臺灣西南海域南海棚坡與高屏棚坡上海底峽谷形貌之比較

Geomorphic comparison of submarine canyons between South China Sea and Kaoping shelf/slope regions offshore southwestern Taiwan

吳依璇(Yi-Hsuan Wu)

指導教授 ： 俞何興

國立臺灣大學/理學院/海洋研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.00963

摘要

在南海最北部，南海棚坡區與高屏棚坡區在此並列成一向南傾斜的盆地，在南海棚坡區和高屏棚坡區之間約以澎湖海底峽谷為界。本研究利用台灣大學海洋資料庫釋出的台灣周圍海域海底地形的資料，辨認出南海棚坡區之海底峽谷形貌，除原本以辨認出的福爾摩沙海底峽谷外，另辨認出12條海底峽谷，在高屏棚坡區除原本已辨認出的壽山、高雄、高屏和枋寮海底峽谷，另辨認出兩條海底峽谷。本研究藉比較台灣西南海域之南海棚坡區與高屏棚坡區的海底峽谷形貌，以瞭解海底峽谷形貌於南海棚坡區與高屏棚坡區之差異，增進瞭解被動與活動大陸邊緣可能對海底峽谷造成之影響，補充台灣西南海域之區域資料，以改善Harris and Whiteway (2011)對於全世界於活動和被動大陸邊緣上海底峽谷形貌的形貌差異比較之研究完整性。
藉海底地形資料找出地形略有起伏的範圍、位置，根據前人對於海底峽谷之定義找出橫剖面呈V形之連續凹陷谷地，並訂定起始點之谷壁起伏須超過50公尺，V形形貌須延續超過1公里的谷地形貌為海底峽谷。由此條件辨認出14條海底峽谷，這些峽谷的谷壁起伏可高達數百公尺，並呈V形形貌，符合前人對於海底峽谷之定義，並以英文字母A至X命名。
整理臺灣西南海域20條海底峽谷形貌差異：發育於南海棚坡區之海底峽谷頭部位置多位於上部陸坡；發育於高屏棚坡區之海底峽谷頭部位置除位於陸坡上外，也具有頭部嵌入大陸棚和頭部與陸上河流相接者；南海棚坡區之海底峽谷間距較緊密，約7公里；高屏棚坡區之海底峽谷間距較寬鬆，約20公里。比較南海和高屏陸坡的縱剖面，發現於水深300至2000公尺之間，南海陸坡坡度(約3°)較高屏(約1.5°)為陡，可能是受陸坡坡度影響，使得坡度較陡的南海棚坡區發育出的海底峽谷數量較多，峽谷間距較緊密。峽谷頭部發育位置的差異可能是因為在南海棚坡區受到構造擠壓抬升影響程度較低，峽谷發育較為緩慢，在高屏棚坡區受到構造擠壓抬升影響程度較高，邊坡較不穩定而崩塌，峽谷發育較為快速，峽谷頭部逐漸向陸遷移，甚至嵌入大陸棚或與陸上河流相接。
由澎湖、高屏和福爾摩沙海底峽谷之縱剖面和標準化剖面形貌相比較，高屏海底峽谷之縱剖面較呈直線形，澎湖和福爾摩沙海底峽谷較呈凹形。高屏海底峽谷整體縱剖面呈一直線形，可能受到地體構造的擠壓抬升影響程度較高，較接近上凸型(convex)的峽谷縱剖面。福爾摩沙海底峽谷上段縱剖面與陸坡縱剖面相類似，整體縱剖面呈一凹形，可能主要沿陸坡原有形貌發育，較接近輕微下凹(slightly concave)的峽谷縱剖面。澎湖海底峽谷上段縱剖面介於高屏海底峽谷和福爾摩沙海底峽谷縱剖面之間，但於下段卻較福爾摩沙海底峽谷縱剖面為下凹，整體縱剖面呈一凹形，可能主要沿陸坡原有形貌發育，較接近輕微下凹(slightly concave)的峽谷縱剖面。比較南海和高屏棚坡區之小型峽谷縱剖面，在南海棚坡區的小型峽谷的縱剖面大多與南海陸坡縱剖面相類似，這些峽谷可能主要沿陸坡原有形貌發育，與福爾摩沙海底峽谷的縱剖面類型相似，較接近輕微下凹(slightly cancave)型；高屏棚坡區的小型峽谷的縱剖面大多呈直線形，與高屏海底峽谷上段的縱剖面相類似，且小型峽谷平均縱剖面下段無明顯坡度減緩的趨勢，可能受到地體構造擠壓抬升影響程度較高，較接近上凸(convex)型的縱剖面。
南海棚坡區的海底峽谷形貌主要受原有的陸坡形貌影響較大，而高屏棚坡區的海底峽谷形貌主要受地體構造擠壓抬升影響程度較高，分別可反映出被動大陸邊緣和活動大陸邊緣上海底峽谷發育的形貌特徵。

