| 摘要: | 為瞭解臺灣西南海域泥貫入體及泥火山之分布，本研究分析了大約1,530公里的震測剖面、高精密多音束水深及底拖側掃聲納與底質剖面等資料。從震測資料辨識出10處呈線性分布之泥貫入體構造，走向約NNE-SSW至N-S，各泥貫入體大約呈平行分布，大部分泥貫入體上部覆蓋年輕的沉積物，部分已出露海床，如貝塚海脊、MD5南端、枋寮海脊(MD8)及小琉球(MD2)等。泥貫入體兩側之地層大都呈傾斜狀，顯示泥貫入體的抬升活動仍相當活躍，惟泥貫入體MD10上部之現代地層約略呈水平無被擾動，顯示近期應無活動。
根據高精密水深資料，在臺灣西南海域首次辨識出13座泥火山，由於泥火山均位於泥貫入體上方，顯示是典型的泥貫入體發育伴隨形成的泥火山。泥火山外觀除MV6為狹長形外，其餘均呈同心圓錐體，高度約65~345 m，錐體底部直徑約680~4,100 m。泥火山坡度甚陡約5.3°~13.6°，顯示泥流是高黏稠度(viscosity)特性。10座泥火山(除了MV8、MV11及MV12)之火山口為陡峭地形，其泥流應屬於較高黏稠度特性，而MV8、MV11及MV12等3座泥火山之火山口頂部呈平坦狀，其泥流應是較低黏稠度泥流。
2011年在臺灣西南海域首次實施泥火山之ROV觀測，從ROV觀測結果發現3座泥火山(MV1、MV5及MV12)活躍噴發，MV1和MV5分別約每3~5分鐘及5~8分鐘噴發一次，而MV12是以連續泥流溢流，並伴隨噴氣柱噴發(約每3~10秒噴發一次)。泥火山之噴發可分為爆發式(explosive)及溢流式(effusive)噴發，MV1及MV5是呈爆發式噴發，顯示其泥流通道可能是低滲透性(permeability)特性，易在泥火山下部蓄積流體壓力，而呈現爆發式噴發；MV12是溢流式噴發，其泥流通道應是高滲透性，泥流可以很快速從泥流通道流出，向四周溢流。就泥火山之大小而言，在深水區的MV9~MV13 (直徑約2,800 m ~ 4,100 m) 比在淺水區的MV1~MV8 (直徑約 680 m ~ 2,200 m) 大的多，另外溢流式噴發的MV12(直徑約4,100 m)也比爆發式噴發的MV1 (直徑約1,250 m)及MV5 (直徑約2,200 m)大很多，顯示在深水區大直徑的泥火山，可能是較早噴發之故，然而溢流式噴發，源源不斷的泥流溢出，也是常見造成大直徑泥火山的因素。另從側掃聲納影像可辨識出在6座泥火山(MV1, MV3, MV5, MV6, MV9及MV10)有呈放射狀之高回散射強度泥流痕跡，顯示臺灣西南海域的泥火山是相當活躍噴發。MV10是13座泥火山中坡度最陡的(~13.6°)，也是高度最高的(約345 m)，並且是唯一在近海水面之海水仍偵測到高甲烷濃度(約200 nL/L)異常之泥火山，顯示MV10是所有泥火山噴發最強烈活躍的。泥貫入體形成主要有三項機制，一是快速沉積速率造成地層中有過飽和壓力存在；二是上新世之蓬萊造山運動之地體構造擠壓應力引發泥貫入體向上貫入抬升，三是大量的氣體降低深部地層沉積物密度，增進浮力作用，持續支持上貫(intrusion)作用，由於泥貫入體區有大量之甲烷氣噴出，顯示氣體在泥貫入體形成機制中扮演相當重要的角色。
在枋寮海脊(泥貫入體MD8)、泥貫入體MD5北端及MV4、MV11偵測到氣體成分是熱成熟氣體混合生物氣來源，顯示泥貫入體是主要熱成熟氣體從深部地層移棲到海床之主要通道。從TowCam及ROV拍攝之海床照片，顯示在海床上有大量之化學自營性生物群落聚集(貝類、菌叢)及自生碳酸鹽礁生成；並發現有許多之逸氣通道(gas seep)、麻坑(pockmark)、噴氣柱(gas plume)及活躍泥火山噴發，以及在泥火山上部之水層有高甲烷濃度異常，因此在研究區內具有相當高之甲烷通量。
高屏上部斜坡的地形主要受控於泥貫入體的發育，泥貫入體的侵入造成一系列的構造高區及斜坡盆地在這些構造高區間形成。由於泥貫入體的抬升，造成峽谷路徑僅能在構造高區之側邊或兩構造高區之間發育，因此高屏峽谷及枋寮峽谷之地形及路徑主要受控於泥貫入體的入侵。
In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ~1,530 km long MCS profiles, multibeam bathymetric data, and deep-towed sidescan sonar images and sub-bottom profilers. Our results show ten quasi-linear mud diapirs, oriented NNE-SSW to N-S directions. Most of the diapiric structures are covered by a layer of young sediments, but some have pierced through thick sediments and exposed to the seafloor, such as the Shell-Tomb Ridge, the south pinnacle of MD5, the Fangliao Ridge (MD8) and Xiaoliuchiu islet (MD2). The disturbance of sedimentary strata on both sides and top of the diapiric structures suggests that the diapirism is active. However, MD10 is probably no more active because modern sedimentary strata deposited above the diapir are undisturbed.
