Characteristics of Clay Minerals from the Taiwan Chelungpu Fault Drilling Project
Date Issued
2011
Date
2011
Author(s)
Kuo, Li-Wei
Abstract
The earthquake is one of the natural disasters that is harmful for safety and wealth of mankind. Seismic faults are the major source to produce earthquakes. Therefore, investigations of the characteristics and mechanisms of a seismic fault could protect people from dangers and damages.
The Chelungpu Fault, Taiwan produced a northward propagating rupture on September 21, 1999 resulting in an Mw 7.6 earthquake. It made thousands of people dead and produced serious destructions in central Taiwan areas. It is necessary to obtain more information on the Chelungpu fault zone properties, and to investigate fault mechanisms via understanding the physical and chemical processes in principal slip zone (PSZ) and/or slip surfaces. The Taiwan Chelungpu-fault Drilling Project (TCDP), therefore, is performed to extract deep materials for the investigation of faulting mechanism.
To realize the possible physical and chemical processes through fault zones and to investigate the faulting mechanism of the Chelungpu fault, the principal slip zone (PSZ) should be identified. In this study we recognize the PSZ of the Chelungpu fault related to 1999 Chi-Chi within its characteristics of clay minerals in the fault zone. The Taiwan Chelungpu-fault Drilling Project (TCDP) Hole-A recovered continuous core samples across the rupture zone of the 1999 Chi-Chi earthquake (Mw7.6). Studying in-situ chemical properties sequentially from fresh-fault-zone materials of the Chelungpu fault provides insight into possible faulting mechanisms. Distinct anomalies of mineral assemblages at the 1111-m fault zone of TCDP Hole-A are found to be: (1) A decrease in clay content within the primary slip zone (PSZ); and (2) A significant decline of illite, disappearance of chlorite and kaolinite, and spike in smectite within the PSZ. Meanwhile, features relating to melting or amorphous material in the PSZ have been observed by SEM and TEM. The results suggest that the PSZ might have experienced generation of glassy materials such as pseudotachylyte by the expense of clay minerals due to strong shear heating, then prompt alteration of pseudotachylyte into smectite. Characteristics of clay minerals and images obtained from electronic microscopes in the PSZ thus imply that pseudotachylyte possibly developed during the 1999 Chi-Chi earthquake, but quickly altered into smectite. This particular phenomenon may explain why pseudotachylyte is rarely found in exhumed hydrated fault zones.
To investigate the coseismic frictional temperature in seismogenic fault zones, we examine the characteristics of clays in the Chelungpu-fault zones with isothermal heating experiments, Scanning Electron Microscope coupled to an Energy Dispersive spectrometer (SEM/EDX), and Thermogravimetry analysis (TGA). In the TCDP case (Taiwan Chelungpu fault Drilling Project), three fault zones of the Chelungpu-fault system were identified at the depth of 1111m, 1153m, and 1,222m (described as FZ1111, FZ1153, and FZ1222 hereafter), respectively. The clay mineral assemblages of FZ1111 show evidence of melting, and the temperature in a ~2 cm band within the black gouge zone is estimated to be from 900°C to 1,100°C by comparing the SEM images of in situ natural samples with those of heated materials, and the finding of no recrystallization of kaolinite-amorphous aluminosilicates-spinel in the fault samples. The clay mineral assemblages of FZ1153 suggested that kaolinite has been broken down by the thermal decomposition/dehydroxylation but chlorite has not. The clay characteristics and results of SEM/EDX and TGA constrain the faulting temperature from 500°C to 900°C, with a spatial distribution up to ~1.3 m. The clay characteristics of FZ1222 indicated that clays were changed by experiencing high temperature acid fluids, instead of thermal decomposition/dehydroxylation processes, and that the temperature is localized in ~2 cm and ranges from 350°C to 500°C, the lowest temperature among three fault zones. The estimates of temperature ranges, and thermal anomaly intervals among three fault zones provide important information and constraints on the physical and chemical processes, coseismic dynamic weakening mechanism, and earthquake energy budget in the future.
Chemical weathering is also an important process to produce clay minerals. We systematically analyze the characteristics of clay minerals from north to south and along the depth of the Chelungpu fault. The Chelungpu fault, with northward propagating ruptures, was created as a result of the Mw 7.6 earthquake which struck Central Taiwan on 21 September 1999. To investigate its true faulting mechanism, we examined the clay mineralogy and major element geochemistry of the host rocks of the Chelungpu fault from four outcrops, Fengyuan (455.3 m in depth), Nantou (211.9 m in depth), and Taiwan Chelungpu fault Drilling Project (2003 m in depth). The outcrops are spanned roughly 70 km along the fault. Mineralogical and chemical results of the host rocks revealed different degrees of chemical weathering, and its intensity could be further understood through the relative clay percentage of smectite, illite chemistry index, and illite crystallinity. These mineralogical proxies combined with the chemical index of alteration (CIA) and the intensity of chemical weathering indicate that the degree of chemical weathering is a function of depth, i.e., the most severe on the surface and the mildest in the TCDP samples. The mineralogical and geochemical data obtained in this study also suggest that chemical weathering, rather than leaching, seems to be the main driving force for the phase change of clay. The amount of smectite produced by chemical weathering varies with depth, and it argues against a previously suggested idea that weak-fault behaviours were caused by the presence of smectite on the surface. The observations of clay mineralogy and major element geochemistry in this study indicate that the presence of smectite in the outcrops may not play a significant role during faulting, and suggest that fault-weakening as a result of the presence of smectite cannot be applied to the Chelungpu fault.
Subjects
the Chelungpu fault
clay mineral
temperature
weathering
Taiwan Chelungpu-fault Drilling Project
Type
thesis
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