Studies on nontectonic geologic structure in Japan: Recent progress and future perspective

Abstract

Nontectonic geological structures are formed near the Earth's surface under the field where gravitational force dominates tectonic stress. This paper reviews studies on nontectonic structures that are related mainly to landslides, active faults, and geomorphic processes in Japan, published over the last two decades. Nontectonic faults are generated by the gravitational movement of rock masses, seismic shaking, and volcanic activity. Landslides cause nontectonic folds, such as buckling and bending folds, which can form gravitationally. Similarly, valley bulging makes anticlines along river channel. Studies on joint have progressed markedly, especially regarding sub-surface sheeting joints including closely developed micro-sheeting or lamination sheeting. It is difficult to discriminate nontectonic structures from tectonic ones in most cases, because both structures coexist in the same environment and, furthermore, polygenic or complex structures of both nontectonic and tectonic origins can exist in the same locality. Also from the standpoint of engineering geology, which applies geological knowledge to human society, it is important to comprehensively clarify the formative mechanism and history of geological structures, including tectonic and nontectonic processes.