臺灣台南外海正斷層以及近代沉積現象研究

A study on normal faults and recent sedimentary features offshore Tainan, Taiwan

莊介瑋(Chieh-wei Chuang)

指導教授 ： 林殿順

國立中央大學/地球科學學院/地球科學系/碩士(2014年)

摘要

台灣台南一帶平原區的構造主要為東西走向的正斷層，其中疑似活動斷層的義竹斷層為向南傾沒的正斷層並由台南陸上延伸至台灣海峽南端。本研究收集處理海研一號986及1033航次，總長約一千三百公里的多頻道反射震測以及海床底質剖面配合五口井測資料作為岩性、年代的控制，解釋台南外海之海域斷層分布、形貌以及活動性。震測資料顯示義竹斷層以南二十公里內發育密集的正斷層，這些斷層的斷距以及側向延續性遠小於義竹斷層並主要為東西走向。其中位於曾文溪出海口外的A1斷層為義竹斷層主要的反向逆斷層，該斷層為東西走向、向北傾斜並且切穿近海床地層，其他斷層的分布則集中於A1斷層與義竹斷層之間，此斷層空間分布與近代地震分布趨勢一致。此外，海床底質剖面分別顯示義竹斷層在海床上的斷層崖以及A1斷層具生長地層的特徵之上盤沉積物影像，顯示義竹斷層與A1斷層應為活動斷層。斷層帶往台灣一側逐漸停止活動，澎湖水道東側的斷層頂點大多終止於中期更新世的侵蝕面，顯示靠近台灣造山帶一側的斷層較早停止伸張滑移，而震源機制也顯示本區域以走向滑移解為主。
由海床底質剖面的變頻聲納的回聲特性分析，可以解析澎湖水道以及其兩側海床的堆積或侵蝕特徵。結果顯示澎湖水道地形起伏通常為近代侵蝕作用造成，與構造活動較無關係；水道東西兩側則以堆積為主。在曾文溪三角洲外海的堆積區中發現三組水道侵蝕現象，可能分別是發育於古曾文溪、現代曾文溪以及二仁溪的異重流侵蝕所形成，高解析反射震測顯示海床底下沒有斷層活動並且有水道往南側向移棲的沉積特徵。

關鍵字

台南盆地 ； 義竹斷層 ； 海域震測

並列摘要

Yichu fault which is a NE-SW trending, south dipping normal fault has been documented as a main structure offshore Tainan, southwestern Taiwan. This fault and faults nearby may be active suggested by seismic activity. High resolution seismic profiles currently indicate that densely active faults spreading about 20 km on the hanging wall of Yichu fault. Most of these faults are E-W trending with extensional displacement, penetrating the upper Pliocene strata at least. A1 fault is the most active antithetic fault, bounding the fault zoon in substance. On the other hand, sub-bottom profile provides fault scarp and growth-strata image upon the seafloor of Yichu fault and A1fault, respectively.
Focal mechanism shows strike-slip stress transition field between the N-S extensional stress in Taiwan strait to the west and the oblique compressional stress on Taiwan orogeny to the East. By the observation, faults cease below Pleistocene submarine canyon system in the Eastern Penghu channel, Taiwan strait. By contrast, in the Western Penghu channel, those fault shows active reaching seafloor almost. The difference of fault geometry may be the result of stress field transiting from extension to strike-slip region.

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