| 摘要: | 由於菲律賓海板塊向西北隱沒到台灣島北部及其外海，因此台灣北部海域的地體架構屬於造山運動後的垮山機制中，從擠壓環境改變為伸張環境，因而形成一系列東北-西南走向的正斷層構造或是原先逆斷層反轉為正斷層的地質構造。在台灣貢寮地區有龍洞斷層、蚊子坑斷層、澳底斷層及枋腳斷層的逆斷層分佈，這些斷層分佈至海岸線，似乎與往海域延伸上的地質構造有相關性。為了要了解此區域陸上斷層往海域延伸的可能性，以及延伸到海域的斷層是否為活動斷層，我們選用適合淺海域作業的火花放電反射震測系統，針對貢寮外海地區進行調查。
我們利用震測剖面中層序地層、震測相及斷層判釋來分析研究區域的地質構造。根據層序地層可辨認出海進面、末次冰盛期不整合面及末次冰盛期前的最大海泛面，除此之外還定義出砂波基底面、同張裂不整合面與聲學基盤面。在震測相中可分為五種類別，分別與沉積環境的改變相關。在斷層分析方面，找到五處明顯的正斷層(分別為斷層a、b、c、d和e)，分佈於陸上斷層的延伸處，斷層僅有幾處有出露海床的情形，但皆有切穿基盤並造成基盤的起伏落差，形成一些半地塹盆地。斷層a為較活躍的生長斷層，在斷層a所形成的半地塹盆地A內，沉積物呈現傾斜樣貌，推測是斷層持續活動所導致。斷層b和斷層c所形成的半地塹盆地B內有兩特徵，第一是末次冰盛期不整合面以下的地層呈現傾斜狀，末次冰盛期不整合面以上的地層較平坦，第二是只有在半地塹盆地B上部的沉積物堆積形式與海水面變化相關，因此認為斷層b和c在末次冰盛期以前較活躍，但這兩個斷層在幾處仍有出露的情形，推測末次冰盛期後斷層活動趨緩，但仍有在活動。斷層d和e因被厚層沉積物覆蓋，推測為早期構造，現在已不活躍。
;The offshore of northern Taiwan is subject to post-collisional collapse and under an extensional regime. Because of the change from a compressional environment to an extensional environment, a series of normal fault structures has been formed. The reverse faults in the Gongliao area of Taiwan, include the Longdong Fault, Wentzukeng Fault, Aodi Fault, and Fangjiao Fault. Those faults are distributed to the coastline and may extend to the offshore area. In order to understand the situation of the fault extension to the offshore area and the possibility of fault activity. We conducted Sparker reflection seismic surveys in the offshore area of Gongliao. The Sparker seismic system is suitable for shallow water surveys and can provide high-resolution shallow structures.
To understand the geological structures of our study area, sequence stratigraphy, seismic facies and faults identification are used to analyze our seismic profiles. According to the sequence stratigraphy, the transgressive surface, the last glacial maximum (LGM) unconformity, and last maximum flooding surface can be identified. In addition, the sand wave base surface, syn-rift unconformity and the acoustic basement are also defined. For next part, there are 5 seismic facies correlated to the different depositional environments.
We have identified 5 normal faults (i.e. Fa, Fb, Fc, Fd and Fe) in the prolongation of the onshore faults. All the identified faults cut through the basement and caused large offsets, forming half-graben basins. Some faults have caused offset of the strata near the seafloor. For the activity of these faults, the strata in the half-graben basin A formed by Fault a, is tilted, which was probably caused by the continuous growth of the fault. The strata in the half-graben basin B formed by Faults b and c, are inclined below the LGM unconformity, but the strata above the LGM unconformity are relatively flat. Otherwise, only the sediments in the upper part of the half-graben basin B show the characteristics of sequence stratigraphy, which means the sediments deposit controlling by sea level change. In summary, we infer that the faults b and c were active before the LGM and relatively stable recently. For faults d and e, because they were covered by thick sediment layers, indicated early structures and not active at all. |
