Abstract
The iron content and the asymmetry of iron and magnesium ions in chlorites are examined for the Chelungpu Fault in Taiwan, which is a seismogenic fault. The samples are collected from the cores drilled for the Taiwan Chelungpu Fault Drilling Project (TCDP, borehole B). Three fault zones are recognized as candidates for the source of seismogenic materials. The fault zones are composed of fractured-damaged rocks, breccia, gray gouge, black gouge, and black material. Chlorite from each type of rock was analyzed by using X-ray diffraction (XRD). The iron content and asymmetry of the iron and magnesium ions in the chlorites were estimated from the XRD peak ratios. The hydroxide and silicate layers of chlorite in the black gouge and black material have low iron contents. Many studies have suggested that a temperature rise occurred at the fault zones. In addition, the temperature rise can result in the production of iron oxides such as magnetite or maghemite, as reported by other studies. However, the temperature rise cannot explain the low value of iron content in the chlorites. Another reason for the low value of iron content is the variation in the pH of the fluid, which can be controlled by radical reactions. Therefore, on the basis of chlorite characteristics, the reactions at the seismogenic fault are due not only to the thermal decomposition resulting from the temperature rise and but also to rock-fluid interactions.
Highlights
The formation of clay minerals within gouges is strongly related to fault activities because gouges are common products of shallow faulting (e.g. Sibson, 1977) and clay minerals are dominant in gouges
Because the clay-rich gouges are often observed in natural fault zones, clay minerals are important to understand fault processes
We focused on the characteristics of clay minerals in seismogenic fault rocks
Summary
The formation of clay minerals within gouges is strongly related to fault activities because gouges are common products of shallow faulting (e.g. Sibson, 1977) and clay minerals are dominant in gouges. We focused on the characteristics of clay minerals in seismogenic fault rocks. The analyzed samples are from the Chelungpu Fault in Taiwan. In the northern part of the Chelungpu fault, the absence of high-frequency radiations has been reported. The frictional heating at the time of seismicity is suggested by former studies of the same fault (Kano et al, 2006; Hirono et al, 2006b; Mishima et al, 2006). Both fluid-rock interaction and frictional heating are expected to control the clay composition along the fault. We use chlorite to examine the issue because characteristics of chlorite can provide thermal and chemical conditions along the fault
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