西南日本における海溝–島弧–縁海系の地殻構造

Abstract

The first attempt to make a crustal-scale cross-section of a trench-arc-backarc sea system has been successfully accomplished from the Nankai trough to the northern margin of the Yamato basin across Southwest Japan, using three seismic profiles: Nankai trough to Japan Sea coast (Ito et al., 2009), Oki trough (Tanaka and Ogusa, 1981), and Yamato basin (Sato et al., 2006). The section contains much new information that will be useful for research on the structural development of the Japanese island arc. The highlight of the information is the substantial difference in the structure between the Outer and the Inner zones. The Outer zone is constructed mainly by N-dipping accretionary complexes in its upper crust with a poorly developed lower crust. On the contrary, the Inner zone exhibits predominantly horizontal structures in its upper crust, beneath which thick horizontal lower crustal laminations occur as in the continental lower crust. Furthermore a 10-km-thick lens-shaped olivine-pyroxene cumulate body underlies the lower crust. The N-dipping Median Tectonic Line juxtaposes two substantially different zones from the upper to lower crusts The section, however, has the following serious defects caused by the poor specifications of the seismic profilings used here: (1) Shallow structures (

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西南日本的海沟—岛弧—缘海系地壳构造

利用南开海槽至日本海海岸(Ito et al.， 2009)、大木海槽(Tanaka and Ogusa, 1981)和大和盆地(Sato et al.， 2006)三个地震剖面，首次成功地完成了从南开海槽到大和盆地北缘横跨日本西南部的海沟-弧-弧后海体系的地壳尺度剖面。这一节包含了许多有助于研究日本岛弧构造发展的新资料。信息的亮点是外部和内部区域在结构上的巨大差异。外带主要由上地壳向n倾斜的增生杂岩构成，下地壳发育不全。与此相反，内带的上地壳主要表现为水平构造，在水平构造的下面，与大陆下地壳一样，出现了较厚的水平下地壳层状。此外，一个10公里厚的透镜状橄榄石-辉石堆积体位于下地壳下方。向n倾斜的构造中线将上地壳和下地壳两个截然不同的区域并置在一起，然而，由于这里使用的地震剖面规格较差，该剖面存在以下严重缺陷:(1)浅层构造(<几公里)成像差，导致地质构造解释困难。(2)日本海沿岸陆上和近海带的地震图像缺失，这也使人们难以理解与日本海开放相关的裂陷过程。因此，一个新的地壳尺度的截面，没有这些缺陷是必要的。幸运的是，近年来地震剖面的理论和技术都取得了巨大的进步。这种情况使我们能够同时沿着陆地和海洋的长地震线进行地震剖面(所谓的“陆海综合地震剖面”)，并提供新的剖面。预计新路段将很快取代旧路段。

本文章由计算机程序翻译，如有差异，请以英文原文为准。