Thirteen mud volcanoes are identified from the multibeam bathymetric data off SW Taiwan. These mud volcanoes generally occur on tops of the diapiric structures. The mud volcanoes are typically cone-shaped, except MV6 which is more elongated. The heights of the cone-shaped mud volcanoes range from 65 m to 345 m, and the diameters at base from 680 m to 4,100 m. These mud volcanoes have very steep slopes (from 5.3° to 13.6°). We suggest that the mud volcanoes are fed by high-viscosity flows. The 10 mud volcanoes (except MV8, MV11 and MV12) with steep cone-shaped craters could be produced by higher viscous flows. In contrast, 3 mud volcanoes (MV8, MV11 and MV12) could be fed by relatively low viscosity flows, resulting in the flat volcanic edifice on the crest.
For the first time, a ROV observation was conducted to observe the mud volcanoes off SW Taiwan in 2011. The results of ROV observations reveal three mud volcanoes (MV1, MV5 and MV12) with active eruptions. The eruption cycles of MV1 and MV5 are about 3~5 minutes and 5~8 minutes, respectively. However, the MV12 is characterized by continuous outpouring of mud together with gas plume (the cycle is about 3~10 seconds). Two modes of eruption (explosive and effusive eruptions) can be identified. The MV1 and MV5 are characterized by explosive eruption, indicating that they could be formed by relative lower-permeability feeder, which can generate excessive pore fluid pressure beneath the mud volcano. The MV12 shows an effusive eruption type is related to high permeability conduit facilitates fluid flow and leads to more effusive eruption. In terms of size, the MV9~MV13 (diameters of 2,800 m to 4,100 m) located at deeper water depths are rather large than the other MV1~MV8 (diameters of 680 m to 2,200 m) located at shallower water depths. In addition, the effusive eruption of MV12 (diameter ~4,100 m) is rather larger than the explosive eruptions of MV1 (diameter of ~1,250 m) and MV5 (diameter of ~2,200 m). The larger cone-shaped mud volcanoes located at deeper water depths could be related to a longer eruption history. However, the large mud volcano is also common associated with effusive eruption. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), imaged by the high backscatter intensity bands from the sidescan sonar images. In general, the mud volcanoes in the continental slope off SW Taiwan are very active. The MV10 is the most active one because its edifice shows the steepest slope (~13.6°) and highest (345 m), and the only high methane concentration (200 nL/L) was detected in water column near sea surface above the mud volcano. The formation of mud diapirs and volcanoes in the study area are ascribed to the over-pressured fluid in sedimentary layers and compressional tectonic forces. Especially, the gas-bearing fluid plays an important role in enhancing the intrusion after the diapirism.
In our study area, the gas source is characterized by a mixture of thermogenic and biogenic gases. As shown in the Fangliao Ridge (MD8), in the north end of MD5 areas, MV4 and MV11, the mud diapir is an efficient pathway for the vertical migration of thermogenic gas fluid from the deeper part of the sedimentary layers. Seafloor photos were taken by ROV observations and TowCam system, which shows the chemosynthetic communities (shells and bacteria mats) and authigenic carbonates are widely distributed on the seafloor. The gas seeps, pockmark, gas plumes and active mud volcanoes eruptions are observed in the study area. In addition, the anomalous high methane concentrations of seawater are detected in the water column above the mud volcanoes. Accordingly, we suggest the high methane fluxes in the study area.
The morphology of the upper Kaoping Slope is mainly controlled by mud diapiric intrusions. The mud diapiric intrusions have resulted in a series of structural highs and three slope basins are formed between the structural highs. Due to the uplifting diapirs the canyon courses have developed along the flanks of structural high or between structural highs. We suggest that the courses and morphologies of the Kaoping Canyon and Fangliao Canyon are strongly dominated by mud diapiric intrusions. |
