利用微構造分析太麻里地區的變形歷史

本研究區位於台灣東南部的太麻里海岸一帶，由於正好處於菲律賓海板塊與歐亞板塊之隱沒-碰撞的過度帶，此處的岩層完整紀錄台灣早期造山階段的特徵和橫移壓縮的過程。本區之岩層以中新世的廬山層為主，由黑色到深灰色的硬頁岩、板岩和深灰色的硬砂岩互層組成，以及複理層沉積物，中間夾有一些透鏡狀砂岩以及散亂的泥球結核；這些地層都經過低度變質其變質度可達葡萄石-綠纖石相，由野外觀察可看出此區已歷經多次的變形作用。本研究除了藉由野外的觀察與構造測量分析外，尚利用板岩內的壓影構造(pressure shadow)構造重建岩石變形歷史，分析壓影構造首先要將採集的岩層的定向標本，在變質岩韌性剪切帶中，一般認為主要葉理面代表剪動面，主要剪動面即為XY面，而剪動面上的伸張線理方向即為有限應變橢球的最長軸方向，亦即X軸方向；分別由劈理面及伸張線理的方向決定三互相垂直的XY面、XZ面及YZ面，觀察這三面薄片中的壓影纖維構造及層面和劈理面的相關性以進行分析；除此之外，並加上磁感率異向性分析，對金崙類背斜之褶皺進行探討。從野外的觀測上，可發現此區普遍存在一組呈次水平的劈理(Sh)和一組成東南東方向的線理(L)。而本研究主要分析五個褶皺露頭之次水平劈理上的壓影構造，欲了解其褶皺和劈理的相關性。分析結果發現：(1) 此區的壓影構造大多是黃鐵礦型態(pyrite type)。仔細觀察薄片上的壓影構造，則發現不論是在XY面上或是在XZ面上甚至是YZ面上都可觀察到非共軸變形，由壓影纖維構造於空間上的分布情形可以推論其形成主要是受控於向東活動的逆衝斷層帶的影響，並記錄到塊體順時針旋轉的現象，且壓影纖維構造於形成之後，仍有受到後期板岩中固定流動(flexural flow)的影響。(2) 由磁感率異向性的分布則可得知於褶皺兩翼的受力情形並不相同，和所觀察到的不對稱褶皺現象相符。(3) 野外的次水平分布的葉理面，以及壓影纖維構造上放射狀的纖維，都可得知此區曾受到垂直方向的壓縮。(4) 從野外觀察到的中視尺度之截切關係加上壓影纖維構造的分析結果可以得知此區的變形歷史。此結果顯示此區的變形歷史相當複雜，推論其反應出在抬升的階段中，三軸變形和垂直壓縮是主要的變形機制。

關鍵字

壓影纖維構造；變形歷史；壓扁效應造成的葉理隱沒-碰撞的過渡帶；太麻里

並列摘要

Since the geometries of syn-tectonic fibers in pressure shadows are considered to be closely related to the deformation paths of host rocks, syn-tectonic fibers have been used to determine fold kinematics and the kinematics of deformation in accretionary prism and fold-and-thrust belt. This study uses pressure shadows to analyze rock deformations in five outcrops of folds with flat-lying foliation (Sh) at Taimali, Taiwan. The study region, which is located at the subduction-collision conjunction, southeastern Taiwan, consists of overturned Miocene deposits having undergone lowest-grade metamorphism and ductile-brittle deformation during the Late Cenozoic Penglai Orogeny. There lies a set of flat-lying foliation (Sh) and ESE-trending stretch lineation ( L ). The deformation history was evaluated using incremental strain indicators and microstructures. Observed phenomena are described as follows: (1) the antitaxial overgrowths of fibrous quartz are developed in pressure shadows of pyrite object; (2) non-coaxial strain was indicated by fibers on both cleavage plane (XY) and cleavage-perpendicular plane (XZ); (3) in the XY plane, the radial fibers over the pyrite are prominent and are of the last deformation stage; (4) the rotation directions of pressure shadows in rocks of different limbs were the same on the XY plane, which indicated the pressure shadows have not been affected by folding; (5) the deformation recorded by pressure shadows on the YZ plane in rocks of different limbs of the Jinlun antiform was symmetrical with respect to the fold axial plane. These results show that the deformation history was triclinic deformation and vertical flattening is strong during exhumation in this region.

並列關鍵字

pressure shadows ； strain history ； flat-lying foliation ； subduction-collision conjunction ； Taimali

參考文獻

楊萬慧 (2007) 應用地形分析方法研究台灣中央山脈東翼地表抬升。國立中央大學地球物理研究所碩士論文。共121頁。

Minster, J.B. and Jordan, T.H., 1979, Rotation vectors for the Philippine and Rivera plates: Eos, Trans. Amer. Geophys. Union, v.60, p.958.

Lu, C. Y., Chang, K. J., Malavieille, J., Chan, Y. C., Chang, C. P., and Lee, J. C., 2001. Structural evolution of the southeastern central range, Taiwan. WESTERN PACIFIC EARTH SCIENCES 1, 213-226.

Anastasio, D. J., Fisher, D. M., Messina, T. A., and Holl, J. E., 1997. Kinematics of decollement folding in the Lost River Range, Idaho. Journal of Structural Geology 19, 355-368.

Bachman, S. B., Lewis, S. D., and Schweller, W. J., 1983. Evolution of a forearc basin, Luzon Central Valley, Philippines. AAPG Bulletin 67, 1143-1162.

國際替代計量

利用微構造分析太麻里地區的變形歷史

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