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1 引言 通过对日本海海底捞取的岩石和ODP样品,进行一系列放射性测定研究,揭示出日本海海底的形成至少为20Ma(如:Kaneoka等,1990,1992)。然而,有关日本海的形成历史,仍存在许多尚未确定的问题。 例如,在127/128航次期间获取的ODP样品,含有许多海底洋脊的岩石,它们可能不能代表形成日本海海底主要部分的岩石类型(Tamaki等,1990)。而且,从日本海盆获取的岩石,由于蚀变作用,甚至~(40)Ar-~(39)Ar分析也未能成功地得到十分可靠的年龄,对它的形成时代仅能给出一个粗略地估计。这样,对于日本海盆地区的形成时代仍然是含糊不清的。 奥尻海脊位于日本海中日本海盆的东北部边缘,与东面的武藏海盆相邻。海脊延伸超过180km,局部高出日本海盆底达2500m(图1)。海脊西侧为陡峭的悬崖,而东侧十分平缓。它沿着日本海的东部边界分布,被解释为一个板块会聚带,代表欧亚和北美板块之间的边界(Kobayashi,1983;)。奥尻海脊是一个外来的构造地块,以一个向东倾斜的主边界逆冲断层和一个向西倾斜的反向逆冲断层为特征(Tokuyama等,1992)。一系列的地震研究表明,奥尻海脊是一个抬升的日本海洋壳碎片(Miyashita等,1989)。例如,多道地震反
1 Introduction A series of radiometric studies conducted on rocks and ODP samples taken from the seabed of the Sea of Japan revealed that seafloor formation in the Sea of Japan is at least 20 Ma (eg, Kaneoka et al., 1990, 1992). However, there are still many uncertain issues about the history of the formation of the Sea of Japan. For example, ODP samples acquired during the 127/128 voyage, rocks containing many ocean ridges, may not represent the type of rock that forms the main part of the Sea of Japan (Tamaki et al., 1990). Moreover, rocks obtained from the Japanese basin failed to obtain a very reliable age due to alteration even by the ~ (40) Ar- ~ (39) Ar analysis, and only gave a rough indication of its formation estimate. In this way, the formation of the Japanese basin area is still vague. The 尻 尻 ridge is located on the northeastern edge of the Japanese basin in the Sea of Japan and is adjacent to the Musashi basin in the east. The ridges extend over 180km and are locally up to 2500m above the bottom of the Japanese basin (Figure 1). The steep cliffs west of the ridge, while the east is very gentle. It is distributed along the eastern boundary of the Sea of Japan and is interpreted as a plate convergence zone that represents the boundary between Eurasian and North American plates (Kobayashi, 1983; The ridges are an exotic tectonic block characterized by an eastward-leveled main boundary thrust and a westward thrusting reverse thrust (Tokuyama et al., 1992). A series of seismic studies have shown that the ridges of the ridges are an uplifted fragment of Japan’s oceanic crust (Miyashita et al., 1989). For example, multiple earthquakes