環太平洋地域の変動帯の砂岩に認められる組成変化とグローバルな環境変化  [in Japanese] Temporal variations in sandstone compositions from circum-Pacific orogenic belts and their relation to global environmental changes  [in Japanese]

日本ならびに北米西岸地域やニュージーランドなど, 環太平洋地域の変動帯の砂岩に認められる組成変化の特徴を検討した。特に, 砂岩組成のうち, 岩石片の相対量の時代的変化に注目した。岩石片の相対量の増減は, 堆積盆での堆積速度の増減や, 火成活動に伴う火山岩片の供給量の増減に対応している。特に中生代以降, これらの変動は海洋プレートの運動に支配されてきている。さらに, 環太平洋地域の変動帯の砂岩には, グローバな海水準変動や, スロトンチウム同位対比ならびに砕屑物や火山物質の供給量の変化など, グローバルな物質量の変動とたいへんよく類似した変動が認められる。本来, これらグローバルな環境変化も基本的にはプレート運動に大きく支配されていると考えられるため, 中生代以降世界で最も大きい沈み込み帯として発達してきた環太平洋地域の変動帯の砂岩の組成変化に, こうしたグローバルな環境変化に対応した変動が記録されているのであろう。

The framework compositions of sandstones in the circum-Pacific orogenic belt show distinct temporal variations since the Paleozoic. In general, lithic sandstones containa greater volume of volcanic rock fragments than quartzose/feldspathic sandstones, and have developed typically in response to episodic activation of arc volcanism which has been controlled by fluctuations in oceanic plate motions at the circum-Pacific convergent margins. At the Japanese convergent margin, the temporal variations in sandstone compositions, represented by fluctuation in the relative ratio of lithic fragments, show a mode very similar to that in sediment accumulation rate. This indicates that the temporal variations in sandstone compositions can provide a detailed record of sedimentary basin evolution at convergent margins. The general trend of temporal variation in the sandstone compositions follows the one in the rate of landward migration of coastal on lap curves of Haq et al. (1988). Furthermore, the general mode of temporal variation in the strontium isotope ratio, and the estimated sediment and volcanic fluxes through the Phanerozoic are also similar to the temporal variations in the sandstone compostions at the Japanese convergent margin and the other circum-Pacific convergent margins, such as Northwestern America and New Zealand. The temporal variations in the sandstone compositions in the circum-Pacific orogenic belts, therefore, appear to record fluctuations in sea-level changes and material cycles through the Phanerozoic which also seem to have been controlled by fluctuating oceanic plate motions.