下部大陸斜面の地質構造と,その起源に関する2・3の問題

Abstract

Development of the accretionary prism is one of the most characteristic features of the Pacific-side slopes of the Japanese Islands. Most of the sediments of the accretionary prisms are composed of terrigenous and hemi-terrigenous materials. The boundary between the oceanic basement and overlying pelagic sediments expected at the lowermost part of the accretionary prism should be merely a conformable or an non-conformable relation. Tectonic influence of the relative movement of the continental lithosphere and the oceanic lithosphere, that is, influence of the activity of the Wadati-Benioff zone, appears as the thrust movement which cut the boundary between the oceanic basement and the overlying sediments. These thrusts extend into the accretionary prisms, and, some of them towards the surface of the sea floor. Earthquakes at intermediate depth under the continental slopes are the direct result of the movement of these thrusts. Acidic and basic magmatic intrusions penetrate some large thrust planes and sometimes these magmas carry continental and oceanic crustal xenoliths. Good examples of the exposure of these continental slope igneous rocks can be found not only on the present slope of the Japanese Islands but also in the Permo-Triassic structural belts in the islands.