DEFORMATION PARTITIONING AROUND PEBBLES

A pebble-bearing phyllite unit can be found near Daejeon, Korea in a regional geologic map. The pebbles are diverse in shape and physical property. Shapes of these pebbles are irregular in size, lithology, roundness and shape from place to place. Lithology of the pebbles is variable form competent quartzite and granite to soft limestone and slate. The matrix is very fine and generally calcareous and phyllitic, consisting of calcite, quartz, graphite, muscovite and biotite. A regional fold exists in the phyllite unit having a general trend of NE-SW. Two unique structural fabrics can be found from the folded rock. One is planar fabrics recognized by the preferential traces of the flatten pebbles and strain shadow quartzes developed around competent pebbles. It shows a general trend of NW-SE. The other structure is NE-SW-trending crenulation cleavages. The former rotates around the latter showing various orientations. Along the first planar structure, limestone and slate pebbles are stretched and flattened, and reveal a strong preference in orientation while some of other pebbles, generally granites and quartzite, are merely deformed and have pressure shadows, symmetric or asymmetric, or sometimes complex. Strike of the earlier structure systematically changes across the second planar structures (crenulation cleavages, small folds and the regional fold) showing more NW-SE trends at the hinge areas. All this varieties in original properties and the combination of the two structures described above might result in the miscellaneous finite strains of the pebbles in this rock unit. This study explains a dramatic correlation between the distribution of the young cleavages and the physical properties (shape, lithology, etc.) of the pebbles through a detailed description of the pebbles and structures.