| 摘要: | 呂宋島到臺灣之間為聚合板塊帶的過渡區域，由南往北馬尼拉隱沒帶的正常隱沒環境已逐漸過渡為臺灣島的初始碰撞造山環境。為了更瞭解此區域的板塊聚合特徵的變化，本研究主要利用2009年TAIGER計畫由R/V Marcus G. Langseth研究船所收集的跨越馬尼拉海溝的多頻道反射震測和沿測線的多波數水深資料，以及R/V Ocean Researcher I的震測資料，探討呂宋到臺灣（18°45’N~21°45’N）海域在構造和沈積上的特徵與變化。 研究結果顯示有一組明顯的半遠洋沈積層序大規模分佈在南海海盆的頂層以及海溝中；在南段（18°45’N~20°N）的海溝區，此頂層層序之上覆蓋較厚的海溝濁流沈積物，但在北部頂層則呈被侵蝕的特徵。在南段的震測剖面（18°45’N~20°N）具有單純的板塊隱沒特徵，在海溝的前端先出現正斷層，甚至有切穿基盤面與海床面的大型正斷層。之後隨著板塊聚合，地層開始受擠壓而變形，越往島弧方向擠壓作用越劇烈。根據構造特徵，由西向東可依序將海溝區及楔形體畫分出六個子區域：正斷層區（Normal Fault Zone）、原逆衝斷層區（Proto-Thrust Zone, PTZ）、疊瓦狀逆衝斷層區（Imbricate Thrust Zone, ITZ）、前端脫序逆衝斷層區（Frontal Out-of-sequence thrust Zone, FOZ）、上部斜坡區（Upper Slope Zone）和背衝區（Back Zone）。海溝與基盤面的隱沒特徵在20.5°N以北變得不明顯；滑脫面構造也不明顯，取而代之的是海溝與楔形體底下出現掩埋的海底山和基盤面高區，增加隱沒的困難度。同時，構造楔形體內的PTZ寬度逐漸變窄，ITZ、FOZ往北變寬廣，而增積岩體的特色也由隱沒擦刮（offscarp）的特性，逐漸轉變為前陸盆地的沈積特色，覆蓋由臺灣造山帶而來的沈積物，這些現象顯示往北碰撞作用加劇。 隨著菲律賓海板塊西北向的聚合，北呂宋海槽的西緣都有地層被擠壓而抬昇（uplift）的現象，但東側的地層都很平靜；最南端的測線（~18°45’N）海槽西側的抬昇現象反應此處為正向聚合，但19°15’N以北海槽西側還有隆起（pop-up）及向西延（west-vergent）的逆衝斷層，顯示19°15’N以北北呂宋海槽西緣應受到斜向聚合的影響而同時有平移和壓縮的作用。由北呂宋海槽內地層變形的位置，顯示平移和壓縮作用產生的變形主要發生在北呂宋海槽西緣和構造楔形體東緣的交接處，而不是海槽東緣和呂宋島弧交接處。呂宋到臺灣之間北呂宋海槽西側的聚合為平移壓縮（transpression）的應力，此種平移壓縮的聚合應力系統應該可以沿北呂宋海槽的西緣向北延伸至臺灣島的花東縱谷位置。 The Eurasian plate (EP) and Philippine Sea plate (PSP) convergence in the region between Luzon and Taiwan islands gradually evolves from normal subduction to initial collision. In order to better understand the structural characteristics of the subduction-collision transition between Taiwan and Luzon islands, we have analyzed the multichannel reflection seismic data and multibeam bathymetry data collected by R/V Marcus G. Langseth in 2009 and by R/V Ocean Researcher I in 2003. The results indicate a clear hemipelagic seismic sequence in the South China Sea (SCS) and Manila Trench. This sequence is dipping eastwards and is covered by thick trench-fill sediments in the south of the trench but has been eroded in the north. Structural analysis shows that the Manila Trench area can be distinguished by six structural zones: a normal fault zone (NFZ), a proto-thrust zone (PTZ), an imbricated thrust zone (ITZ), a frontal out-of sequence thrust zone (FOZ), an upper slope zone (USZ) and a back-thrust zone (BZ). Obvious subduction-related characteristics can be found in the south profiles between 18°45’N and 20°N, better than in the northern profiles between 20.5°N and 21°45’N. To the north, the décollement beneath the trench and frontal wedge is not clear and is disrupted by several buried seamounts or basement highs. Moreover, the width of PTZ becomes narrower, but ITZ and FOZ become wider. The subduction accretionary prism also gradually evolves into the foreland basin system receiving sediments from the Taiwan orogen. Because of the strong convergence between PSP and EP, the uplift of stratigraphic strata has occurred at the west side of the North Luzon Trough (NLT). To the north of 19°15’N, pop-up, uplifted structures, and west-vergent thrusts can be observed in the west side of NLT from MCS and bathymetry data, which indicates both thrusting and left-lateral strike-slip faulting due to the oblique plate convergence. But the strata are relatively flat in the east side of NLT, suggesting that the plate deformation mostly happen to the west of Luzon volcanic arc. This transpressional convergence along the west side of NLT might extend to the southern Longitudinal Valley off SE Taiwan. |
