奇美海底峽谷與深海扇系統的形貌、構造及震測特徵

Morphology, structures and seismic characters of the Chimei Canyon-Fan system offshore eastern Taiwan

謝雨寰(Yu-Huan Hsieh)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.03142

摘要

本研究利用多頻道反射震測資料及高解析度水深資料研究奇美海底峽谷及深海扇的地形、鄰近構造及震測層序，以了解奇美海底峽谷及奇美深海扇谷的成因。奇美海底峽谷為發育在呂宋火山島弧東翼，位於臺灣東部秀姑巒溪出海口外海的一個形貌非常奇特的海底峽谷，其寬度非常寬廣(平均寬度9公里)且谷底地形非常平坦，往東流經呂宋火山島弧弧坡，並在海底峽谷末端形成一個面積很大的深海扇谷系統。奇美深海扇現為一不對稱的深海扇，深海扇北邊已沿著深海水道崩塌，位於深海扇之上的深海水道接續奇美海底峽谷往東發育，最後匯入花蓮峽谷。
在構造上，奇美海底峽谷似乎除了濁流的切蝕外，也受到兩側谷壁抬升，使得現今峽谷谷壁與谷底的高度差距將近950公尺。震測剖面及臺灣東部外海呂宋火山島弧與花東海盆的基盤等深分布圖顯示奇美海底峽谷係沿著呂宋火山島弧間的地形低區發育，同時奇美海底峽谷也被一系列北北東─南南西走向、向東逆衝的逆衝斷層截切，其中一條規模最大的逆衝斷層位於呂宋火山島弧坡底，分隔奇美海底峽谷(變形帶)與奇美深海扇(未變形帶)。奇美海底峽谷內老的地層已變形、破碎，年輕的沉積物平整地沉積在其上，形成一個不整合面。現今這些較年輕的沉積地層已傾斜，並且地層被海床截切，暗示奇美海底峽谷底下的構造仍在活動，將地層抬升傾斜。雖然許多逆衝斷層經過奇美海底峽谷，但現今奇美海底峽谷的地形非常平坦，並沒有觀察到斷層形成的地形高差。
沉積層序上，我們在深海扇地區辨識出八層震測層序。最下部兩層層序將花東海盆的地形低區填平，其震測相多為遠洋席狀覆蓋震測相與位於呂宋火山島弧周圍的坡前填充震測相，被解釋為弧陸碰撞前島弧周圍安靜的深海遠洋沉積環境。中層的震測層序開始出現雜亂無章及丘狀的深海扇上部扇震測相及平行的下部扇震測相，此層序被解釋為弧陸碰撞後的沉積層序。最上部的五層震測層序顯示明顯的上部扇天然堤及深海水道震測相，推論這些層序形成於成熟弧陸碰撞階段。
本研究認為在弧陸碰撞之前，島弧後方並沒有奇美海底峽谷與深海扇的形成，只有一些堆積在呂宋火山島弧周圍的沉積物與遠洋沉積物。當呂宋火山島弧開始增積至亞洲大陸時，造山帶沉積物可能沿著呂宋火山島弧中間的地形低區傳輸至島弧後方的海盆沉積，形成奇美海底峽谷及深海扇。隨著造山活動轉趨劇烈以及越接近造山帶，奇美海底峽谷與深海扇也越加明顯。一部分沉積於呂宋火山島弧之上的深海扇隨著島弧一起抬升，其中沉積在島弧地形高區的沉積物受弧陸碰撞影響而褶皺變形，沉積在奇美海底峽谷內的沉積物則受斷層作用而破碎，在被年輕的沉積物覆蓋形成平坦的谷底。深海扇的南半邊因位於島弧地形高區後方而免於峽谷侵蝕，北半邊的深海扇由於緊接著海底峽谷發育，已遭受侵蝕而不見原深海扇形貌。

