南沙群島鄭和環礁北邊海域的地殼特徵

Abstract

本研究使用ORI-1068航次收集來的一條約217公里長的多頻道反射震測剖面，搭配重力與磁力資料等，來探討南海南沙群島鄭和環礁北邊海域的地殼特徵。震測剖面位於鄭和-道明海槽之中，由西南到東北大致分成海山區、太平島北邊海域以及火成岩侵入體區。海山及火成岩侵入體侵入體擾動海床沉積物。另外，在太平島北邊海域觀察到碳酸鹽平台邊緣強振幅與弱振幅反射重複交互出現的反射特徵。衛星重力反演的Moho深度大約在13-19公里之間，表現出拉薄的大陸地殼特徵，重力模型的下部地殼模擬出2.97 g/cm3的高密度層。磁力異常在海山及侵入體區呈現80-160 nT的高值，在太平島北邊海域為-35到-180 nT的低值。磁力模型在海山區以及火成岩侵入體區也顯現較高的磁感率，在太平島北邊海域呈現低磁感率。 地殼下部模擬出高密度層，可能是底侵作用所造成的。再者參考ODP Site 1143鑽井資料認為火山及火成岩侵入體不是在張裂時期生成的，也沒有發現到同張裂火山流造成的向海傾斜的反射特徵（Seaward Dipping Reflector；SDR）。本研究認為鄭和環礁區域不能以二分法區分為火山型或岩漿缺乏型邊緣，而是屬於中間型的被動大陸邊緣。 另外鄭和環礁構造走向呈東北－西南向，與南海西南次海盆張裂方向垂直，再加上太平島北邊海域表現為花崗岩的磁感率，以及沒觀察到類似火山及火成岩體的反射特徵，本研究認為鄭和環礁可能是建立在張裂時期造成的大陸地殼斷塊（fault block）之上，而非建立在火山的高區之上。

In this study, we analyzed multichannel reflection seismic profile, gravity and magnetic data along a 217 km long line, to discuss the crustal characters in the northern offshore area of the Chenho Reefs in southern South China Sea. The roughly SW-NE trending seismic profile lies in the Chenho-Daoming trough north of the Chenho Reefs. We divide the seismic profile into three sections from southwest to northeast: seamount area, northern offshore of the Taiping Island and igneous intrusion area. All the seamounts and igneous intrusions disturbed the shallow sediments beneath the seafloor. The seismic reflection images in the northern offshore of the Taiping Island are characterized by several sets of strong reflections with amplitudes dropping off westward. These reflections show reciprocal stacking patterns which are typical seismic expressions at the edge of carbonate platforms. The Moho depth is about 13-19 km by inversion of satellite gravity data, suggesting a thinned continental crust. Gravity modeling reveals that a high density layer（2.97 g/cm3）lies in the lower crust. The magnetic anomalies in the seamount area and igneous intrusions area show high values about 80 to 160 nT, whereas -35 to -180 nT in the northern offshore of the Taiping Island. Magnetic modeling also reveals that the magnetic susceptibilities of the seamount area and igneous intrusion area are higher than that of the northern offshore area of the Taiping Island. In this study, a high density layer which may be caused by underplating is simulated in the lower crust. Referring to ODP Site 1143, we suggest that the volcanoes and the igneous intrusions were not formed during the rifting but were developed after the rifting stage. In this study, we propose that an intermediate mode of rifting to fit characteristics of the Chenho Reefs, which is different from either volcanic margin or magma-poor margin. The structural trend of the Chenho Reefs axis is perpendicular to the rifting axis of the Southwest Subbasin in the South China Sea. Moreover, the magnetic susceptibility of the crustal material in the northern offshore area of the Taiping Islands suggest a granite basement. The seismic features of seamounts and igneous intrusions do not appear in the northern offshore area of the Chenho Reefs. We propose that the basement beneath the Chenho Reefs should be a continental fault block caused by the rifting of the South China Sea, rather than volcanic seamounts.