Deformation bands in the Eoil Basin, Gyeongju, Korea: Field occurrence and structural characteristics

Abstract

Due to brittle deformation, fractures and deformation bands may form in non-porous rocks and porous rocks, respectively, and some of them may evolve into faults. We conducted structural observation and material analysis on deformation bands in the semi-consolidated, porous sediments of the Eoil Basin, Gyeongju, SE Korea, to understand their structural evolution into faults. According to the outcrop-scale observation, the deformation bands are shear bands in terms of kinematic classification. Most of the identified shear bands are several millimeters wide, and the apparent shear displacement along them is less than 10 cm. Microstructural observation of the typical shear band confirms that it has a protocataclasis structure with a high clast vs. matrix ratio despite grain size reduction due to mechanical crushing. At the outcrop, the shear band's width appears to increase as the apparent displacement increases, and a small number of the shear bands change to faulted deformation bands (or faults) which are as wide as up to similar to 3 cm and have slickensides (or smooth slip surfaces). The increase in the band's width with the displacement increase may be due to the strain-hardening behavior associated with the protocataclasis structure. In the faulted deformation bands, principal slip zones (PSZs) with a width of several tens of mu m, where shear strain is localized, are developed. The PSZs are composed of ultrafine grains of quartz, feldspar, and clay minerals (mostly smectite). The observation implies that the formation of ultrafine particles by ultracataclasis or the formation of clay minerals with low frictional strength may contribute to the strain-softening behavior and the shear localization in the PSZs (or the evolution of the shear bands into faults), whereas the development of the protocataclasis structure in the shear band of small displacement may cause the strain-hardening behavior and increase in the band thickness.