關鍵字

海底峽谷 ； 深海扇 ； 深海扇谷 ； 震測層序 ； 反射震測剖面 ； 弧陸碰撞 ； 東臺灣

並列摘要

The Chimei submarine canyon is located on the Luzon arc offshore east Taiwan and delivers sediments from Taiwan orogen to deep-sea basin behind the arc to form the Chimei submarine canyon and fan during Taiwan arc-continent collision. The Chimei submarine canyon shows a very distinct morphology and has a wide (9 km on the average) and very smooth bottom. A large submarine fan-valley system was formed at its foot and the northern part of the submarine fan has been eroded. The Chimei submarine canyon starts from the Hsiukuluan River mouth, and runs across the Luzon arc. Meanders in the Chimei fan-valley system follow the Chimei submarine canyon eastward and merges into the Hualien Canyon near the western end of the Ryukyu Trench off NE Taiwan.
In this study, we use multichannel seismic reflection profile data together with high resolution bathymetry data to study the topography, basement structures and seismic sequences along the canyon path and in the distal fan. We try to understand how do the Chimei submarine canyon and the Chimei submarine fan-valley form. Based on seismic reflection data, we suggest that sediments were delivered from the orogenic belt into the Huatung Basin across the topographic low of the Luzon arc to form the Chimei submarine canyon-fan system. The Chimei submarine canyon became U-shaped from structural uplift and submarine erosion processes. A series of NNE-SSW trending and east-vergent thrusts run across the Chimei submarine canyon. An larger east-vergent thrust fault lies at the foot of the eastern flank of the Luzon arc, which separates the canyon (over deformation zone) from the fan-valley (over undeformed zone). In spite of many thrusts run across the Chimei submarine canyon, the canyon floor is flat and smooth at the present time.
Eight seismic sequences are recognized in submarine fan-valley system. The two lower sequences show sheet-drape and slope-front fill seismic facies which fill the basement low. They are interpreted to be older deep sea sediments deposited before arc-continent collision. The middle sequence shows chaotic and mounded upper fan seismic facies and continuous parallel lower fan seismic facies that were deposited after arc-continent collision. The five upper sequences show a very clear upper fan levee and channel seismic facies that formed during advanced arc-continent collision.
We suggest that before arc-continent collision there could be no canyon nor submarine fan formed. Deposition east of the Luzon arc consisted of slope-front sediments. As the Luzon arc began to accrete on the Asian continental margin, sediments were delivered across the topographic low of the Luzon arc, to form the initial Chimei submarine canyon and fan behind the Luzon arc. The Chimei submarine canyon and fan-valley grew from continued sediment supply during advanced arc-continent collision. Part of the submarine fan which deposited on the Luzon arc was uplifted by thrust fault during advanced arc-continent collision. Sediments which deposited on the topographic high of the Luzon arc are folded, while sediments which deposited in the Chimei submarine canyon are faulted. Southern part of the submarine fan behind the topographic high of the Luzon arc was protected from erosion thus better preserved, while northern part of submarine fan at foot of the Chimei submarine canyon has been eroded.

並列關鍵字

Submarine canyon ； submarine fan ； submarine fan-valley ； seismic sequences ； seismic reflection profiles ； arc-continent collision ； eastern Taiwan

參考文獻

林聖心，2010。北呂宋島弧東側之構造特徵。國立臺灣大學海洋研究所碩士論文，共79頁。

Alam, M., Alam, M.M., Curray, J.R., Chowdhury, M.L.R., Gani, M.R., 2003. An overview of the sedimentary geology of the Bengal Basin in relation to the regional tectonic framework and basin-fill history. Sedimentary Geology 155, 179–208.

Babonneau, N., Savoye, B., Cremer, M., Klein, B., 2002. Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan. Marine and Petroleum Geology 19, 445-467.

Barrier, E., Angelier, J., 1986. Active collision in eastern Taiwan: the Coastal Range. Tectonophysics 125, 39-72.

Bruce, C.H., 1973. Pressure shale and related sediment deformation: mechanism for development of regional contemporaneous faults. American Association of Petroleum Geologists Bulletin 57, 878-886.