臺灣─琉球隱沒與碰撞交界地區的構造特徵

Abstract

琉球隱沒系統最西端是隱沒與碰撞的複合帶，位在琉球海溝末端與台灣島交會邊界。琉球島弧西端弧前地區有一系列的弧前盆地，由東向西分別是東南澳(East Nanao Basin)、南澳(Nanao Basin)及和平海盆(Hoping Basin)；由於菲律賓海板塊與歐亞板塊斜向聚合，加上西側台灣造山帶的阻擋，位在弧前地區最西側的和平海盆向北移退凹入琉球島弧(Ryukyu Arc)，變形最為劇烈，有複雜的構造演化史。地貌上和平海盆呈不規則狀，因新城海脊(Hsincheng Ridge)橫切過海盆中間而可分成南北兩部份，海盆北部為一向北凹陷琉球島弧的封閉地形；而海盆南部則因基盤隆起為一高區地形，稱為和平高區(Hoping Rise)。本研究主要是利用TAICRUST與TAIGER計畫所收集到的多頻道反射震測資料，對古和平海盆中的沈積層，進行其空間分佈與構造特徵的描述。反射震測剖面顯示和平海盆中的沈積物主要可分為兩層，年輕的近代沈積層上覆於一變形的古沈積層(稱為古和平層)之上。 位在年輕的和平海盆沈積物與弧前盆地基盤之間的古和平層，主要分布在和平海盆東半部及新城海脊之下。本研究將古和平層分成三區進行探討，分別是和平海盆北部、新城海脊下方以及和平高區之上，並觀察到不同區域的古和平層呈現不同的變形現象。古和平層變形並非單一作用導致，地層淺部變形主要受沈積作用造成，其包括古和平層發生切蝕現象、鄰近新城海脊重複發生侵蝕與填充作用；深部地層變形主要是構造作用導致，其影響包括古和平層向西南下傾、位在和平峽谷的厚度最大，以及增積作用造成沈積物翻轉。這說明了和平海盆演變過程中可能受到不同階段之變形作用，其包括隱沒的菲律賓海板塊連接到台灣造山活動之間造成的轉變與因斜向聚合的隱沒板塊導致的深部構造。 本研究推論古和平層在台灣造山活動開始之前就已經存在，其沈積物可能源自當時的大陸邊緣。古和平層主要經歷過兩階段的變形作用，第一階段即古和平層下傾，此時古和平層下傾同時並接受沈積，其下傾作用來源於深部地殼的構造影響；第二階段的變形是和平海盆南部的基盤隆起，其造成了古和平層反覆遭受侵蝕與填充及南側增積作用等一系列弧前破壞。此外，琉球弧前最西側主要受到壓縮作用影響，而橫切和平海盆的新城海脊位處在兩相反活動方向的地塊之間，該海脊為受到壓縮應力後所造成的沈積物塊體。

The Ryukyu forearc region offshore eastern Taiwan is the area where the Ryukyu subduction zone terminates against the Taiwan Island. The Hoping Basin is a small forearc basin located just east of the Taiwan Island. Due to the northwestward convergence between the Philippine Sea and the Eurasia plates, there are complex seafloor topography and large crustal deformation in the Hoping Basin area. This study describes the sediment distribution in the Hoping Basin, especially focuses on a deformed strata, called the Old Hoping Strata, in the Hoping Basin and the western end of the Ryukyu forearc area. From the analyses of deep seismic reflection profiles of the 1995 TAICRUST and 2009 TAIGER cruises, together with other multichannel seismic reflection profiles collected by the R/V OR1, we have mapped the distribution and deformation of the Old Hoping Strata in detail. The Old Hoping Strata deformed as the results of complicated subduction-collision processes in this region, and the structures observed could shed lights on how the subducted Philippine Sea slab collides with the Taiwan Island. By compiling the reflection seismic images, we characterize the Old Hoping Strata deformation by two factors. The first one is submarine sedimentary processes, as seismic images reveal truncated surfaces and channel cut-and-fill features in the Old Hoping Strata. The second factor is the tectonic processes. We observe a 3000-m subsidence of the dipping Old Hoping Strata lying over the Ryukyu Arc slope, and complex folds and fault systems developed in the Old Hoping Strata on top of the Hoping Basement Rise. This strata also shows upturned basin flank on the side against the accretionary wedge. The thickest strata is located beneath the Hoping Canyon and adjacent to Hsincheng Ridge. We hypothesize that the flat lying Old Hoping Strata formed before the subduction-collision processes started in this region. It began to deform when the subducted Philippine Sea slab collided with the Taiwan Island, and infer that the accreted Yaeyama Ridge, the Taiwan block, and the subducted Luzon Arc (the coastal Range) all contributed to the Old Hoping Strata deformation. Very negative graity anomalies (< -300 mGal) and very thick (> 5 km) crustal velocity strata in the Hoping Basin area further strengthen the idea that the 3000-m subsidence of the Old Hoping Strata was caused by the sinking of the Philippine Sea plate at its western edge and tearing of the Eurasia plate here.