臺東海槽北緣之海陸域地質特徵、接連與構造機制

Abstract

由於歐亞大陸邊緣與呂宋島弧之間的斜向聚合，使得臺灣成為研究隱沒系統轉變為碰撞系統的天然實驗室。而臺灣東南區域，陸域的海岸山脈以及近岸海域的臺東海槽，被認為是了解初始弧陸碰撞的重要過渡帶。為了瞭解臺東海槽最北端和海岸山脈最南端之間的構造發育，我們進行了陸域野外調查以及海域多頻道反射震測，透過資料探討海陸域地體架構的連接關係，進一步瞭解區域演化歷史。 陸域研究部分，我們將焦點放在海岸山脈最南端的永豐斷層，以及位於永豐斷層西南側、與永豐斷層走向接近平行的利吉斷層。在縱谷內部，永豐斷層為利吉層與八里灣層的邊界，走向近南北向，為向東傾的逆斷層，然而在海岸山脈最南端，永豐斷層的位置與走向尚未定論，接觸關係也存在不同的解釋，如構造接觸或沉積接觸，根據野外調查，我們認為永豐斷層走向約為東西向，並且更傾向將其解釋為具有活動性的構造邊界。利吉斷層同樣為走向近南北向的逆斷層，並且在海岸山脈最南端同樣有向東南方轉彎的趨勢。海域研究部分，本研究震測資料範圍主要涵蓋緊鄰陸域的臺東海槽。地層部分，利用震測地層學的方法，我們將剖面中不同的震測相進行區分，並且與陸域海岸山脈的都鑾山層、蕃薯寮層與八里灣層、卑南山礫岩、利吉層等地層進行對比。構造部分，根據水深與震測資料，顯示臺東海槽最北端存在數個呈現東西走向的階狀地形，從震測剖面來看這些階狀地形是由一系列轉換正斷層所構成的花狀構造。 我們認為臺東海槽北緣的東西走向的轉換斷層，與縱谷斷層與綠島-蘭嶼斷層的左移運動有關，即菲律賓海板塊與歐亞板塊的走向滑移相對運動，促使了縱谷斷層與綠島-蘭嶼斷層的左移運動，造成兩條斷層之間產生平移拉張環境，進而在臺東海槽北緣形成拉張盆地。除此之外，根據海域近岸區的擠壓構造特徵與正斷層震源機制分布，結合區域GPS速度場資料，我們提出海岸山脈最南端的小規模逃逸構造模型，由於弧陸碰撞產生的擠壓環境，促使位於花東縱谷內的利吉層有往東南海域逃逸的趨勢。

Due to the oblique convergence between the Eurasian continental margin and the Luzon Arc, Taiwan serves as a natural laboratory for studying the transition from subduction to collision systems. In southeastern Taiwan, the Coastal Range and the adjacent offshore Taitung Trough are considered crucial transitional zones for understanding the initial arc-continent collision. To understand the tectonic development between the northernmost Taitung Trough and the southernmost Coastal Range, we conducted terrestrial field investigations and offshore multi-channel seismic surveys. By examining the data, we explore the connections between terrestrial and marine tectonic frameworks, providing insights into the regional evolutionary history. For the terrestrial research, we focused on the Yungfeng Fault and the Lichi Fault at the southern end of the Coastal Range. Within the Longitudinal Valley, the Yungfeng Fault forms the boundary between the Lichi Mélange and the Paliwan Formation, trending nearly north-south and acting as an east-dipping reverse fault. However, at the southern end of the Coastal Range, the position and trend of the Yungfeng Fault remain unresolved, with varying interpretations of the contact relationships, such as structural contact (Huang et al., 2018; Chen & Wang, 1996) or depositional contact (Lai et al., 2021). Based on field investigations, we suggest that the Yungfeng Fault trends approximately east-west and is more likely to be interpreted as an active tectonic boundary. The Lichi Fault is also a north-south trending reverse fault and shows a tendency to bend southeastward at the southern end of the Coastal Range. For the marine research, our seismic data mainly cover the Taitung Trough adjacent to the terrestrial area. We distinguish different seismic sequences S1, S2, S3, S4, and S5 in the profiles and correlate them with terrestrial formations such as the Tuluanshan Fm., Lichi Mélange, Fanshuliao Fm., Paliwan Fm., Peinanshan Congl., and modern fluvial alluvium of the Coastal Range. According to bathymetric and seismic data, the northernmost Taitung Trough exhibits several east-west trending stepped topographies, which are identified as flower structure composed of a series of transfer normal faults. We interpret these east-west trending transfer faults as resulting from the left-lateral motion of the Longitudinal Valley Fault and the Lutao-Lanyu Fault. The strike-slip relative motion between the Philippine Sea Plate and the Eurasian Plate has resulted in the left-lateral motion of the Longitudinal Valley Fault and the Lutao-Lanyu Fault. The left-lateral motion of the Longitudinal Valley Fault and the Lutao-Lanyu Fault create a transtensional environment between the two faults, leading to the formation of a pull-apart basin at the northern Taitung Trough. Additionally, considering the compressional structural features nearshore and the distribution of normal fault focal mechanisms, combined with local GPS velocity field data, we propose a small-scale extrusion model for the southern end of the Coastal Range. The compressional environment generated by arc-continent collision results in the southeastward extrusion of the Lichi Mélange from the Longitudinal Valley towards the southeastern offshore area.