On the Serpentine Rocks in Kaizuka landslide area Chiba Prefecture, Japan

Abstract

The Kaizuka landslide area is situated on the easternmost part of so-called Mineoka Mountainrange, which trends along a tectonic line running in E-W directions from Kamogawa City to Hota Town in southern Boso Peninsula. The area is composed of Oligocene Mineoka group, Serpentine and basaltic rocks which are intruded into them.
The author found a specimen of boring core which is constituted from different two kinds of rocks, although both are serpentine petrographically. The one is pale-green in color and rather soft and the other is serpentinized ultramafic igneous rocks, considered from existence of relic minerals internally altered into sezpentinized olivine and non-altered pyroxene.
As results, the author concluded that serpentine of the area is not a single geological unit but is divided into two units which are different in geological age. The one is serpentine proper and the other is ultramafic igneous rocks (basaltic rocks.) In addition, it is notable that accumulation of stress are observed along a contact zone between above-mentioned two rocks.