清水斷層之淺部構造; Investigating the Shallow Structures around the Chinshui Fault, West-central Taiwan

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题名:	清水斷層之淺部構造;Investigating the Shallow Structures around the Chinshui Fault, West-central Taiwan
作者:	蕭瑞文;Jui-Wen Hsiao
贡献者:	地球物理研究所
关键词:	清水斷層;大肚台地;泰安斷層;Chinshui fault;Tatu terrace
日期:	2002-06-20
上传时间:	2009-09-22 09:53:59 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	論文提要內容：清水斷層位於台灣中部大肚台地之西緣，與南邊八卦台地西緣之彰化斷層及北邊后里台地西緣之大甲斷層，位置形態相當類似，其東邊之台地均為在厚約2公里之頭嵙山層內發生逆衝作用，使地層繞曲抬昇而形成，而逆衝前緣則發生斷層，故合稱為大甲–清水–彰化斷層。文獻資料指出此台地西緣之序列斷層較類似地下隱藏之「盲斷層」作用所產生（蕭寶宗, 1968; 張錫齡, 1971），此盲斷層引發之逆衝作用抬昇上述之三個台地，造成背斜，並在其西緣產生斷層，加速台地之上昇。本研究採用淺層反射震測法在清水斷層可能存在位置處施測，企圖找出該斷層的正確位置，並探討其錯動機制與特性。野外總共實施了五十多條測線，其中三分之二為短測線之廣域佈線，用以觀察淺部地層構造變化，瞭解背斜構造與斷層之關係。結果指出： (1) 清水斷層為深部泰安斷層所穿切而分為南北二段，北段連至大甲斷層系統（大甲至清水），南段連至彰化斷層系統（沙鹿至田中），此二段成雁行排列。 (2) 大甲–清水斷層東側之副斷層：鐵砧山斷層（或大甲東斷層及橫山斷層）雖為地調所（2000）歸類為第二類活動斷層，但本研究震測資料顯示這些斷層可能均不存在，其地下地層水平，地形上所見之變異係為台地抬升後，經古河道大量侵蝕所產生，為局部性之侵蝕溝而已。 (3) 從震測剖面看來，大肚台地西緣之地下地層並無錯動破裂現象，僅在斷層附近造成褶皺彎曲，形成背斜構造中較陡的西翼，故本研究認為斷層的破裂只發生在較深處位置，形成盲斷層形式，斷層彎曲褶皺（fault-bend-fold）的模式，可用以說明斷層與背斜發育之關係。 (4) 沿大肚台地西緣之地層彎曲具明顯的南北側向變化，彎曲量在南段由南往北遞減，但在北段則恢復向北遞增，此中間之段落，淺部彰化斷層之發育受深部泰安斷層的屏障作用，地層變化趨緩，近乎水平。 (5) 在地層彎曲處，由於近地表地層與較深部地層起伏一致，表示構造之活動至今尚未停止，清水斷層活動性仍應嚴加注意，不容忽視。 ABSTRACT The Chinshui fault is located at the western border of the Tatu terrace, west-central Taiwan. It is believed to be a continuous part of the Changhua fault extenting from the Pakua terrace in the south and also connecting to the Tachia fault developed along the Houli terrace in the north. These serial faults: the Tachia-Chinshui-Changhua fault represent the most western thrust fault system along the deformation front of western Taiwan foothill area. Photogeologic studies indicated that it is an active fault with apparent lineation and fault scarps. The seismic profiles provided by the petroleum company reveal that a standard ‘fault-bend fold’ may exist under the anticlines of the terraces. This study attempts to use the shallow reflection seismic method to map the shallow structures around the fault and to examine its relationship with the surrounding fold structures. This data may also be able to explore the generation mechanics of the terraces and the faults. Over 50 seismic lines are distributed along the boundary between the plane and the terrace. It is surprised to find that most seismic sections are composed of flat layers even inside the area of low-level terrace. The places where the sedimentary layers start to bend are very close to the boundary of the Toukoshan formation exposed at the high-level terrace. The smoothly bending layers on the seismic sections support the model of fault-bend-fold. The following conclusions can be drawn: 1.The Chinshui fault can be segmented into two parts: the southern and the northern portions by the Ta-An fault which bisects them in the middle, close to the town of Shalu. These two portions of faults form an en-echlon arrangement of the faults. 2.The accompanied back-thrust fault of the main Chinshui fault, i.e., the Teichenshan fault, Tachiatung fault and the Hungshan fault, may not exist. The underground seismic images of these faults are quite flat, not show a single bit of evidences of bending. Their fault-like shapes on the surface may be resulted from the erosion of the rivers and the drainages, not from the fault activity. 3.The bending of the structural layers near the fault indicates that they are of the fault-bend-fold type. A hidden ‘blind’ thrust may exist at deep places and above which arises the fold as a result of thrusting. The theory of the fault-bend-fold nicely explains the fault behavior along the fault. 4.Apparent fold angle variation occurs along the south-north direction of the fault. The degrees of folding decrease from the south to the north in the southern portion, but increase again toward the north in the northern portion. It seems that the layer variations are largely reduced in the middle where the Tai-An fault passes. This deep sited fault acts as a prohibitor which may have relaxed the fold stresses in this area. 5.In certain places, the layers near the surface have the same degrees of folding with the deep layers. This means that the process of the folding is quite young and its reactivity in the future should not be under estimated.
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