| 摘要: | 台灣位處於歐亞板塊與菲律賓海板塊之板塊邊界上，呂宋島弧向西北方向碰撞歐亞板塊被動中國大陸邊緣，持續且活動之造山作用使台灣成為研究山脈演化的絕佳地點。台灣造山帶中的板岩帶包含兩個地質單元─雪山山脈與脊樑山脈板岩帶，雪山山脈由沉積在被動大陸邊緣上的始新世至漸新世沉積物所構成，脊樑山脈板岩帶由廣布於被動大陸邊緣上始新世至中新世沉積物所構成。由於這兩個地質單元特徵十分不同，前人提出不同的模型來解釋雪山山脈與脊樑山脈板岩帶的接觸關係，假整合、斷層、縫合帶等等。本研究藉由野外構造觀察、碳質物拉曼光譜地質溫度計所得最高變質溫度、斷層擦痕古應力分析、薄片微構造等結果來探討此邊界的性質，更進一步重建板岩帶的構造演化歷史。從野外構造觀察發現，雪山山脈的劈理傾角較陡；而脊樑山脈的劈理傾角較緩，甚至在某些露頭可以觀察到兩期劈理。本研究總共觀察到四個斷層露頭，分別是脆性、脆韌性以及韌性剪切帶的露頭。由斷層擦痕古應力分析發現兩期的事件，早期的東─西向張應力與晚期的西北─東南向壓應力事件。由變質溫度的結果顯示，雪山山脈的變質溫度較高(321~372°C)，脊樑山脈板岩帶的變質溫度較低(223~280°C)，兩者之間夾有一小塊中間變質溫度的區域(281~320°C)。本研究認為變質溫度的差距是由斷層錯動所導致的，主斷層在近地表發展出分支斷層，使得中間變質溫度區域的產生。觀察薄片微構造發現，共軸壓力影出現在雪山山脈，非共軸壓力影出現在脊樑山脈以及雪山山脈靠近斷層剪切帶附近，顯示研究區域有兩種不同的變形模式。綜合上述結果提出本研究區域的構造演化模型，雪山山脈地層原先沉積在被動大陸邊緣的雪山槽內，而遠洋的中新世廬山層在晚中新世隱沒並進入增積楔形體下方，之後雪山槽也開始隱沒受到褶皺變形作用並在造山楔下方獲得最高變質溫度，最後脊樑山脈盧山層受到擠壓與內雪山山脈碰撞在一起，兩者接觸關係為梨山斷層。梨山斷層向東逆衝活動使雪山山脈比脊樑山脈板岩帶多抬升將近3公里，因此出露較高變質溫度的岩石。;Taiwan sits at the convergent plate boundary between the Eurasian and the Philippine sea plate, and the Luzon arc collided northwestward with the passive Chinese continental margin of the Eurasian plate. Continuing and active mountain building processes makes Taiwan a modal locality for studying mountain building processes. The slate terrane of the Taiwan mountain belt including two geological units, the Hsuehshan Range(HR) and the Backbone Range slate belt(BRSB), both are composed by Eocene to Miocene passive margin sequences. Due to the contrasting characteristics of the two units, previous studied proposed different models to interpret the contact relationship between the Hsuehshan Range and the Backbone Range, including disconformity, fault, and plate suture. This study combines field observations, peak metamorphic temperature from Raman spectroscopy of carbonaceous material(RSCM), slickenside paleostress analysis and thin section microstructure to investigate the property of this boundary, and then reconstructing the tectonic evolution of the slate terrane. According to the field observations, the dip of foliation in the Hsuehshan Range is steeper than the Backbone Range slate belt, even a crenulation foliation in the Backbone Range slate belt. In this research, we found four fault outcrop, including brittle, brittle-ductile, and ductile shear zone outcrop. The slickenside paleostress analysis shows there are two event, early E-W extension and late NW-SE compression. According to RSCM results, the metamorphic temperature of the Hsuehshan Range(321~372°C) is higher than Backbone Range slate belt(223~280°C) and small region of mid-temperature(281~320°C) between them. We consider that the fault offset was responsible for the observed metamorphic temperature gap, and the mid-temperature region caused by near surface branch faults. According to the microstructure results, we found the coaxial pressure shadow in the Hsuehshan Range and the non-coaxial pressure shadow in the Backbone Range slate belt, representing different deformation pattern in the two regions. Combining all of the data, we propose the structure evolution of the research area. The strata of the Hsuehshan Range deposited in the passive continental margin Hsuehshan Trough, and the pelagic Miocene Lushan Formation subducted and get into the accretionary wedge in the late Miocene. Then the Hsuehshan Trough subducted and folded and also achieved the maximum metamorphic temperature under the orogenic wedge. In the end the Backbone Range Lushan Formation juxtaposed with inner Hsuehshan Range strata, the contact relationship between them is the Lishan Fault. Because of the Lishan Fault east vergent thrusting, the Hsuehshan Range was more exhumed ~3 km than the Backbone Range slate belt and exposes rocks with higher metamorphic temperature. |
