| 摘要: | 台北盆地位處台灣北部，為一年輕的構造盆地，過去十餘年來，中央地調所利用鑽井資料做了許多的研究，本小組亦利用淺層反射震測結合鑽井資料，描繪了盆地內淺部第四紀地層的基本形貌，其下的第三紀基盤面也有初步的認識。台北盆地本身屬於一個半地塹盆地，在基盤面較深的西北地區，過去本小組使用EWG-Ⅲ撞擊式震源得到之震測剖面，對較深的基盤面無法清楚掌握，本研究利用新購的MiniVibe震盪震源車，獲取數公里長的1秒連續剖面，該震源能量大頻域寬，穿深能力較佳，可以有效偵測基盤面深度及各地層。 本研究的測線集中在盆地西北部分，包括3條短測線及3條長測線：關渡地區測線（GuanDu）長度約2公里，使用MiniVibe施測，利用關渡一號井以及剖面中反射面的交角不整合，將基盤面定出，其深度與過去利用EWG-Ⅲ所得到深度相當，但底下的第三紀地層在新剖面可以清楚辨識。五股蘆洲地區，蘆洲變電所周圍的4條短測線(LR1-LR3)為EWG-Ⅲ施測所得，信號清晰，可看出數層的第四紀地層。疏洪道測區環境空曠，利用MiniVibe 獲得一條品質極佳的7公里連續1秒之剖面（SuHon），經與近鄰的五股一號比對井，可清楚的判識基盤深度及各地層細部構造，此剖面為到目前為止，台北盆地所獲最為完整的連續震測剖面。另外，在華中橋之長測線(2公里，HuaChung)則看到明顯的不整合面及第三紀基盤之構造。 本研究獲得以下成果：1）確定台北盆地基盤深度最深不超過700公尺（五股一號井WK-1基盤深度679m)，在蘆洲五股地區維持一大範圍的600公尺深度；2）山腳斷層為正斷層，斷層上盤可看到明顯的盆地陷落所造成的地層拖曳，斷層傾角約50?60度；3）盆地底下的崁腳斷層造成基盤面深度急劇變化，斷層西北側形成傾角約10度的基盤斜面，寬約700公尺；4）基盤面以上的第四紀地層，不同年代有不同傾角：板橋層（40?25萬年前沈積）傾角6度、五股層（25?10萬年前）傾角3度、景美層（10?3萬年前）則近乎水平，此意味台北盆地形成之早期，斷層活動極為活躍，使盆地不斷陷落，但10萬年來則趨於平緩，而今日之地表地形亦反映地下構造變化，代表盆地仍處於活動階段。5）基盤面以下的第三紀地層大致向東南傾斜，傾角20?30度。在崁腳斷層兩側地層有激烈變化，東南側上盤之五指山層逆衝至下盤的大寮層及石底層之上，並在上盤地層造成背斜褶皺，該背斜向西南延伸即為出露地表的山仔腳背斜。6）基盤地層形成盆地陷落前的原始地形面，故其岩性亦影響基盤面的深度變化，且以崁腳斷層為分隔界線，西北深東南淺。盆地陷落過程，主要改變盆地較深的西北部分之地層，顯示山腳正斷層影響範圍比先前預測的為小。 Taipei basin is located in the northern Taiwan. It is a young tectonic basin. Over the past ten years, the Central Geological Survey (CGS) has conducted numerous investigations by drilling over 20 boreholes. Our group has also used the shallow reflection seismic method, combining with the borehole data, to describe the subsurface Quaternary structures and the Tertiary basement variation of the basin. In this study, we used the newly purchased MiniVibe sweep source to obtain an one-second profile with several kilometers long. The sweep source can generate signals with large energy, wide frequency range, and good penetration. Therefore, we can explore the basement depth and individual strata efficiently. The survey lines are concentrated on the northwestern basin, including four short and three long survey lines. The GuanDu line is about 2km long and obtained by the MiniVibe source. Utilizing KT-1 well and the angular unconformity on the profile, we can decide the basement structure. The depth matched with the profile obtained by the EWG-Ⅲ in the past, and the Tertiary strata below the basement can be well recognized in the new profile. In the WuKu-LuChow region, four short profiles were surveyed by the EWG-Ⅲ. The signal of these profiles is distinct and can be used to identify the Quaternary strata. In this open-space area, we also obtain a very long line by using the MiniVibe source. This long seismic profile has superior signals, which might be the most outstanding profile obtained in the Taipei basin up to now. This Suhon seismic line is 7 km long, which shows quite obvious basement variation and detailed Quaternary layers. In addition, the long profile (HuaChung, 2km in length) of HuaChung Bridge also shows distinct unconformity and Tertiary structures. This investigation obtains the following results: 1) The deepest basement was confirmed in the Taipei basin not over 700m. (The basement depth is 679m in WK-1.) The basement depth in the Wuku-LuChow region keeps around 600m in a wide range. 2) The SanChiao fault is a normal fault and the hanging wall shows distinct drags caused by the basin depression. The dip angle of the fault is about 50-60 degree. 3) The KanChiao fault obviously affects the basement shape. It forms an 10-degree slope of the basement surface, spanning in a range of 700m. 4) The dip angles of Quaternary strata indicate that the faulting was active in the early stage and become less active in the past 10Ma. 5) The lithological characters of diverse strata below the basement influence the basement depth variation, and the KanChiao fault under the basin also affects the basement relief. |
