台湾东部海岸山脉乐合弧前盆地层序：记录活跃斜向弧陆碰撞之构造演化

Stratigraphy of the Loho Forearc Basin in the Coastal Range, Eastern Taiwan: Recording Tectonic Evolution of Active Oblique Arc-Continent Collision

台湾东部海岸山脉弧前盆地层序由西侧的利吉混杂岩和东侧的残留弧前浊积层序组成,记录了北吕宋火山岛弧与欧亚大陆边缘碰撞的构造演化。在海岸山脉中段,利吉混杂岩不仅分布于乐合弧前盆地西侧,也出露于残留弧前层序中央。本文通过野外地质调查及微体化石研究,论述乐合弧前盆地内各地层单元的时空展布特征及其构造控制机制。综合浮游有孔虫与钙质超微化石研究结果,利吉混杂岩泥质基质可限定在早上新世4.3~3.4 Ma内,而残留弧前浊积层序沉积在晚上新世3.4~3.0 Ma,说明利吉混杂岩不是与残留弧前浊积层序同时异相的滑塌堆积,出露于盆地中心的利吉混杂岩也不是沉积于向斜轴部的最年轻地层。在乐合弧前盆地,年老的利吉混杂岩总体上分布于年轻的残留弧前层序西侧,该地层展布特征类似于台湾东南海域北吕宋海槽内受向东背冲构造控制的花东海脊–残留弧前盆地。这说明乐合弧前盆地早期沉积层序在不晚于3.4 Ma时同样受到背冲构造的控制,被变形抬升为花东海脊地形高区。而位于东侧的残留弧前盆地在3.4~3.0 Ma间继续沉积正常浊积层序。乐合弧前盆地的地层展布特征不仅记录了初期弧陆碰撞阶段的向东背冲构造,也记录了成熟期弧陆碰撞阶段的向西逆冲构造。在成熟期弧陆碰撞阶段,花东海脊被进一步向西剪切为利吉混杂岩,残留弧前层序及火山岛弧向西逆掩于利吉混杂岩之上,形成海岸山脉。由于河流的侵蚀作用使得部分利吉混杂岩以构造窗的方式出露于盆地中心。

Forearc basin sequences in the Coastal Range include the highly sheared Lichi Mélange in the west and the remnant stratified forearc sequences in the east. They demonstrate the tectonic evolution of the oblique collision between the North Luzon Arc and the Eurasian Continent margin. In the middle part of the Coastal Range, the Lichi Mélange crops out not only in the west of the Loho forearc basin, but also in the center of the remnant forearc sequences. Detailed field survey and micropaleontological study are conducted to understand the temporal and spatial distribution characteristics of the sedimentary sequences in the Loho forearc basin and the mechanism responsible for such characteristics. Based on micropaleontological study integrating both planktonic foraminifera and calcareous nannofossils, the mélange unit and the remnant forearc sequences in the Loho forearc basin can be assigned to the Early Pliocene（4.3–3.4 Ma） and the Late Pliocene（3.4–3.0 Ma） in age, respectively. This result indicates that neither the Lichi Mélange is an olistostromal facies coeval to the remnant forearc sequences nor the Lichi Mélange exposed in the basin center represents the youngest strata overlying the remnant forearc sequences. In the Loho forearc basin, the older Lichi Mélange generally lies west of the younger remnant forearc sequences. This sedimentary framework resembles that of the Huatung Ridge-remnant forearc basin in the deformed North Luzon Trough off southeastern Taiwan in the initial arc-continent collision zone. This indicates that arc-ward backthrusting might have occurred in the western part of the Loho forearc basin before 3.4 Ma. The lower forearc strata were thus deformed and uplifted as the pop-up Huatung Ridge in the west, while sedimentation continued in the remnant forearc basin in the east during 3.4–3.0 Ma. The distribution characteristics of the sedimentary sequences in the Loho forearc basin record not only the eastward backthrusting in the initial arc-continent collision stage, but also the westward thrusting in the advanced arc-continent collision stage. In the advanced arc-continent collision stage, the backthrust Huatung Ridge was further thrust westward and deformed into the Lichi Mélange which was in turn overthrust westward by the remnant forearc sequences and the volcanic islands. Part of the Lichi Mélange was then exposed as a tectonic window in the center of the remnant forearc sequences due to river erosion after formation of the Coastal Range in the last 1 Ma.