臺灣東北海域海床線形與火山特徵

Seafloor Lineaments and Volcanic Features off Northeastern Taiwan

鍾承峻(Cheng-Chun Chung)

指導教授 ： 許鶴瀚

共同指導教授 ： 張日新(Jih-Hsin Chang)

國立臺灣大學/理學院/海洋研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103964

摘要

海床的形貌反應了不同地質及構造作用的影響，臺灣東北方海域位於東海陸棚與陸坡之過渡帶，東側與南沖繩海槽相鄰。過去研究指出，此區域的構造作用是由臺灣造山帶的後造山垮塌以及沖繩海槽弧後擴張等張裂作用所主導，但其細部的海床形貌特徵與新期構造作用之關聯性卻少有討論。本研究使用高解析度的多音束水深以及多頻道反射震測資料，檢視臺灣東北方海域棚坡區的海床形貌特徵與地層構造型態，並討論其分布特性與形成機制，以及區域構造作用對於海床形貌的影響模式。
透過多音束水深資料，將研究區域內的海床形貌進行特徵辨識後，區分為崖狀線形特徵、脊狀線形特徵以及環狀特徵。依據地形分析結果，研究區域內線形特徵的主要走向為東北-西南向、次要走向為西北-東南向，主、次要走向的線形特徵具有不同的特性。主要走向之線形特徵廣泛分布於研究區域內，延伸長度可達10公里以上，且大多屬於崖狀線形特徵。次要走向的線形特徵分布於北方三島區以及基隆內陸棚區內，延伸長度皆在10公里以下，且大多屬於脊狀線形特徵。環狀特徵大多分布於基隆內陸棚區與北方三島區內。其平均高程差(47.1公尺)比線形特徵(33.5公尺)要大。透過震測剖面資料結果顯示，崖狀線形特徵的下方地層構造大多可以對應至切穿海床的正斷層構造。脊狀線形特徵除了也可對應至切穿海床之正斷層構造，並且有觀察到依循斷層向上延伸的強振幅訊號特徵，可以指示為火成岩體向上侵入。
綜合多音束水深與震測剖面資料的分析結果，認為崖狀線形特徵為區域構造作用而導致的正斷層構造，脊狀線形特徵可能為裂隙噴發或是侵蝕殘餘之線形岩脈，環狀特徵可能由破火口陷落或是斷塊旋轉而導致海床崩塌地形。而根據線形特徵在走向和長度上的差別，東北-西南向線形特徵發育自既有的東北-西南向斷層系統，而西北-東南向線形特徵可能為主要斷層系統伴隨的次生構造，此為接力斜坡(Relay Ramp)之構造特徵。本研究結果說明了臺灣東北方海域棚坡區海床形貌目前正處於張裂環境下，並受到活躍的斷層作用與火成活動所影響。

關鍵字

臺灣東北海域 ； 海床形貌特徵 ； 新期構造 ； 多音束水深資料 ； 多頻道反射震測 ； 火成活動

並列摘要

Seafloor morphology reflects the effect of the tectonic processes. Located between the shelf and slope at the southernmost East China Sea and adjacent to the South Okinawa Trough to the east, offshore northeastern Taiwan is currently dominated by an extensional state caused by the post-collision collapse of the Taiwan orogenic belt and the back-arc spreading of the Southern Okinawa Trough. However, the relationship between the seafloor morphologies and neotectonics received little attention. In this study, high-resolution bathymetry data and multi-channel seismic data offshore northeastern Taiwan are used to analyze the distribution and formation of the seafloor morphologies and subsurface structures, and to discuss the effect of neotectonics on the seafloor morphologies.
Based on the seafloor morphologies revealed by high-resolution bathymetry data, cliff-shaped lineament, ridge-shaped lineament, and rim-shaped features are identified. The results of geomorphological analysis suggest that the strike of the dominating and subordinate lineaments are in NE-SW and NW-SE, respectively. The dominating strike lineaments can extend over 10km and are widely distributed in the whole study area. Most of them are cliff-shaped. The subordinate-strike lineaments are mainly distributed in the Keelung Inner Shelf area and the Three Islets Off Northern Taiwan area. Most of them are ridge-shaped, with less than 10 km in length. Most of the rim-shaped features are distributed in the Keelung Inner Shelf area and the Three Islets Off Northern Taiwan area, with larger average reliefs (47.1m) are than those of the lineaments (33.5m). Based on our seismic profile analysis, we find that normal faults are common to cut through the seafloor beneath the cliff-shaped lineaments. In addition to normal faults, high-amplitude reflectors are found to extend along the faults beneath the ridge-shaped lineaments, suggesting that the intrusion is likely to be controlled by fault structures.
Take advantage of integrated bathymetry and seismic data, we propose that the cliff-shaped lineaments result from regional normal fault structures and the ridge-shaped lineaments are plausible to result from the fissure eruption or relict dikes. The rim-shaped features could result from the caldera and collapses of the crater, or alternatively, the result of the fault-block rotation. According to their differences in strikes and lengths, we propose that the NE-SW lineaments result from the regional NE-SW fault structures, the NW-SE lineaments are local and subordinate to the NE-SW fault system. We ascribe these dominating and subordinate structure assembly to the result of a regional relay ramp structure. We suggest that offshore northeastern Taiwan is in a state of extension, and the seafloor morphologies are currently impacted by active faults and volcanic activities.

並列關鍵字

offshore northeastern Taiwan ； seafloor morphology ； neotectonics ； multi-beam bathymetry data ； multi-channel seismic data ； volcanic activity

參考文獻

Acocella, V. Funiciello, R. (2006). Transverse systems along the extensional Tyrrhenian margin of central Italy and their influence on volcanism. Tectonics, 25(2). https://doi.org/10.1029/2005TC001845

Belousov, A., Belousova, M., Chen, C.-H. Zellmer, G. F. (2010). Deposits, character and timing of recent eruptions and gravitational collapses in Tatun Volcanic Group, Northern Taiwan: Hazard-related issues. Journal of Volcanology and Geothermal Research, 191(3), 205–221. https://doi.org/10.1016/j.jvolgeores.2010.02.001

Boggs, S., Wang, W. C. Chen, J. C. (1974). Textural and compositional patterns of Taiwan Shelf sediment. Acta Oceanogr. Taiwanica, 4, 13–56.

Boggs, Wang, W.-C., Fletcher S., L. Chen, J.-C. (1979). Sediment Properties And Water Characteristics Of The Taiwan Shelf And Slope. Acta Oceanographica Taiwanica, 10, 10–49.

Casalbore, D. (2018). Volcanic Islands and Seamounts. In A. Micallef, S. Krastel A. Savini (Eds.), Submarine Geomorphology, 333–347. Springer International Publishing. https://doi.org/10.1007/978-3-319-57852-1_17