| 摘要: | 台灣島位於歐亞大陸板塊與菲律賓海板塊交界處，菲律賓海板塊以每年約70mm的速度向西北推進，斜向碰撞歐亞大陸板塊，使得台灣島上為擠壓的環境，因此高山林立，其中以中央山脈與雪山山脈為主。然而，在雪山山脈的南段卻發育了一系列的山間盆地，在高山為主的環境下為何會產生這些陷落的盆地，一直是地質學家想要了解及探討的問題。過去許多學者對於埔里盆地的形成提出一些見解，但因缺乏直接的鑽井證據或地球物理資料而一直無法有定論。 本研究綜合炸測以及淺層反射震測，試圖描繪出埔里盆地的第三紀基盤形貌以及第四紀的沉積層位態。 研究結果發現：埔里盆地基盤最深不超過600公尺，成淺碟狀，在盆地西側較深。基盤以上之盆地堆積層分為淺部100公尺之礫石層及下面之砂層及泥層。淺部礫石層則東側較深。盆地內第四紀堆積層P波速度2200m/s，S波速度950m/s。基盤P波速度4300m/s，S波速度2000m/s。基盤為白冷層，上段為厚石英質砂岩，下段為厚頁岩。基盤構造呈二背斜一向斜，波長約4公里，波高約1公里。基盤以上之盆地堆積層走向近南北，而下部砂層泥層在東西向具有與基盤同樣的起伏，代表在沉積過程中砂、泥層不斷受到構造作用而變形。至於上部之礫石層近乎水平，代表其沉積時構造運動已趨緩。除兩側的限制斷層(眉原斷層與地利斷層)，盆地內未發現明顯的斷層。綜合研究結果推測埔里盆地的成因乃為早期受到壓縮而產生曲漥作用，屬於構造作用，近期則為河川侵蝕作用影響地表礫石層。 The Taiwan island is located in the convergent zone between the Eurasian continental plate and the Philippine sea plate. Due to the plate collision, there rise a lot of high mountains on the island. The main mountain chains include the Central Range and the Hsuehshan Range. However, in this compression environment, there exist some basins in the southern portion of the Hsuehshan range. Several researches have been conducted to interpret the causes of this basin group. Due to lack of drilling and geophysical data, many ambiguities still exist about the structure and the development processes of these basins, especially the largest one, the Puli basin. In this study, we use the high-resolution shallow seismic reflection to explore the Puli basin. The targets are to figure out the forms of the Tertiary basement and the Quaternary formations of the basin, and basing on these to investigate the mechanisms which may generate the basin. After combining many observations, we obtain the following conclusions: (1) The deepest basement in the Puli basin dose not exceed 600m, and is deeper in the western portion. The top 100m of the basin sediment is dominant with the gravel and the deeper part with the sand and the mud. (2) The velocity of the P wave in the Puli basin is 2200m/s, and the S wave, 950m/s. The velocity of P wave in the basement is 4300m/s, S wave is 2000m/s. Because of plenty of gravels, the velocity of S wave is relatively high, which induces strong groundrolls and significantly interferes the seismic field work. (3) The basement rock belongs to the Paileng Formation. It is revealed to have 2 anticlines and 1 syncline with the wavelength about 4km and the height about 1km. (4) The sediment layers within the basin are not flat, but with apparent dipping angles. Furthermore, the layers tilt more in deeper places, which implies continuous compression keeps occurring during the sedimentation processes. The upper gravel layer is nearly horizontal, which may show a stronger erosion control by the surface drainage system. (5) There are not apparent faults discovered in the basin. (6) The mechanism of down warping may be dominant in the early stage of the basin, however, in the later stage it is controlled by the river erosion. |
