| 摘要: | 南中國海（South China Sea）位處東亞區域，為西太平洋最大的邊緣海之一，其東界為馬尼拉海溝，西、南、北界皆為歐亞大陸板塊所圍繞，而其南北緣則屬於被動大陸邊緣（passive continental margin）的架構。根據Tapponnier等人提出的新生代東亞地體構造運動模型，因為印度板塊向北撞擊歐亞大陸板塊造成中南半島向東南脫出及紅河斷裂帶的形成導致了南海的南北向張裂。南海海洋地殼磁力條帶的定年研究指出南海張裂起始約在37個百萬年前，結束於15個百萬年前。 本研究以高解析多音束（multibeam）水深為主，配合淺部地層剖面資料為輔，針對南海北坡的被動大陸邊緣區域進行了較詳細的地球物理調查與分析。其中在海床面上發現了許多大陸邊緣海床特徵，如大陸斜坡上的海底峽谷（submarine canyon）、火成物質形成的條狀地形高區、海底火山群、海床的侵蝕崩塌構造（slump structure）、大規模的海底山崩（submarine landslide）等。條狀火成地形高區與海底火山群分別位於大陸斜坡坡腳及東沙島一帶，經由震測資料與前人研究的結果比對後，我們發現部份火成地形高區是沿著張裂構造侵入的入侵火成岩體，因此其走向與張裂的正斷層構造平行，另一部份的火成地形高區其走向則與張裂構造不一致，因此我們認為條狀火成地形高區乃是海洋地殼張裂後的火成入侵作用所形成；而海底火山群我們則認為其形成與區域的火成活動造成的地殼抬昇有關， 可能為東沙隆起時期的產物（~ 5 Ma）。另外海床上的侵蝕崩塌構造與海底山 崩其成因為海底峽谷的侵蝕作用及斜坡上的重力作用。 地下剖面的部份，在淺層底質剖面（sub-bottom profile）中我們發現了廣泛分布於淺層沉積物中的聲波透明帶層（acoustic transparent layer），其分布位置皆位於地形坡度由高坡度急遽轉變為低坡度處如大陸斜坡坡底與海底峽谷底部，根據對其分布位置以及沉積特性的分析，我們認為其成因為海底碎屑流（debris flow）的堆積。震測剖面的部份我們使用了九條單頻道震測剖面，來自美國國家地球物理資料庫NGDC，以及引用前人研究所發表的20條多頻道震側剖面，除了比對地表特徵外，我們也利用了這些剖面重新定義了南海北坡的基盤面深度及沉積物厚度，將兩者與當地構造比對後皆有不錯的吻合度。 在東沙隆起的西南緣大陸斜坡，其海床上我們發現了一系列的串狀麻坑構造（pockmark structure），其外型之巨大為世所罕見（直徑數百至千餘公尺）。麻坑所在的區域位於中國大陸南海天然氣開發的重點海域『神狐區』，根據中國大陸的鑽井研究指出這一帶的海底地層蘊藏著豐富的天然氣水合物資源，因此我們判斷研究區域內的麻坑構造其形成原因與地層中的游離天然氣向上移棲而破壞海床面有關。另外我們將麻坑的大小走向與麻坑所在區域的地形參數相互比對，發現多數的麻坑構造其形貌受到地形參數如坡度及斜坡走向影響，可能與大陸斜坡上的侵蝕滑陷（slump）活動有關，當斜坡發生侵蝕滑陷活動時，地層中的游離天然氣層其上方荷重壓力隨即改變，游離天然氣因此沿著荷重壓力最小處（海床上的侵蝕滑陷痕跡）向上移棲破壞，形成了大規模的串狀麻坑，且其外型與分布位置與侵蝕滑陷構造一致，此一發現也間接證實了南海北坡區域所蘊藏的天然氣能源潛力。 South China Sea (SCS) is located in the East Asia, which is one of marginal seas in western Pacific. The northern and southern margin belong to passive continental margin and the eastern boundary of SCS is located along the Manila Trench. According to Tapponnier et al., the collision of Indian Plate and the Eurasia Plate was started in early-middle Cenozoic and caused the southeastward escape of Indochina block, eastern escape of China block and formed the left-lateral Red River Fault zone. The block and fault movements could cause the spreading of SCS. Based on the result of magnetic lineation dating, SCS started spreading in 37 Ma and stopped in 15 Ma. The major method of this study is using multibeam bathymetry to identify the detailed seabed features on the northern margin of SCS. The seabed features including submarine canyons, lineate topographic highs, submarine volcanoes, slumping structures and submarine landslides. The lineate topographic highs are located near foot of the continental slope. Comparing seismic profiles with the past studies, we believe the volcanism highs were formed by the post-rifting mantle magma intrusion. The grouping volcanoes are locating at the Dongsha Rise area. Dongsha Rise movement was a zone of igneous activity, the igneous activity made the crust uplift and created submarine volcanoes in this area. We propose that the volcanoes were formed in the same igneous activity with Dongsha Rise (~ 5 Ma). The submarine canyon erosion formed the slump structures on the continental slope. Based on sub-bottom profiles, we have several acoustic transparent zones (ATZ) in the shallow sediment in our study area. All the ATZ are located in three places: continental slope foots, channel of submarine canyons and foot around the volcanism highs. We believe that the deposition of debris flow can account for the ATZ layers. We have used 9 single-channel seismic profiles (SCS) from NGDC (National Geophysical Data Center) and 20 multi-channel seismic profiles (MCS) in order to compare with bathymetry and the structures. In the other hand, we made newly sediment thickness and basement depth distribution figures by these seismic profiles. In the southwestern slope of Dongsha Rise, we have found a lot of lineate pockmarks. The sizes of pockmarks are variant, about hundreds to thousand meters in diameter. According the drilling studies, this area could have a rich potential of gas hydrate. Hence, we suggest that the pockmarks are related to the free gases. By calculating the parameters of pockmarks shape and bathymetry, we noticed that the slope gradient and contour trends of bathymetry control the size and the trend of most of pockmarks. Besides, there are many slumpings and two landslide structures in the same area. We suggest that the slumping structures may reduce the weighting of the underneath gas-hydrate, and therefore release the gas. The upward migration of free gas may finally form the pockmark structures on the seafloor. |
