南縱海槽之構造形貌與沉積過程

Abstract

南縱海槽位於典型隱沒帶過渡到碰撞帶上的弧前盆地，西邊為恆春半島(馬尼拉弧溝系統的增積楔)，東邊為花東海脊，南邊為北呂宋海槽(馬尼拉弧溝系統的弧前盆地)，北邊緊接花東縱谷。由GPS資料顯示現今菲律賓海板塊以每年7到9公分的速度朝西北移動，以及由發生在恆春半島與蘭嶼、綠島之間頻繁的地震，皆可知臺灣東南海域正受到強烈的聚合作用。東南海域陸源沉積物供應量大且陸上河川侵蝕速率高，南縱海槽充填著來自中央山脈、海岸山脈及恆春半島的沉積物，形成一個水深相對較淺(介於400到1400公尺)且平坦的盆地，整個盆地位於海平面之下，沉積物不易受到侵蝕，非常適合用以研究初始碰撞帶的構造與沉積過程。 本研究利用多音束水深資料與多頻道反射震測資料，探討南縱海槽的地形、構造與沉積地層，以更加了解這個位於板塊縫合帶的沉積盆地。精細的水深資料可以更加瞭解南縱海槽的地形起伏，並且辨識出多條現生的深海水道。水深資料結合ArcGIS軟體做坡度分析，以辨識出南縱海槽邊坡海底滑塌所發生的位置。接著利用ECHOS軟體處理多條於TAICRUST計畫中所收集的長支距反射震測剖面，於海槽中辨識斷層、掩埋古水道、成長地層等構造特徵。 從地形形貌上，南縱海槽可分為三個地形特徵明顯不同的區域:北邊為海底峽谷與海底水道明顯集中的區域；南邊為深海扇區；西邊為斜坡滑塌(slump)區。本研究於南縱海槽北邊的區域辨識出現今活躍的水道有兩個，分別為南縱海槽帶來大量陸源沉積物，最終水道地形消失於南縱海槽南段。南縱海槽西側斜坡的滑塌現象明顯，在水深地形圖與坡度圖上可見弧形馬蹄狀地貌，為滑塌的標準地形特徵。南縱海槽南邊為深海扇地形，其分布因受地質構造的制約呈不規則狀，而不是呈典型輻射狀或圓錐狀的扇形。過去認為南縱海槽陸源沉積物主要由卑南溪帶入，但本研究認為應該是由太麻里溪、利嘉溪與知本溪透過兩條海底水道傳輸。構造上，由於海盆內沉積地層平緩且多數地震發生於更深部的地殼內部，因此推測南縱海槽主要沉積較年輕的造山沉積物為主。反射震測剖面顯示在南縱海槽東側有明顯的成長地層(growth strata)，此成長地層朝西傾斜，年輕的成長地層其傾斜角度較老的地層為緩，可能為沉積物一邊沉積而南縱海槽東邊的花東海脊一邊抬升的結果，顯示南縱海槽已隨著花東海脊變形抬升。由震測剖面我們辨識出許多淺層正斷層，並認為其為沉積物荷重所造成，海槽的西側斜坡上則辨識出階梯狀的反射層，其為滑塌的震測特徵，本研究並推論出一邊坡重複滑塌再堆積模型來模擬不同階段的邊坡堆積。本研究也從反射震測剖面中辨識出泥貫入體(mud diapir)的現象。

The Southern Longitudinal Trough (SLT) is a N-S trending elongated trough located between the southeastern part of the Central Range and the Luzon Arc offshore southeastern Taiwan. It is situated in the incipient arc-continent collision zone, and the GPS measurements suggest that 60% of the plate convergence is taken up by the crustal deformation in this region. So it is important to understand the structural features and their developments in SLT. In this study, we use multichannel seismic reflection profile data together with high resolution bathymetry data, to study the topography changes and their relations with basement structures in SLT. Morphology, the Southern Longitudinal Trough is enclosed by the Hengchun Peninsula at the western side and the Huatung Ridge at the eastern side. Large amount of orogenic sediment were delivered from Taiwan mountain belt and formed submarine channel system in the Southern Longitudinal Trough. There are two main channels in this channel system: one channel starts from Taimali river onland, the other starts from Lijia river onland. Submarine channel system is more obvious in the northern area of the Southern Longitudinal Trough where is closer to mountain belt, whereas submarine plane is more clear in the southern area. Some sediments seem to overflow out of the Southern Longitudinal Trough into the Taitung Canyon through the topographic gaps of the Huatung Ridge. The seismic stratigraphy reveals that the Southern Longitudinal Trough is filled by growth strata, which may also form the Huatung Ridge, and the western tilting growth strata are getting flat from deep to shallow. Two main seismic sequences are recognized in the Southern Longitudinal Trough. The lower seismic sequence is strongly deformed with the basement, therefore it is interpreted to be the pre-collision sequence. The upper seismic sequence shows continuous parallel seismic strata and onlap pattern upon the lower sequence. The tilting angle of the strata in the upper sequence reduce from deep to shallow and became flat near surface. Some normal faults developed in the upper sequence but those normal faults stop at the top of the lower sequence. We interpret those are characters of syn-collision sequence. Large amount of orogenic sediments deposits in the Southern Longitudinal Trough and form growth strata when the Southern Longitudinal Trough was uplifted by convergence. Those normal faults in the upper sequence probably resulted from overloading of overlying strata.