| 摘要: | 2018年花蓮地震（Mw 6.4）誘發米崙斷層破裂，並導致數百人傷亡。米崙斷層上一次破裂發生於1951年的地震亦導致米崙斷層破裂，顯示此斷層活躍且短再現週期的特性。由於目前未在地表發現米崙斷層露頭，使我們對其斷層特徵與可能之斷層特性缺乏理解。幸運的是，2020年米崙斷層鑽井研究計畫（MiDAS）的執行，成功獲得米崙斷層帶的斷層岩芯，使我們可以一窺米崙斷層之構造特徵。
米崙斷層帶呈現非對稱性構造，包含斑狀片岩與綠色泥構成的破裂帶（damage zone），厚約29公尺，以及灰色、黃色、黑色與深灰色斷層泥共同構成的斷層核芯（fault core），厚約20公尺。本研究針對斑狀片岩與綠色泥之來源及其蝕變過程進行詳細的分析與討論。本研究使用水熱實驗、熱重分析（TGA）、X光繞射分析（XRD）、X光螢光分析（XRF）以及拉曼光譜分析對米崙斷層帶全區樣本進行實驗分析，期盼建構更完整的斷層帶資訊，並進一步探討發生於斷層核芯與破裂帶的作用。根據岩芯觀察，破裂帶被認為是水－岩反應的產物，斑狀片岩風化後的基質與風化前的岩石呈現網狀裂隙（anastomosing occurrence），顯示流體沿岩石裂隙滲入所導致水－岩反應。結果顯示，斑狀片岩風化前後礦物相一致，綠色泥風化前後礦物相類似，但未風化岩塊識別出更多種礦物，意味著綠色泥原岩可能包含不同的岩石原岩。另外，斑狀片岩與綠色泥風化前後化學組成有明顯差異，表示流體在此長時間的劇烈作用。
由於分析樣品的岩石組成，礦物與化學特徵與花蓮豐田地區出露的玉里帶岩石相似，我們推測米崙斷層可能由玉里帶（Yuli belt）岩石構成。針對為何玉里帶岩石存在於米崙斷層中，我們提供了可能的解釋，如崩移的傾瀉層（olistostrome）或是「聖誕樹形」構造（Christmas tree structure）等機制，但仍需更多證據佐證。
;The 2018 Hualien earthquake (Mw 6.4) triggered the Milun Fault rupture and resulted in hundreds of casualties. The 1951 earthquake also caused the Milun Fault rupture, indicating that the Milun Fault is active and characterized by a short recurrence interval. To date, the outcropped Milun Fault has not been recognized in the field, limiting our understanding of its structural and mechanical characteristics. Fortunately, the implementation of the Milun fault Drilling and All-inclusive Sensing project (MiDAS) in 2020 successfully obtained fault core samples from the Milun Fault zone, allowing us to investigate its internal structural features.
The Milun Fault Zone exhibits an asymmetric structure, characterized by 29-m-thick altered spotted schist and non-cohesive serpentinite in the damage zone, and 20-m-thick foliated grey to black gouges in the fault core. In this study, we focused on the origin and alteration processes of the spotted schist and green gouge. We conducted hydrothermal experiments, thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence (XRF), and Raman spectroscopy on samples across the entire Milun Fault Zone, aiming to provide a more comprehensive understanding of the Milun fault zone and to further investigate the processes occurring within both the fault core and damage zone. From the observations of cores, the damage zone is considered as the product of fluid–rock interactions. The altered spotted schist exhibits an anastomosing occurrence, suggesting the presence of fluid-relevant interaction along the fractures. Our results show that the mineral assemblages of the spotted schist remain consistent before and after alteration, while the green gouge shows comparable mineral assemblage but contains a wider range of minerals in the unaltered samples, implying that its protolith may include multiple rock types. In addition, both spotted schist and green gouge display significant differences in chemical composition before and after alteration, suggesting long-term and intense fluid-rock interaction.
Based on the lithological, mineralogical, and geochemical characteristics of the Milun damage zone, which are similar to those of the Yuli Belt exposed in the Fengtian area of Hualien, we suggest that the Milun Fault Zone may be composed of rocks derived from the Yuli Belt. Here, we provide possible mechanisms for the presence of Yuli Belt rocks within the Milun Fault zone including olistostrome by slumping or the development of a "Christmas tree" structure; however, more evidence is needed. |