關鍵字

海底峽谷 ； 形貌比較 ； 大陸邊緣 ； 臺灣西南海域

並列摘要

In the northernmost South China Sea (SCS), two facing margins of SCS shelf/slope and Kaoping shelf/slope are juxtaposed forming a triangle-shaped basin with an open end to the south. Along the basin axis, the Penghu canyon separates these two juxtaposed shelves/slopes.
Examing bathymetric data from deep-sea region off southwestern of Taiwan, we define submarine canyons using criteria such as relief of canyons over 50m at head and the V-shaped cross-section. This study identified twelve new canyons named by A through V on SCS slope and two new canyons named as W and X on Kaoping shelf/slope. Comparing morphology of canyons on SCS shelf/slope and Kaoping shelf/slope, we try to understand the difference in morphology between canyons on these two regions, which are passive and active margins, respectively. Through this study, we wish to improve the completeness of study of Harris and Whiteway (2011) with comparison of geomorphic differences in canyons on passive and active margins.
Differences between canyons in these two regions are as following. On SCS shelf/slope, canyon heads most developed on upper slope with closer spacing. On Kaoping shelf/slope, canyon heads not only developed on slope, but also on shelf or head connecting the river with wider spacing. By comparing the longitudinal profiles of SCS and kaoping slope, we found SCS slope is steeper than that of kaoping between 300m~2000m in water depth. The greater numbers of canyons with closer spacing on SCS slope may be related to the steeper slope. Shelf-indenting and river-connecting canyons on Kaoping shelf/slope may be influenced by tectonic uplifting. Frequent slumping may happen on Kaoping slope to help the development of canyons. Canyon heads may shift up slope and then indent the shelf or connect the rivers.
Comparisons between the longitudinal profiles of Penghu, Kaoping, and Formosa canyons show that the profile of Kaoping canyon being a linear to slightly convex. It is may be affected by tectonic uplifting. The Penghu and Formosa canyons are characterized by longitudinal profile with shape of concave to slightly concave pattern. They developed following the slope gradient. The upper profile of Penghu canyon may be affected by tectonic uplifting, making the profile steeper upward. Comparisons between longitudinal profiles of small canyons on SCS and KP shelves/slopes show that profiles of canyon on SCS shelf/slope resemble the profile of SCS slope, with shapes of slightly concave pattern, and are like the profile pattern of Formosa canyon. The profiles of canyon on Kaoping shelf/slope show linear to slightly convex shape, which may be affected by tectonic uplifting and are like the profile of Kaoping canyon.
Geomorphic characteristics of canyons on SCS shelf/slope may be affected by morphology of inherited slope. The morphology of canyons on the Kaoping shelf/slope are mainly controlled by regional tectonics. Morphological contrasts between canyons on the SCS shelf/slope and Kaoping shelf/slope may be related to the characters of these two passive and active margins, respectively.

並列關鍵字

submarine canyon ； geomorphic comparison ； continental margin ； offshore southwestern Taiwan

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