由海底地震儀資料探討南海北坡被動大陸邊緣地殼速度構造

Crustal Velocity Structure of the Northern South China Sea Passive Continental Margin from Ocean-Bottom Seismometer Data

陳紀辛(Ji-Xin Chen)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.01825

摘要

本研究使用2012年5月海研一號研究船（R/V Ocean Researcher 1）999航次一條在南海北坡通過大陸棚、大陸斜坡炸測的多頻道反射震測剖面與沿測線佈放的海底地震儀資料，探討南海東北部大陸邊緣的地殼構造。由海底地震儀資料建立的速度模型顯示莫荷面深度的變化：大陸棚下方地殼厚度約20公里深，較典型大陸地殼薄，速度構造特徵較接近大陸地殼速度，推測可能是被拉張變薄的大陸地殼構造。大陸斜坡處地殼厚度減薄為13公里厚，速度模型中發現有許多向陸方向傾斜的低速帶，對應多頻道震測剖面有類似向陸傾斜方向的正斷層特徵以及基盤向海方向傾斜的地層反射界面，而速度構造顯示此區接近基盤的沈積物速度變化較大，與火山型被動大陸邊的特徵相似；速度模型中下部地殼的速度有異常高速層的出現，地殼速度介於大陸地殼與海洋地殼速度之間，但較接近大陸地殼速度性質。在下部大陸斜坡到海洋盆地之間，莫荷面深度向海方向漸淺，發現有些許向陸方向傾斜的正斷層，速度構造顯示較接近海洋地殼速度。推測大陸棚下方的地殼可能是大陸地殼，直到大陸斜坡轉變為大陸地殼性質的過渡帶，下部大陸斜坡向海方向為海洋地殼性質的過渡帶。火成活動集中在南海北坡較南邊的區域，有向北地殼厚度漸薄的趨勢，顯示南海北坡可能歷經過初期的拉張，且拉張應力方向不同，可能是在擴張時期的拉張地函對流岩漿對大陸地殼的移除作用，導致地殼變薄。

關鍵字

海底地震儀 ； 被動大陸邊緣 ； 向海傾斜強反射 ； 過渡帶 ； 波線追跡 ； 高速層 ； 低速區

並列摘要

This study uses a multichannel seismic（MCS） and combined ocean bottom seismometer（OBS） profile data to investigate crustal structures of the passive continental margin in the northern South China Sea. The profile runs in a NW-SE direction across the continental shelf and continental slope in northeast South China Sea. The velocity model derived from the OBS data shows depth variation of Moho: Crustal thickness beneath the continental shelf is about 20 km deep, which is thinner than the typical continental crust. Velocity structure shows characters are closer to that of the continental crustal, thus the continental shelf is underlain by stretched and thinned continental crust. Crust thins from continental shelf to continental slope, where the crustal thickness is 13 km. We found several landward dipping low velocity zones in the upper crust of the velocity model, which correspond to landward dipping normal faults, while SDRs（sea dipping reflectors） are observed in basement on the multi-channel seismic reflection profile. Seismic velocity varies a lot in the lower part of sediment layer near the basement, characteristic of volcanic passive continental margin. A high velocity layer is observed in the lower crust, and the characters of the velocity profile fall between that of typical continental crust and typical oceanic crust, but are more similar to continental crust. Moho depth becomes shallower in lower continental slope toward oceanic basin. Based on the crustal velocity model constructed, we suggest that the continental crust lies beneath the continental shelf, thinned and stretched continental transitional crust lies beneath the continental slope, and transitional oceanic crust lies below the continental slope and oceanic basin. Igneous activities are observed in the southern part of the study area and the crustal thickness thins northward there, suggesting that northeast SCS experienced extension during the initial opening phase, and probably there were stresses developed in different directions. Crustal thinning may be caused by mantle convective removal of continental crust before seafloor spreading developed in northeast SCS.

並列關鍵字

OBS ； passive continental margin ； SDR ； ray-tracing ； HVZ ； Low velocity section

參考文獻

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