埔里盆地之震測研究 摘要 台灣板塊運動以壓縮為主，抬升許多崇山峻嶺，但在擠壓的環境中， 雪山山脈內部卻出現了一系列的山間盆地，從埔里往南到銃櫃。高山之中 如何產生這些盆地？盆地底下構造為何？如何納入周圍高山的造山運動？ 這些都是重要且有趣的地質問題。 本研究使用「震盪震源車(MiniVibe)」施測三條長測線(每條約1~3.5 公 里，總長7 公里)，獲取基盤的主要形貌。另外，亦採用「EWG-III 彈簧式 震源」，施測十餘組短測線(每組交叉二條或單一測線，每條約700~800 公 尺)，散佈盆地範圍，全面測繪埔里盆地的三維構造，包括基盤面以上的第 四紀地層及基盤的第三紀地層。綜合反射震測資料以及2002 年的炸藥折射 實驗結果，可以正確描繪埔里盆地的地下形貌及其上的堆積層，並推估可 能的構造成因。 結果顯示埔里盆地第三紀基盤面成碟狀，其中最深處在中間略偏西， 最深可達到720 公尺，整體而言，基盤面向西微傾斜。基盤面以上的第四 紀沈積層分為兩層，上層主要為礫石層，厚約200 公尺，位態趨近於水平。 下層為礫石及砂泥層，呈大幅度下凹，與上覆的礫石層呈現交角不整合， 該礫石-砂泥層具有與基盤同樣的彎曲起伏，代表在沉積完成後，與基盤一 起受到「曲窪作用」而變形。基盤屬於白冷層，地質圖標示其上部為石英 岩質-長石質砂岩，下部為厚層硬頁岩，震測顯示基盤內部之白冷層呈現兩 背斜一向斜構造，基盤面最深在白冷層地層背斜處，推論為基盤地層受侵 蝕時，到下部硬頁岩切深加快所致。另外，在東側測得地利斷層的明顯證 據。盆地內的P 波速度在沈積層為3000m/s，在基盤為4500m/s，S 波速度 則分別為1600m/s 及2700m/s。推估埔里盆地基盤為東西擠壓及南北擠壓所 共同造成的曲漥作用所形成，也影響到下部第四紀地層下彎；但最近數萬 年來，活動趨緩，僅在地表堆積水平200m 厚的礫石層。 Reflection Seismic Studies in the Puli Basin Abstract The Taiwan island is located at the convergent boundary between the Eurasian plate and the Philippine sea plate. Due to plate collision, major compressions build up high mountains. However, in this compression dominated environment, there exist several basins in the southern portion of the Hsuehshan Range. How are these basins created in the mountain? What are their structures? How to integrate the basin structure with the periphery mountain orogeny? All these are important, challenging and interesting geological problems of Taiwan tectonics. In this study, we use the high-resolution shallow seismic reflection to explore the Puli basin. Two kinds of seismic sources are adopted: 1）’MiniVibe’: three long lines (each about 1~3 km) with MiniVibe-sources are obtained to exhibit the basic feature of the Tertiary basement; 2) ‘EWG-III’: ten groups of short lines (each contains two crossing lines with 700m long) are used to describe the 3D structure distribution. In addition, the data from a wide-angle refraction survey carried out in 2002 is also involved to set up constrains of the basin structure. The purposes are to find structures of the Quaternary stratum and the Tertiary basement and to investigate the mechanisms which generate the basin. The results show that the basement shape of Puli basin is like a shallow disc. The deepest part has a depth of about 720m near the center with a little shift to the west. The Quaternary formations above the basement contain two layers: a 200m-thick top flat layer dominated by gravels and a deeper 400m-thick curved layer with gravels interlaced with mud and sand. The deep gravel-mud-sand layer forms an angular unconformity with the top flat layer. The similar curved shape of this layer with the basement implies a synchronic structural movement. The basement belongs to the Baileng formation which has a Quartzitic-feldspathic sandstone at the top and a heavy argillite in the lower part. The reflection profiles demonstrate that the Baileng basement possesses two anticlines and one syncline. Moreover, no apparent evidences show traces of faults inside the basin except those on two sides such as the Deli fault which is quite clear in the seismic section. The P wave velocity is found to be 3000m/s in the top deposit layers and 4500m/s for the basement. The S wave velocities are 1600 m/s and 2700 m/s, respectively. It is inferred that the Puli basin is formed by the ‘down warping’ mechanism due to the compressions from both the west-east and north-south directions. In the later several ten thousand years, the tectonic movement may be ceased and a 200m flat layer is deposited at the top.
