台灣西南海域下枋寮盆地的震測地層分析

Abstract

台灣西南海域位處於弧陸碰撞前緣與被動性大陸邊緣的交接處，地形上為狹窄的高屏陸棚與向西南傾斜的高屏陸坡。下部陸坡區的主要構造為受隱沒碰撞作用所形成的逆衝斷層與褶皺，而上部高屏陸坡區地形地貌之分布則深受呈北北西-南南東走向的泥貫入體以及呈東北-西南走向的海底峽谷構造控制。高屏陸坡上廣泛分佈著斜坡間盆地，為陸源沉積物輸送到深海過程中保存沉積物之重要空間。 下枋寮盆地位處於台灣西南海域高屏陸坡區域的枋寮峽谷下游處，是一由八座泥貫入體包圍所組成的斜坡內盆地。為了瞭解下枋寮盆地受陸源沉積物輸送過程之影響，本研究中使用了多筆二維反射震測資料以及一筆三維震測資料對一個位處於枋寮峽谷下游並匯集進下枋寮盆地的層序與構造進行詳細的描繪，以探討下枋寮盆地的構造發展過程與沉積特徵。我們在此盆地中的沉積層中分出六個地層單元並辨識出八個貫入體構造。透過這些層序與貫入體構造間之關係，我們建立了一個下枋寮盆地形成的演化過程模式。盆地南緣的逆斷層發育形成第一期褶皺(Stage B)，推測貫入體D6 也是順著此逆斷層形成而在盆地南緣形成。接著斷層活動暫停，覆蓋沉積物Unit B於上(Stage C)，形成第一期進覆不整合面，第二期斷層發育時(Stage D)，同時沉積Unit C形成第二期進覆不整合面，貫入體D3發育造成的陡坡而引發第一次的海底崩塌活動(Stage E)，其後，Unit D沉積，水道從東側入侵，並填充水道沉積物Unit E形成主要的沉積過程(Stage F)，而後盆地發育期間，盆地西緣的泥貫入體D2形成並增加坡度，再度造成海底崩塌(Stage G)。 D4的出現中止了盆地西側的水道沉積物；而沉積中心也在此時因D4的發育逐漸向西移動。最後，近代沉積的Unit F覆蓋在所有層序之上(Stage H)，形成目前所看到的下枋寮盆地沉積堆疊與構造分佈情況。

Taiwan is located at the junction between the Ryukyu and Luzon Arcs in the northwest Pacific, and the region offshore southwestern Taiwan is an incipient collision zone where the accretionary wedge has obliquely impinged on the passive continental margin of the South China Sea. Imbricated fold-and-thrust structures are observed in the lower Kaoping slope domain and mud diapirs, extending in a NNE-SSW direction, emerge through thick orogenic sediments in the upper Kaoping slope domain. Mud diapirs and canyons run across the upper slope domain in NE-SW direction, represent the source of sediments and spatial variation of the intraslope basins distributing in the upper Kaoping slope domain. In this study, we use a 3D seismic and many 2D seismic profiles to reveal the development and sedimentation of the Lower Fangliao Basin, locating in the southern portion of the Fangliao Canyon where it merges into a nearly filled submarine canyon and then join the Kaoping Canyon. We interpreted six depositional units, eight mud diapir structures and one plunge fold in the Lower Fangliao Basin. The unit boundaries have relatively continuous reflections throughout the basin. We propose a 8-stage conceptual model to explain our observation for the evolution of the Lower Fangliao Basin. In this model, Unit A folded during thrust folding (Stage B), and Unit B deposited here during Stage C. The thrust fault was active again and Unit C deposited at the same time (Stage D). The first time occurrence of mass transport deposits (MTDs) were caused by D3 uplifting (Stage E). As the H4 occurrence of erosional surface from eastern Lower Fangliao Basin, channelized sedimentation became dominant and formed Unit E here during Stage F. The D2 and D4 developed simultaneously (Stage G), ceased the channelized sedimentation and induced MTDs to form from D2’s direction. This migrating uplift resulted in shifted depocenters from western to eastern side, and then shifted back to eastern side after D4 developed. Unit E deposited above all units in the end (Stage H).