| 摘要: | 地調所規劃的「臺灣東北部礦產地質調查」4年期計畫，是以台灣東北部海域的火山島及海底火山區域為調查重點。本團隊工作是使用多音束水深、底拖聲納與磁力資料，並判釋水層噴氣柱分布，用以瞭解調查區域內海床地貌、淺部地質構造、火成岩體、熱液噴氣柱等與火成作用相關的特徵分布。 在海底地形成果方面，本團隊已完成新、舊多音束水深資料的處理，並彙編出解析度5 m的台灣東北海域海底地形圖資，資料分布範圍約3700 km2，最遠至約東經122度30分的棉花峽谷上游西側海域。並完整涵蓋本計畫調查區域內的4座火山島，包含基隆嶼、花瓶嶼、棉花嶼及彭佳嶼；以及8座海底火山，包含基隆海底火山與目前編號SV1-SV7的海底火山，且在這些海底火山周圍區域也都有規模較小的海底火山出露。綜整過去3年的地形成果，可以發現這些海底火山分布區域都有劇烈的侵蝕作用，並伴隨許多塊體崩落的地貌特徵。 底拖聲納成果方面，本團隊已完成約1191 km長的新、舊底拖聲納資料處理與分析。綜整過去3年的底拖聲納結果顯示，基隆海底火山區域與野柳線形，具有明顯的火成作用與熱液活動相關特徵，在基隆海底火山附近的淺部地層中，流體通道分布廣，水層中噴氣柱分布密集。在海底火山東側地形凹陷區域，疑似有熱液作用沿地層滲入形成的礦脈，且水層中近海床的構造特徵有噴氣柱現象；在野柳線形則觀察到疑似熱液沿著斷層構造向上移棲，且水層中有噴氣柱現象；SV1海底火山與花瓶嶼則發現淺部地層中疑似熱液作用沿地層滲入形成的礦脈，該區域也發現許多淺層斷層構造；在SV2東側的小海底火山，具有疑似熱液作用沿地層滲入所造成的強反射震測相；在SV7區域地層中則發現有火成岩性質的聲波基盤及火成岩體特徵。然而，在這些海底火山與火成作用區域的海床上，大多為侵蝕、沙波與塊體崩落的地貌，並無發現顯著與熱液活動相關的地貌特徵。 藉由蒐集既有與新收集台灣東北海域的海上磁力資料，本團隊整編出最新的台灣東北海域磁力異常圖。根據磁力異常資料發現，有一明顯磁力異常高區位於台灣東北外海，並大致以海底火山群SV3-SV6為中心分布，並以SV1、SV2、SV7和棉花嶼為其西方及北方之邊界並往東延伸；在此高區的北側及西側磁力異常逐漸降低，在SV7以北有較大範圍磁力異常低區。此外，在基隆嶼及基隆海底火山也為一磁力異常低區。但從磁化強度分布來看，其高區則是以近SV7為中心並往西北方向分布，大致以位於其西側及東側的SV2和棉花嶼、彭佳嶼為界，顯示此區底下應有大量的火成岩體等較高磁源體的分布。磁化強度低區則分布在其南側，海底火山群SV1及SV3-SV6則介於兩者之間，而基隆嶼和基隆海底火山則分布在此低區的西南緣。配合增強解析訊號可發現零階解析訊在較強的訊號位置與高磁化強度區域相符，大部份的海底火山附近具有較高的零階解析訊號強度，代表這些區域的火成作用造成磁力訊號變化較大導致，其中SV1處的解析訊號最強。 此外，本團隊於110年度利用大量既有單音束水深的水層資料進行熱液噴氣柱的判釋，當下結果發現噴氣柱廣泛的分布在本計畫調查區域內。然而，經過處理與檢視新收集的單音束水深資料，並進行部分同測線的側掃聲納與底質剖面的水層訊號比對。根據上述透過三種資料不同時段或類型的水層訊號交叉比對結果，發現基隆嶼及基隆海底火山區域的噴氣柱數量、正確性與110年度既有資料判釋結果相當一致。然而，花瓶嶼以及SV1-SV7等海底火山區域，重新判釋後的噴氣柱數量與正確性卻遠低於110年度既有資料的判釋結果。推測原因，與這些區域實際上的魚群、海洋生物聚集以及懸浮層隨海流漲退而產生升降的變化有關。因為這些情況非常容易造成在單一時段的測線上，水層生物或懸浮層聚集訊號被誤判為噴氣柱。 為了延續過去三個年度的調查成果與此4年期計畫的目標，瞭解臺灣東北部海域火成作用、熱液活動分布範圍的地質、地貌、磁力等特徵，本（112）年度計畫將以彭佳嶼周邊及棉花嶼以北的海域為調查研究重點。藉由彙整和分析調查區內既有的高解析底拖聲納與海底地形資料，提供詳細的海底地形圖資、地貌特徵與淺部地層構造。並綜合磁力資料分析結果，瞭解該區域火成作用的分布特性。也將協助處理其他團隊新收集的多音束水深與單音束水深資料，除了持續彙整海底地形圖資外，也需仔細比對調查區內熱液噴柱的正確性。最終將透過系統性的彙整，提出本四年期計畫台灣東北部的火成作用區域分布情況，並詳述各區域內的地形地貌、淺部地層構造與磁力特性等圖表，提供地調所規劃未來調查與探勘計畫的參考。 ;The four-year project “Geological investigation of mineral deposits in northeastern Taiwan－High-resolution sonar and magnetic surveys” planned by the Central Geological Survey, MOEA (CGS) focuses on the volcanic islands and submarine volcanic areas in the northeastern offshore Taiwan area. The project team uses multibeam bathymetry, deep-towed sonar and magnetic survey data, and interprets the gas plume distribution in the water column to understand the distribution of igneous features such as seabed morphology, shallow geological stratigraphy, igneous bodies and hydrothermal vents in the survey area. In terms of seabed morphology results, the project team has completed the processing of new and old multibeam bathymetry datasets and compiled a 5-meter resolution bathymetry map of the northeastern offshore Taiwan, covering an area of about 3,700 km2, as far as the western area of the upper Mian-Hua Submarine Canyon at 122.5°E. The bathymetry map also covers four volcanic islands in the survey area, including Keelung Islet, Huaping Islet, Mianhua Isled, and Pengjia Islet, and eight submarine volcanoes, including Keelung Submarine Volcano and the submarine volcanoes numbered SV1-SV7, and there are smaller submarine volcanoes exposed in the areas around these named/numbered submarine volcanoes. The seabed morphology results of the past three years show that these submarine volcano areas have been extremely seabed erosional process, accompanied by many blocks collapse features. In terms of deep-towed sonar results, the project team has completed the processing and analysis of about 1,130 km of new and old deep-towed sonar datasets. The deep-towed sonar results for the past three years show that the Keelung Submarine Volcano area and the Yehliu Lineament have obvious characteristics related to igneous and hydrothermal activities, and the shallow strata near the Keelung Submarine Volcano have a wide distribution of fluid passages and a dense distribution of gas plumes in the water column. In the topographic depressions on the eastern side of the submarine volcanoes, it is suspected that hydrothermal process penetrated along the strata to form veins, and there are features of the near-seabed such as the phenomenon of gas plumes in the water column. In the Yehliu Lineament, upward migration of hydrothermal fluids along the fault structures and the presence of gas plumes in the water column were observed. In the SV1 submarine volcano and the Huaping Islet, veins suspected to be formed by hydrothermal fluids penetrating along the strata were found in the shallow strata, and many shallow faults were also found in this area. In the eastern part of SV2, the small submarine volcanoes have strong reflection seismic phases suspected to be caused by hydrothermal infiltration along the