Characteristics of clay mineralogy in the fault zone of the TCDP and its implication

Abstract

The 1999 Chi-Chi earthquake (Mw7.7) induced large ruptures at or near the surface and thus provides us a unique opportunity to study the thrust fault zone after a major earthquake. The materials in deep fault, recovered by the Taiwan Chelungpu-fault Drilling Project (TCDP) are less weathering by shallow or surface processes and enable us to determinate in-situ physical, mechanical and chemical characteristics in the fault zone. Meanwhile, clay reactions such as illite-smectite transformation are not only to estimate the maturity of diagenesis in sedimentary basin, but also to use as heating indicator of faulting in the fault zone. The mechanism of S-I reaction dominantly relies on four variables: 1) time, 2) heat, 3) the availability of K+ and 4) fluids, and the heat is the major factor in this transformation during the faulting. This may give us a hint to estimate the thermal history in the fault zone. The two main fault zones located in 1111m and 1153m deep, respectively, can be identified in the TCDP cores. Based on the XRD data, two characteristics of clay mineralogy can be distinguished in these two fault zones: 1) smectite-rich just occurs within a narrow black materials in 1111 fault zone, and other clay minerals are totally disappeared (only few illite); 2) all kaolinite were gone within a board area in the 1153 fault zone. The former, smectite-rich zone is impossible to be retrograded from illite due to lacking the smectite-illite mixed-layer clays occurred in this zone. It may be devitrified from glasses which were detected by the TEM. Meanwhile, ultra-fine particles in this zone are also identified by the TEM. These evidences, i.e. glasses, ultra-fine particles and smectile-rich zone, suggest that the black materials occurred in the 1111 m deep may be the candidate slip zone of the 1999 Chi-Chi earthquake and provide the information to calculate the fractured and heat energy produced by this event. The consumption of kaolinite occurs in the temperature ranging from 450°C to 550°C and the temperature of chlorite transformed to other minerals is higher than 800°C based on the TGA experiments. Therefore, all kaolinite gone and the existence of chlorite in the 1153 fault zone indicate that this fault zone has been suffered from heating events of 450°C- 800 °C in past earthquakes.