Factors controlling the color of fault gouges: an example from the Geumwang Fault, South Korea.

Abstract:

Particle size and mineral composition are two major factors controlling the color of fault gouges. The gouges of the Geumwang Fault exhibit several colors, although they originated from one protolith, granite and show a similar grain size. The NE– to NNE-striking right-lateral Geumwang Fault is one of the major strike-slip faults in South Korea. The fault core is composed of cataclasite (about 0.6 m thick) and fault breccia (about 28 m thick). The cataclasite and breccia contain about 20 fault gouge layers with their thickness ranging from several mm to several m. The colors of fault gouges include black, dark gray, gray and greenish gray. The black gouges comprise fragments of quartz, feldspar, calcite, dolomite, siderite and graphite, and matrix of illite, quartz and siderite. The dark gray gouges consist of similar minerals to black gouges, but the amount of siderite and graphite is much smaller than the black gouges. The gray gouges consist of fragments of quartz, feldspar, muscovite, calcite and dolomite, and matrix of illite and quartz. The greenish gray gouges show similar mineral composition to the gray gouges except the existence of chlorite in matrix. Since the size of fragments and matrix in all the gouge layers are almost the same, the color difference must reflect their compositions. The dark color of the gouges results from graphite content since their color becomes darker with increasing graphite content. Graphite (~ 50 μm in size) is euhedral and appear to be exotic since the granite protolith is absent of this mineral. Calcite occurs as fracture-filling minerals in wall-rock granite and decarbonation of calcite might be a source of graphite in the gouges. Since this reaction requires a relatively high temperature (650-750ºC), the darker colored gouges may be a product of seismic slip.