| Abstract: | 本研究藉由野外調查和實驗室觀察來描述、分析和解釋初鄉斷層擴展褶皺相關的變形條帶。變形條帶是分佈於台灣中西部褶皺逆衝斷層帶前緣的集集攔河堰下游的晚上新世至早更新世的卓蘭層砂岩中常見的次生構造。初鄉斷層為一東傾左移逆斷層其上盤出現一列褶皺。這褶皺列由半波長有數百米的溪洲子向斜和頂溪洲背斜所組成，向斜為褶皺軸俯衝軸面直立兩翼平緩的褶皺，背斜則為褶皺軸俯衝軸面傾斜的兩翼緊密不對稱褶皺。變形條帶出現在淘選度普通的細粒至粗粒砂岩層中，大部分在背斜西翼和向斜東翼的B區，產狀多為1米長以上，且厚可達2厘米，約呈三組，各組的位態分別為035°/45°SE（組1）、135°/53°SW（組2）和000°/~85°W至85°E（組3）。我們的調查結果顯示，組1的位態與初鄉斷層的位態為次平行，第2、3組的銳角平分線與頂溪洲背斜褶皺軸幾乎垂直，後者意味著近乎平行於最大收縮方向。此外，出現在初鄉斷層跡和溪洲子向斜軸之間區域（C區）的變形條帶在顯微鏡下明顯可辨，但在露頭上則相形模糊，因為它們很薄（僅幾毫米）且也短（<3 dm），並且常轉變成破裂。然而，在頂溪洲背斜的東翼（A區），變形條帶不是與層面近乎平行（類型1），就是靠近小斷層且與之近乎平行（類型2）。類型2是所謂的碎裂變形條帶 (cataclastic DBs)，特徵為具破碎細粒礦物呈帶狀，而類型1是分解變形條帶 (disaggregation DBs)主要是透過顆粒流動 (granular flow) 的方式進行重組顆粒，亦即顆粒與顆粒藉由滾動和滑動而產生條帶狀的變形。B區的變形條帶是溶解膠結條帶 (solution and cementation bands)，特徵為主要由粘土礦物組成，但仍保留了部分的粉碎細粒石英，可能是因為於溶解或膠結過程中產生大量的粘土礦物，僅保留碎裂變形條帶中粉碎的石英細粒，也因此推測碎裂變形條帶應是溶解膠結條帶的前身。最後，區域C中的變形條帶是滑移變形條帶 (slip DBs)，具有滑移面的特徵，在顯微鏡下和露頭上皆很容易觀察到與其相關的滑移面。由以上對研究區的幾種變形條帶的產狀描述、組構及與頂溪洲背斜的幾何關係，本研究認為除了分解變形條帶外，其他幾種變形條帶的形成皆與初鄉斷層擴展褶皺作用相關，控制這些變形條帶發育的主要因素包括岩性、與斷層的接近程度，以及在斷層引致褶皺的位置。綜合言之，A區的變形分解帶可能在岩層褶曲之前形成，初鄉斷層的擴展使得頂溪洲背斜發育，過程中緊隨其後出現三組碎裂變形條帶，隨後大部分的碎裂變形條帶演化為B區的溶解膠結變形條帶和C區的滑移變形條帶，最後向斜才形成。;This study describes, analyzes, and interprets Chushiang fault propagation fold-related deformation bands (DBs) based on field and laboratory observations. DBs are common structural elements found in the late Pliocene to early Pleistocene Cholan sandstones distributed downstream of the Chichi weir along the Choshui River in the frontal part of the fold-thrust belt in central-western Taiwan. One fold train appears in the hanging wall of the east-dipping Chushiang left-lateral reverse fault. It is composed of a plunging, upright, gentle Shizhoutzu syncline and a plunging inclined asymmetric, tight Dingxizhou anticline with a wavelength of hundreds of meters. The DBs occur in moderately sorted, fine-to-coarse grained sandstone layers. Most of them appear long (> one meter) and thick (up to 2 cm) in area B, which is bounded by the anticline axial surface at one side and the syncline axial surface at the other and fall into three sets with orientations of 035°/45°SE (set 1), 135°/53°SW (set 2), and 000°/~85° W to 85°E (set 3), respectively. Our investigation shows that the orientation of the set 1 is sub-parallel to the Chushiang fault orientation. Besides that, the acute bisector of the sets 2 and 3 is sub-perpendicular to the Dingxizhou anticline fold axis, sub-parallel to the inferred direction of maximum contraction. In addition, DBs that appear in the area between the Chushiang fault trace and the Shizhoutzu syncline axial trace (area C) are obvious under microscopic but ambiguous on the outcrops because they are thin (few mm) and short (< 3 dm), and commonly evolve into fractures. However, in the eastern limb of the Dingxizhou anticline (area A), the DBs appear either sub-parallel to the bedding plane (type 1) or close to some minor fault with an orientation parallel to the fault trace (type 2). The type 2 are cataclastic DBs with the characteristic of crushed fine-grained minerals in the bands while the type 1 are disaggregation bands with the characteristic of grain reorganization through a process referred to as granular flow, this is grains are deformed by rolling and sliding. The DBs in area B are solution and cementation bands, which are dominated by clay minerals likely resulting from dissolution or cementation processes but remain the characteristic of crushed fine-grained quartz, likely inherited from cataclastic DBs, i.e. their counterpart in the past. The DBs in area C are slip DBs with the characteristic of slip surfaces easily identifiable as discontinuities within the band under the microscope and on the outcrops. Such evidence including the DB occurrences and textures indicates the origin of these DBs except the disaggregation bands is highly related to the Chushiang fault propagation fold activity. The factors controlling their development include lithology, proximity to the fault, and position on folds induced by faulting. In conclusion, the disaggregation bands in area A likely formed before the emergent anticline, three sets of cataclastic DBs followed during the anticline development, and most of cataclastic DBs evolved into the solution and cementation bands in area B and the slip bands in area C during the post-anticline (syncline forming) phase. |
