臺灣西南海域增積海脊受流體活動所造成之海床特徵

Seafloor Features Induced by Fluid Activities on Accretionary Ridges off Southwestern Taiwan

陳姿婷(Tzu-Ting Chen)

指導教授 ： 劉家瑄

共同指導教授 ： 蘇志杰(Chih-Chieh Su)

國立臺灣大學/理學院/海洋研究所/博士(2020年)

https://doi.org/10.6342/NTU202000192

摘要

臺灣西南海域的增積岩體受到擠壓的應力，產生許多褶皺與逆衝斷層，也提供了流體移棲的管道。本研究利用水下無人載具(autonomous underwater vehicle，簡稱AUV)所收集的高解析水深資料與回散射(backscatter)影像，來了解於西南海域三個增積海脊上受流體活動所造成的海床特徵分布。兩種特殊海床形貌分別為位在海脊兩側斜坡的彗星狀孔洞(comet-shaped depressions，簡稱CSD)與多分布於海脊軸部的海底隆堆(pingo)。由淺至深的地層剖面協助本研究評估增積岩體中構造對於流體移棲與其海床形貌的影響，其剖面顯示海床下具有許多切過海底仿擬反射訊號(BSR)至淺部的傾斜高振幅反射層面(high amplitude reflector packages)與煙囪(seismic chimneys)、裂隙等震測特徵，推測(1)流體能沿著這些傾斜層面向淺部移棲、(2)隆堆下方具有大量的流體填充。本研究同時藉由水下遙控載具(remotely-operated vehicle，簡稱ROV)以及拖曳式海床觀測系統(Ocean Floor Observation System，簡稱OFOS)所收集的海底影像、岩石標本、管蟲以及壓式短岩心(push core)，搭配AUV高解析地形來進一步分析流體滲漏的形貌演化與差異，其結果顯示(3)隆堆上岩石標本的低δ13C值(−39.9 and −44.3 per mil VPDB)，為受到甲烷氣移棲而產生的自生性碳酸鹽礁、(4)具有淺硫酸鹽-甲烷界面(4公分)的黑白色菌叢位於冷泉系統兩側，可望作為海床滲漏的特徵、(5)冷泉系統兩側具有旺盛的冒氣特徵與生物富集並隨地形而有所差異，可能為流體向外側遷移的證據。另一方面，在搭配本研究建立的海床底質分類法後，我們分析了具流體滲漏特徵的海床沉積物環境與其形貌差異，並建議(6)這些孔洞的尺寸受裂隙與出露層面的分布較沉積環境影響大。最後，CSD與隆堆的形貌與形成機制也指示(7)這兩種受流體活動所造成的海床特徵有著不同的滲流速率，海底隆堆是隨時間緩慢而連續地發展，但彗星形孔洞可能是受到地震事件觸發所形成。因此，彗星狀孔洞有可能指示著在短時間釋出大量氣體的事件，值得後續投入更多的研究。

關鍵字

水深 ； 天然氣水合物 ； 流體移棲 ； 自生性碳酸鹽礁 ； 水下載具 ； 底質分類

並列摘要

Folds and thrust faults developed in accretionary wedge off southwestern Taiwan may provide conduits for fluid migration from deep strata to seafloor as there are many seafloor seepage features formed in this area. High-resolution bathymetric data and backscatter images collected using an autonomous underwater vehicle (AUV) over three accretionary ridges are analyzed to better understand the characters, distribution and development of the observed seepage features. Two types of seepage features are revealed: comet-shaped depressions (CSD) occur on flanks of the ridges studied, and pingos mostly occur on or near the apexes of these ridges. In addition, these features overlie areas where multichannel seismic reflection profiles show bottom simulating reflectors (BSR) together with seismic chimneys and high amplitude reflector packages along some dipping strata extending from below BSR to near seafloor. We suggest that (1) these CSD are related to gas seepages as the fluid possibly migrates along the dipping strata and then blowouts on the seafloor where the dipping strata were cut at seafloor, forming these depressions; and (2) there is a large volume of fluid accumulated under the pingos. In addition, seafloor images, rock samples, tubeworms, and push cores have also been collected by ocean floor observation system (OFOS) and remotely-operated vehicle (ROV) to evaluate the evolution of seepages. Our results show that (3) the rock samples have low δ13C values, indicating the source of methane-rich fluid; (4) the microbial mat on the side of a cold seep could be an indicator of seepage feature because the depth of the sulfate-methane interface from the push cores taken at the microbial mat site is less than four centimeters; (5) strong flares with enriched biological communities are on both sides of the cold seep system, providing the evidence of lateral fluid migration. Based on analyzing the sub-bottom profiles using the automatic seabed classification method, we also suggest that (6) structures have more influence on the seepage characteristics than types of seafloor sediments. Finally, we surmise that (7) two types of seepage features were formed at different seep rates, with pingos developed slowly and continuously over time, while CSD formed in discrete events, perhaps triggered by large earthquakes. Our results suggest more studies should be carried out for these seepage features.

並列關鍵字

Bathymetry ； Gas hydrate ； Fluid migration ； Authigenic carbonate ； Underwater vehicle ； Seabed classification

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