| 摘要: | 台灣造山帶形成於歐亞板塊與菲律賓海板塊的斜向弧－陸碰撞，自中新世晚期蓬萊造山運動之展開，台灣山脈逐步由北向南發展。位處隱沒至碰撞造山過渡帶的脊樑山脈板岩帶南段，過去為被動大陸邊緣至南中國海上的中新世沉積物，經過多相變形與淺變質作用在山脈兩翼有著不同程度的葉理發育及構造變形特徵，常作為探討早期台灣造山帶構造歷史之關鍵。然而前人提出不同的構造演化模型解釋山脈南端東緣出露的板岩構造特徵，皆缺乏熱變質約制而忽略變質作用在構造演化中的角色。
由於泥質岩類沉積物富含碳質物，本研究將採用碳質物拉曼光譜地質溫度計測定板岩的最高變質溫度，整合野外露頭構造與顯微構造觀察的分析結果，嘗試解析這些不同構造變形發生時其處於弧－陸碰撞初期之造山岩楔內的隱沒深埋情況。
本研究整合了徐乙君 (2008)在太麻里－金崙溪沿岸南北向剖面採集製作之樣本，先確認碳質物結晶度之測定結果沒有明顯受到韌性剪切行為與雷射光的不同入射方位影響，再新增南田與太麻里溪產業道路東西向剖面之樣本，共計103筆溫度資料。實驗分析所得變質溫度與野外觀察南北向構造剖面之整合，指示了太麻里地區中新世廬山層地層在構造埋深階段以「同褶皺加溫」的變形模式變質，超過盆地埋深時的加溫，再由測定岩層之變質溫度換算層厚，建立了峰變質狀態時地層於倒轉後又已有30°傾斜的褶皺變形量。而等變質溫度線在向東伸向構造變形、第二期葉理發育與向北伸向構造變形間的相對關係分析顯示，在達到變質峰值前且次生葉理為未變形狀態時，層面已受到輕微褶皺變形，搭配野外調查沒有觀察到向東伸向的偃臥褶皺出露在南北向的太麻里海灘，排除區域構造受到旋轉分量控制。本研究認為是由於向北伸向變形在向東伸向變形未結束之際加入一起作用，於葉理發育前以及變質峰值前岩層已受到南北向擠壓變形，使得野外露頭有葉理面傾角以及等變質溫度線緩於層面之地質現象。
根據RSCM量測數據及所作推論，本研究提出初期弧－陸碰撞之台灣造山帶構造演化模型：中新世廬山層隨被動大陸邊緣隱沒進入造山岩楔的過程，由底部滑脫面演變成的向東伸向逆衝褶皺帶，造成地層倒轉形成等斜偃臥褶皺；當向北伸向變形加入時，岩層透過底部加積作用進到造山楔底部，此時積累的構造荷重對等斜偃臥褶皺產生壓扁作用，同時南北向應力擠壓持續作用，直到褶皺變形導致層面與葉理面分別傾斜30°和15°時，在造山楔下方約10公里深處達到最高變質溫度，之後掘升至山脈南端東緣出露並在同高程出現~30℃的溫度差距。
;Taiwan orogenic belt is formed by the oblique arc-continent collision between the Eurasian and Philippine Sea plates, triggering the Penglai orogeny since the late Miocene and leading to the southward propagation of the orogen. The southern part of Backbone Range slate belt (BS), located at the transition from subduction to collision, is composed by Miocene passive margin to pelagic cover series and experiences different degrees of tectonic deformation and metamorphic histories, which make it as an exceptional place to study the early tectonic history of the Taiwan orogeny. However, previous mountain building models proposed to explain this structural deformation of the southern Taiwan range lacked thermal-metamorphic constraints, thus devoid of the role of metamorphism in the orogenic processes.
As pelitic photolith is rich in carbonaceous material, the abundant slates are chosen in this study to determine their peak metamorphic temperatures through a geothermometer (Raman spectroscopy of carbonaceous material, RSCM). Together with results of macroscopic and microscopic structural observations and analyses, an attempt is made to decipher the primary thermos-tectonic trajectory of these rocks concerning the relative timing and environment of the deformation stages within the early orogenic wedge during the arc-continent collision.
In addition to the results of north-south sampling along the Taimali-Jinlun coastline by Hsu (2008) which also help confirm that both aseismic shear strain and incident angle of laser beam relative to the C-axes of carbonaceous material have an insignificant effect on CM crystallinity, new sampling is extended to the east-west Nantien and Taimali industrial roads, resulting in 103 RSCM peak temperature data in total. According to analyses of RSCM-T and macroscopic structural observation, the Miocene Lushan Formation in the southeastern BS is found to have undergone synorogenic metamorphism, and was folded with limbs tilted at ~30˚ during the peak state as suggested by the field RSCM-T isograde. Overprinting relationship among the overturned folding, main S2 foliation, and RSCM-T isograde is established, and north-vergent deformation is observed to have commenced prior to both peak metamorphism and the end of east-vergent backfolding, as evidenced in the gentler dips of RSCM-T isograde and backfolding-associated S2 foliation.
Inferred from the RSCM result, a tectonic evolution model of the Taiwan orogenic wedge growing in the early stage of arc-continent collision is proposed. Following the subduction of the Miocene Lushan Formation deep beneath the accretionary wedge, east-vergent backthrusting started to affect the rocks resulting in overturned strata within recumbent backfolds. Backfolding and formation of associated S2 foliation continued with prograde subduction and was joined with north-vergent folding before peak state metamorphism, when the rocks were incorporated into the base of the wedge through basal accretion. During the peak state at a depth of ~10 km, the overturned bedding and foliation planes observed across the north-vergent folds are deduced to be tilted at 30° and 15°. Afterwards, the slate was exhumed to the southeastern edge of the mountain range with further north-vergent folding and resulted in RSCM-T isograde folding exhibiting ~30℃ difference at the same elevation. |
