WELL-DOCUMENTED TRAVEL HISTORY OF MESOZOIC PELAGIC CHERT IN JAPAN - FROM REMOTE OCEAN TO SUBDUCTION ZONE
WELL-DOCUMENTED TRAVEL HISTORY OF MESOZOIC PELAGIC CHERT IN JAPAN - FROM REMOTE OCEAN TO SUBDUCTION ZONE
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DOI：
10.1029/90tc02134
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发表时间：
1991-04-01
期刊：
影响因子：
4.2
通讯作者：
ISOZAKI, Y
中科院分区：
文献类型：
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作者：
MATSUDA, T;ISOZAKI, Y
日本西南部的米诺-坦巴带是东亚侏罗纪科迪勒型造山链的一段，主要由中上侏罗统俯冲-增生杂岩组成，其中三叠系和下侏罗统层状放射虫硅质岩作为大型异地单元产出，结构上与中上侏罗统碎屑岩夹层。高分辨率微化石(牙形刺和放射虫)研究发现，在硅质岩单元中的平均沉积速率非常低，约为0.5g/cm2/1000yr，类似于现代远洋沉积在公海环境中积累的沉积速率。米诺-坦巴带层状放射虫硅质岩平均沉积速率低，生物硅纯度高，连续沉积持续时间长(>；50m.y.)，沿走向范围广(>；1000公里)，最好的解释是远洋环境中的远洋沉积，因此，所谓的起源于较小的边缘盆地是站不住脚的。在硅质岩序列最上部，从层状硅质岩到上覆硅质泥岩的岩性向上变化，表明大洋板块逐渐向海沟靠岸。这些硅质岩和粗粒陆源碎屑的构造夹层是俯冲带通过双重底侵作用增加的第二特征。根据米诺-坦巴带三叠纪层状硅质岩的原生地层学和野外产状，提出了理想化的大洋板块地层学和科迪勒型层状硅质岩从产生于洋中脊到消亡于俯冲带的概括性旅行史。
The Mino-Tanba belt in southwest Japan, a segment of the Cordilleran-type orogenic chain of Jurassic east Asia, is composed mainly of a Middle-Upper Jurassic subduction-accretion complex in which Triassic and Lower Jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with Middle-Upper Jurassic clastic rocks. High-resolution microfossil (conodont and radiolaria) research has identified very low average sedimentation rates of about 0.5 g/cm2/1000 yr in the chert units, similar to those of modern pelagic sediments accumulated in open ocean environments. Judging from the low average sedimentation rate, high purity of biogenic silica, long duration of continuous deposition (> 50 m.y.), and wide along-strike extent (> 1000 km), the bedded radiolarian cherts in the Mino-Tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments; accordingly, the alleged origin in smaller marginal basins is untenable. Upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench. The tectonic interlayering of these cherts and coarse-grained terrigenous clastics is a secondary feature that was added through duplexing-underplating in the subduction zone. On the basis of the primary stratigraphy and field occurrence of Triassic bedded chert in the Mino-Tanba belt, newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a Cordilleran-type bedded chert from its birth at a mid-oceanic ridge to its demise at a subduction zone.