台灣車籠埔斷層深鑽計畫之黏土礦物特性

Abstract

地震一直是人們最注意的天然災害之一，因為其對我們的生命安全以及財富有極大的影響。其中，地震斷層，就是地震最主要的來源之一，對地震斷層的活動特性及其機制有進一步的瞭解，就能增加對人們的保障，以及減少可能的損失。 車籠埔斷層於西元1999年發生錯動，造成數千人死亡，以及數萬人無家可歸。所以，對車籠埔斷層作詳盡的研究是有其必要性的，若能瞭解斷層帶與圍岩之間的物理化學作用，將對其斷層機制可以有更深入之瞭解。臺灣車籠埔深鑽計畫，便油然而生。 要探討車籠埔斷層之斷層帶可能產生之物理化學作用，就必須先確定主要滑動帶之位置。唯有研究主要滑動帶的特性，才能瞭解其錯動可能之機制。所以，本文第一個工作就是利用黏土礦物的特徵判斷可能於1999年產生錯動之主要滑動帶。由於地震斷層滑動會釋放大量的摩擦熱，並且可能在滑動帶產生高溫造成岩石熔融，使礦物相改變。觀察斷層帶礦物相的特徵之後，發現黏土含量在位於深度1,111公尺處大量減少，而且礦物相與圍岩迥然不同，說明在此深度的斷層帶可能是921集集地震的滑動面。 再者，黏土礦物有不同溫度的崩解反應，所以本文的第二個研究，便是利用其不同的熱崩解特性對車籠埔斷層之三個斷層帶（深度分別為1,111、1,153、1,222公尺）曾經遭受的溫度作初步的估計，而所得到之溫度估計可以作為計算摩擦熱最重要的參數，溫度，提供一個良好限制。研究結果顯示主要滑動帶曾經達到的溫度最高，其次是深度1,153公尺的斷層帶，最低則是深度1,222公尺的斷層帶。 當然，風化作用也是產生黏土礦物另一個重要的過程，所以本文的第三個工作，便是將車籠埔斷層的黏土礦物特徵由南到北，由淺到深作系統性地分析，並提供風化作用對黏土礦物可能產生之影響作定性及定量的描述。其結果顯示，深鑽計畫中的材料可以忠實的呈現或保持滑動帶曾發生的物理或化學作用，而地表的材料則否，同時也說明深鑽計畫的重要性。

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.