臺灣北部造山帶磁性組構與古地磁之研究

Abstract

磁感率異向性因可作為瞭解區域應變的良好工具，已廣泛應用於造山帶演化的分析。過去研究調查指出臺灣北部造山帶的區域構造發育於最大變質溫度之前，為了瞭解造山過程中溫度和變形間的關聯性，本研究沿北橫公路至太平山採集低變質度的定向樣本，除了以磁感率異向性實驗、溫度－磁感率實驗及磁滯曲線來分析磁性組構外，並以熱去磁方法獲得古地磁相關資料，重建早期弧陸碰撞的北部造山帶應變演化過程。 研究結果顯示磁感率橢球體的K1軸多呈東北-西南向，意味著擠壓方向大致和現今的板塊運動方向相符，而磁性參數中的異向性及變形強度由西向東逐漸增強，且在靠近斷層帶處數值相對較高。根據前人之磁性組構的分類，應變由小至大可分為TypeⅠ-Ⅵ六類，利用臺灣北部造山帶採樣剖面的地質特性及磁性組構分佈的結果可將研究區分為A至D四個構造區域，其特徵分述如下：(A) 西部麓山帶至劈理發育處－此區開始受應變作用影響，K1趨於集中在東北－西南向，磁感率橢球體偏向扁平狀，磁性組構應屬TypeⅡ。(B) 劈理發育處至檜山斷層下盤－K3開始受劈理影響，磁感率橢球體主要為扁平狀，磁性組構介於TypeⅡ-Ⅲ間。(C) 檜山斷層下盤至梨山斷層－此區磁性組構於三光向斜至中嶺背斜開始呈現雪茄狀，K3明顯受劈理發育而呈帶狀分布，磁性組構介於TypeⅢ-Ⅳ。(D) 梨山斷層至太平山－受強烈劈理作用，推測應增強至TypeⅣ以上，雖異向性與變形強度較大，但K3仍集中於鉛直方向而非水平方向，應是此區所受動力機制和雪山山脈不同，導致應變過程不連續。 於雪山山脈北部的中嶺背斜進行殘磁之褶皺測試，由熱去磁結果顯示此區磁黃鐵礦所記錄之特徵殘磁時期為褶皺事件之後，可推論底侵作用可能為增溫的方式，而溫度降至磁黃鐵礦的居禮溫度之下便無明顯的褶皺作用，之後雪山山脈逐漸抬升冷卻，同時受到剝蝕作用影響，最終形成現今所見西村層出露之地貌。Anisotropy of magnetic susceptibility (AMS) can be regarded as a useful tool for understanding the finite strain pattern of regional deformation so that it is generally applied to decipher the evolution of mountain belt. Previous studies suggested that the overprint of maximum metamorphic temperature postdated the regional deformation across the northern Taiwan mountain belt. In order to evaluate the interrelationship between maximum metamorphic temperature and deformation during mountain building, we collected oriented samples of low-grade metamorphic rocks from the Northern Cross-Island Highway to the western Backbone Range. In addition to the magnetic fabrics inferred from the study of magnetic susceptibility anisotropy, experiments of temperature-function magnetic susceptibility, hysteresis loop, and thermal demagnetization were also conducted to gather paleogeomagnetism in data. This study can provide insights into reconstructing the strain evolution of northern mountain belt during late Cenozoic arc-continental collision. Current results show that NE-SW orientation of K1 axes of magnetic ellipsoids indicates northwest-southeast compression, which is consistent with current plate convergence direction. Both AMS and deformation intensity increase from the west to the east with abnormally strong intensity and oblate strain near the major reverse faults, suggesting the amount of final strain increases across the mountain belt. According to the characteristics of the magnetic fabrics, magnetic fabrics can be classified as six stages from Type I to VI with increasing strain. The study area can be divided into four fabric domains A to D based on geological aspects and characteristics of magnetic fabrics across northern Taiwan mountain belt. Domain A is from the West Foothills to the cleavage front. Rocks in this area begin to be influenced by horizontal tectonic strain. K1 of ellipsoid is in northeast-southwest orientation, indicating NW-SW compression. The shape of ellipsoid is oblate. Magnetic fabric belongs to Type II. Domain B is bounded by the cleavage front and the footwall of the Kuaishan Fault. The distribution of K3 orientation started to be affected by cleavage. The shape of ellipsoid is mainly oblate. Magnetic fabric is classified as Type II-III. Domain C is located between the footwall of the Kuaishan Fault to the Lishan Fault. The shape of ellipsoid from the Sankuan Syncline to the Chungling Anticline gradually converts to prolate. The distribution of K3 becomes a girdle in NW-SE orientation that clearly is influenced by cleavage development. Magnetic fabric is treated as Type III-IV. Domain D is bounded by the Lishan Fault and the Taipingshan. Due to distinct cleavage development, tectonic strain in this domain presumably should be augmented to more than TypeIV. Though both anisotropy and deformation intensity are increased, the direction of K3 is still concentrated in vertical, not in horizontal. The result might be the reflective of discontinuous strain response to different kinematic mechanisms between the Backbone Range and the Hsueshan Range. Thermopaleogeomagnetic records of pyrrhotite remanence on both limbs of the Chungling Anticline of northern Hsueshan Range failed the fold test, indicating the existence of post-fold exhumation. It means underplating could be the mechanism to form dynamic metamorphism and while the metamorphic temperature was cooled down below the Curie temperature 320℃ of pyrrhotite, there was no remarkable folding deformation anymore. After that, the Hsueshan Range was uplifted passively and eroded to crop out the Hsitsun Formation.