| 摘要: | 呂宋島位於歐亞板塊與菲律賓海板塊之間的聚合帶上，歐亞板塊沿馬尼拉海溝向東隱沒，形成馬尼拉隱沒帶系統。該系統所對應的弧前盆地受陸上活動斷層及海山隱沒作用等構造因素影響，呈現複雜的地殼變形特徵。在北緯 16° 至 18° 的弧前盆地內，有美岸高區（Vigan High）與史都華灘（Stewart Bank）兩處顯著的海域地形隆起，是本研究的主要探討區域。為了瞭解該區的構造演化，本研究整合分析多項地球物理資料，包括 2022 與 2023 年由勵進調查船（R/V Legend）取得的空氣槍多頻道反射震測（MCS）、電火花高解析反射震測（Sparker）與多音束海底地形資料，及2014 年海研五號（R/V Ocean Researcher 5）所取得之空氣槍多頻道反射震測剖面。
根據研究區域所觀察到的構造特徵，可初步區分為兩大區域。一、美岸高區以逆斷層與走向滑移斷層為主，顯示受壓與剪動作用控制。美岸高區由一系列的逆斷層與走向滑移斷層構成，斷層系統幾何特徵呈現典型的突起構造（pop-up structure），推測可能為走向滑移斷層於斷層彎曲處（restraining bend）所產生之擠壓帶。從斷層走向來看，可能為菲律賓斷層系統於海域的延伸部分。二、史都華灘則主要由正斷層發育，顯示受拉張等機制主導。史都華灘的震測資料及水深資料顯示該區具有陡峭的斜坡與局部隆起的構造，並伴隨多組分布於斜坡且向兩側傾斜的正斷層。從地形上來看，推測可能是海山隱沒至增積岩體之下所引發的構造變形與南海中洋脊隱沒有關。
整體而言，美岸高區和史都華灘之構造型態對比顯著，顯示馬尼拉弧前盆地在不同區段會受到不同的應力影響，形成不同的構造演化歷程。
;Luzon Island is located along the plate boundary between the Eurasian Plate and the Philippine Sea Plate. In the west of Luzon Island, the South China Sea lithosphere has been subducting eastward beneath the Philippine Sea Plate along the Manila Trench, forming the Manila subduction system. The forearc area is tectonically complex and highly active, and this study focuses on onshore active faults and seamount subduction. Based on the bathymetry, two prominent structural highs, Vigan High and Stewart Bank, are observed between 16° and 18°N.
To understand the geological evolution and deformation patterns, this study analyzes multiple geophysical datasets, including air-gun multichannel seismic reflection profiles, high-resolution sparker seismic data, and multibeam bathymetric data. These datasets were collected by the R/V Legend in 2022 and 2023, and by the R/V Ocean Researcher 5 in 2014.
Based on the seismic profiles, the study area can be divided into two tectonic domains. First, the Vigan High is primarily characterized by thrust and strike-slip faults, which forms a pop-up structure. The deformation is controlled by compressional and shear forces. This deformation is interpreted as a restraining bend developed along a bending segment of a strike-slip fault, possibly an offshore extension of the Philippine Fault System. Second, the Stewart Bank is dominated by normal faulting, indicative of extensional deformation.
Numerous outward-dipping normal faults are distributed along the slopes. Based on the bathymetry, those normal faults are likely produced by the subduction of seamounts beneath the accretionary wedge. This may be related to the subduction of the South China Sea extinct mid-ocean ridge.
Overall, the contrasting structural characteristics between the Vigan High and the Stewart Bank suggest that different segments of the Manila forearc basin are affected by distinct stress regimes. This indicates a heterogeneous stress field and a complex tectonic evolution in the Manila forearc, at least between 16° and 18°N. |
