台南外海淺水區之地形特徵

Abstract

變形前緣是一種地質上的分界線，在台灣，它區分了兩種受到不同應力作用的地質區。變形前緣線以西的地區受到伸張應力的影響；而在變形前緣線以東的地區則是受到擠壓應力的作用。所謂的變形前緣，其定義為在台灣的造山帶中，屬於西部麓山帶的山腳線，即是一條造山運動中最西部的逆衝斷層線。在台灣造山運動組成的逆衝斷層構造帶中，由最高的山脈往西，出現數條南北向的逆衝斷層，這些斷層越往西部越年輕，斷層面傾角越小，活動力越高，斷層錯動所產生的災害也越大，因此西部麓山帶會有周期性的地震災害產生，了解活斷層的位置對於日後台灣因應地震災害方面會有重大的幫助。根據前人研究，台灣西南外海的構造分佈，將台南出海的構造線連接到自馬尼拉海溝向北延伸的變形前緣中，但在目前尚缺乏解析度較高的台灣西南地區近岸處的地球物理資料情況之下，對於結果還需要更嚴謹的討論。 在本研究中，我們想要對於前人所描述的台南外海變形前緣位置進行一次地球物理探勘來確認其位置。藉由高解析度的多音束測深系統我們可以初步得知此地區的地質構造分布，再利用3.5kHz震測剖面的資料來了解該地區的地下構造，辨識出兩側受不同應力影響所產生的構造上相異的表現。我們在研究區的西北方觀察到到了正斷層的序列，而在其東南方則是辨識出受到應力擠壓所產生的地層彎曲之現象，將此分界的點連結在一起，似乎不屬於前人所定義之變形前緣構造線的位置。

In geological point of view, deformation front separates two different regions received unlike stress. In Taiwan, the region west of the deformation front is impacted by extensional force. In the other hand, east of the deformation front is impacted by contractional force. The deformation front is defined as the frontal boundary of Taiwan western foothills. In other words, it is the westernmost limit thrust fault in Taiwan orogenic belt. According to the previous studies, they connect the extension line of Manila Trench north forward until Tainan offshore, then toward Taiwan. But now, there is lack for high-resolution nearshore data. Therefore, we need to be more rigorous discussion of the result. This study investigated area is located in Tainan offshore, modern topographic map cannot be seen any deformation front geomorphological features in the region. This is because of more rugged terrain outside the edge of the continental shelf. And on the shelf due to the lack of detailed bathymetric data drawn topographic map shows the regional sea-bed water depth of 140 meters or less, is very flat. However, some unpublished data shows that can be observed in the nearshore seabed some obvious linear collapse with the seabed, can also see local small cone topography, seismic profiles can also be observed in the formation, there are some horizontal movement of the deposited layer of the high water content, these the small tapered terrain is the product of these high water content of sediment removed upwardly. Therefore, we can assert that, in the the Tainan coast of the sea bed, is under the influence of forces, these forces will affect the very shallow surface of the sea-bed sediments, easily can be observed from the stress state of the sea bed sedimentsout whether there is a so-called fault deformation front after.