中部。在Aamami-Oshima卡萨里半岛东南约20千米处的Kikai-jima，其海洋阶地的高度及其由钙质阶地沉积物得出的绝对年龄揭示了过去130,000年来的快速隆升(1.5-2毫米/年)(Konishi等人，1970)。由于琉球弧与日本西南部外弧具有相似的地质和地球物理背景，日本西南部最外带的龟开-吉马的快速隆升被解释为与地震有关的变形(Yonekura，1975)，这一点得到了严格的研究。本文旨在阐明滨海-大岛县晚第四纪垂直形变的模式，并提供一些与上述地震地壳运动相比较的资料。大岛县的地貌面大致可分为：低、中、高海岸阶地和最高的低起伏侵蚀面。低阶地(L)由全新世海相阶地、断浪阶地和冲积平原组成，海拔不超过几米。中部阶地(M1和M2)主要发育在Kasari半岛海拔10-60米的地方，在Amami-Oshima的其他地方几乎看不到，除了在Koniya以东的Honohoshi海滩观察到的8米高的带有近圆形砾石和红壤的次要波浪切割阶地。中间阶地的轮廓总体上是连续的，没有任何悬崖或扣点，向大海缓缓倾斜。然而，在卡萨里半岛东海岸，它们可以分为两个层次：较高的(M1)和较低的(M2)。M1阶地的阶地沉积物一般是覆盖着红壤的薄海相砾石，但在泰良、川内和小池等一些小地峡中存在着40多米厚的山谷充填沉积物。这些沉积物的上部由海相砂砾组成，底部由粘土沉积组成，其中含有大量角砾岩和插入白色凝灰岩层的植物碎片。M2梯田是切割M1梯田的耐磨平台，梯田砾石薄，覆盖红壤。高阶地(H)主要发育在Amami-Oshima的东北部(主要包括在图2所示地区)，该地区被V形山谷高度剥离，但仍保留非常平坦的原始阶地表面。它们现在的高度在海拔180到340米，大概可以分为两到三层。在它们上面没有观察到梯田沉积。低起伏侵蚀面广泛发育在北大岛及其附属岛屿的西半部，海拔250-400米。从40多米深的大型海侵证据和中下阶地序列推断，M1阶地是末次间冰期海侵形成的，日本关东南部称之为“下水海侵”，形成于公元前120,000-130,000年(Mchida和Suzuki，1971)。根据原ML阶地海岸线的海拔分布推算出1000年以来：(1)卡萨里半岛一直向西北方向隆升和倾斜(图2)。位于Kasari半岛东南20千米的Kikai-Jima的Hyakunodai梯田与Amami-Oshima的ML梯田是同一时期的，海拔220米。因此，倾斜区域似乎从卡萨里半岛延伸到该岛。
Amami-Oshima is located at the central part of the Ryukyu Arc which extends from Kyushu to Taiwan. In Kikai-Jima at about 20 kilo-meters south-east off Kasari Peninsula of Amami-Oshima, rapid uplifting (1.5-2 mm/y) during the past 130, 000 years was revealed by the altitudes of its marine terraces and their absolute ages derived from the calcareous terrace deposits (KONISHI et al., 1970). As the Ryukyu Arc has similar geological and geophysical settings to the outer arc of Southwest Japan, the rapid uplifting in Kikai-Jima was interpreted as earthquake related deformation (YONEKURA, 1975) which has been critically studied in the outermost belt of Southwest Japan. The purpose of this paper is to clarify the mode of the vertical deformation in the late Quaternary period in Amami-Oshima with relation to that in Kikai-Jima, and to present some data for comparison with above mentioned seismic crustal movements.The geomorphic surfaces of Amami-Oshima are roughly classified as follows : low, middle and high coastal terraces and the uppermost low-relief erosion surface. The low terrace (L) consist of Holocene marine terrace, wave-cut bench and alluvial plain, altitude of which does not exceed several meters high. The middle terraces (Ml and M2) are mostly developed in Kasari Peninsula at the height of 10 to 60 meters above sea level and scarcely observable in the other part of Amami-Oshima, except a minor wave-cut bench of 8 meters high above sea level with subrounded gravel and red soil observed at Honohoshi Beach, east of Koniya. The profiles of the middle terraces are, in general, continuous without any cliff or knick point in them and gently inclined seaward. However, in the east coast of Kasari Peninsula they can be divided into two levels : the higher (M1) and the lower (M2). The terrace deposits of M1 terrace are generally thin marine gravels covered with red soil, but valley fill deposits more than 40 meters thick exist inside some small isthmus such as at Taira, Kawauchi and Ogachi. The upper parts of these deposits consist of marine sands and gravels, and the basal parts of them consist of clayey deposits with a lot of breccia and fragments of plants into which a white tuff layer is inserted. M2 terrace is abrasion platform cutting M1 terrace with thin terrace gravels covered with red soil. The high terraces (H) are mainly developed in the northeastern part of Amami-Oshima (mostly included in the area of Fig. 2), which are highly dissected by V-shaped valleys but still preserve very flat original terrace surfaces. They are now at the height of 180 to 340 meters above sea level and probably can be divided into two or three levels. Terrace deposits are not observed on them. The low-relief erosion surface is widely developed in the western half of Amami-Oshima and its dependant islets at the height of 250 to 400 meters above sea level.From the evidences for a large transgression more than 40 meters deep and from the sequence of the middle and low terraces, M1 terrace is inferred to have been formed by the transgression in the last interglacial period, which is called “Shimosueyoshi transgression” in South Kanto, Japan, and dated 120, 000-130, 000 years B. P. (MACHIDA and SUZUKI, 1971).The mode of the vertical deformation during the last 120, 000 or 130, 000 years inferred from the distribution of altitudes of former shoreline of Ml terrace is as follows : (1) Kasari peninsula has been uplifting and tilting north-westward (Fig. 2). Hyakunodai terrace in Kikai-Jima 20 kilo-meters south-east off Kasari Peninsula, which is contemporaneous with Ml terrace in Amami-Oshima, is at the height of 220 meters above sea level. Therefore the tilted area is seemed to extend from Kasari Peninsula to this island.