strata, and the strata in the SV7 area have igneous basement and igneous body features. However, the seafloor in these submarine volcanic and igneous areas is mostly characterized by erosion, sand waves, and block collapse, and no significant features related to hydrothermal activity are found. By collecting the existing and newly collected magnetic data in the northeastern offshore Taiwan, the project team compiled the newest magnetic anomaly map of the northeastern offshore Taiwan areas. According to the magnetic anomaly data, there is an obvious high magnetic anomaly area located off the northeastern Taiwan, and it is roughly distributed by the submarine volcanic group SV3-SV6 as the center, with SV1, SV2, SV7 and the Mianhua Isled as its western and northern boundaries and extended to the east; the magnetic anomaly gradually decreases to the north and west of this high area, and there is a larger area of low magnetic anomaly north of the SV7. In addition, there is also a magnetic anomaly low area in Keelung Islet and Keelung Submarine Volcano. However, from the distribution of magnetization intensity data, the high magnetization area is centered near the SV7 and distributed to the northwest, roughly bounded by SV2, Mianhua Isled, and Pengjia Islet, which are located to the west and east, showing that there should be a large number of igneous bodies and other higher magnetic sources under this area. The low magnetization intensity area is located in the south side, and the submarine volcanoes SV1 and SV3-SV6 are in between, while Keelung Islet and Keelung Submarine Volcano are located in the southwest edge of this low area. The enhanced analytic signal shows that the stronger zero-order analytic signal is consistent with the high magnetization intensity area, and most of the submarine volcanoes have higher zero-order analytic signals, which means that the pyrogenic effect in these areas causes the magnetic signal to vary greatly, and the strongest analytic signal is at the SV1. In addition, in 2021, the project team used a large amount of existing single-beam bathymetry data to interpret the possible hydrothermal gas plumes in the water column, which were found to be widely distributed in the investigation area of this project. However, after processing and examining the newly collected single-beam bathymetry data, we conducted a partial comparison of the side-scan sonar and sub-bottom profiler signals from the same survey line. Based on the above-mentioned cross-comparison results of the water column signals of different time periods or types from the three data types, it was found that the number and accuracy of the gas plumes of Keelung Islet and Keelung Submarine Volcano area were quite consistent with the results of the existing data in 2021. However, the number and accuracy of the re-interpreted plumes in the submarine volcanic areas of Huaping Islet and SV1-SV7 were much lower than those of the 2021 data. The reasons for this are assumed to be related to fish or marine life accumulation in these areas and changes in the rise and fall of the suspended layer in response to tidal effects. Because of these conditions, it is very easy to misinterpret the signal of pelagic or suspended layer aggregation as a gas plume in a single time survey line. In order to continue the survey results of the past three years and the objectives of this 4-year project, and to understand the geological, morphological, and magnetic characteristics of the distribution of igneous and hydrothermal activities in the offshore northeastern Taiwan areas, the current (2023) year project will focus on the offshore area surrounding Pengjia Islet and the north of Mianhua Isled. By compiling and analyzing the existing high-resolution deep-towed sonar and seafloor bathymetry data in the survey area, we will provide detailed topographic information, morphological features of the seafloor, and shallow stratigraphic structure. The results of the magnetic data analysis will also be integrated to understand the distribution of igneous properties in the survey area. We will also assist in the processing of new multibeam and single-beam bathymetry data collected by other project teams, and will continue to compile seafloor bathymetry data and compare the correctness of hydrothermal vents in the survey area. Finally, through systematic compilation, we will present the distribution of igneous areas in northeastern offshore Taiwan for this four-year project, and detail the topography, shallow stratigraphic structure, and magnetic properties of each area for the reference of the CGS in planning future surveys and exploration projects. |
