結晶片岩の大規模岩盤クリープ性地質構造 関東山地三波川帯大谷地区を例として

Abstract

The term mass rock creep structure is here used for structures resulted from long -term gravitational rock slope deformation without a continuous sliding surface. Mass rock creep structures have been known implicitly, to be a kind of mass movements, but have been scarcely studied scientifically partly because those have been taken to be of little impor-tance, in geology, to academic research or investigation of mineral resources. The mass rock creep structure, however, is not only important in structural or engineering geology but also important in the technology for natural disaster prevention, because mass rock creep is sometimes a precursor of large landslide.This paper describes a large mass rock creep structure of crystalline schist, which have been only poorly described till now, on a mappable to microscopic scales at Ogai district in the Sambagawa terrain in the Kanto mountainous land.Ogai district is located on a dip slope of a first-order fold, where basement rocks is mainly composed of pelitic schist. The schist, made up of quartz thin layers and pelitic thin layers, is probably originated from chert laminite. The macroscopic investigation on a mappable scale revealed that the rock masses at Ogai district have deformed gravitation-ally and disintegrated into three blocks rotated each upon a vertical axis. The strike trace of bedding schistosity have eventually deformed into box fold-like pattern in a plan view, and a large landslide topography (1 km wide and 1.5 km long) has been formed.This macroscopic mass rock creep occurred at first as the formation of creep zones within the creeping rock mass. The creep zones are those where straining and fracturing of the rock masses due to the mass rock creep are intensive. The creep zones at Ogai district are classified into three groups ; high-angled creep-fracture-zone, low-angled creepfracture-zone, and creep chevron folds. The low angled fracture-zone contains partial, discontinuous, sliding surfaces of the incipient stage. These zones and folds are basically made by slip of small rock masses along schistosity and by fracturing of quartz thin layers. The high-angled creep-fracture-zones as well as low-angled ones, are separated into two parts: phyllitic and massive. The zones contain scarcely clayey materials. The phyllitic and massive parts change to each other gradually. While the high-angled fracture-zone is not sharply bounded with the hanging and foot-walls, the low-angled fracture-zone is sharply bounded only with the hanging wall forming a sliding surface. The creep chevron folds are the type of buckle folds with several to 20cm wavelength. The folds were generated by a compressional force acting parallel to the schistosity of pre-fold stage. Quartz thin layers in the axial part of the folds are fractured intensively and the layers in the limbs have structure resembling pulled-apart structure of sedimentary rocks with the intervening open cracks 5-20μm wide.The mass rock creep structures like those at Ogai district may be very common in the areas where foliated rock masses such as crystalline schist, slate, and chert laminite are distributed.