利用反射震測探討宜蘭平原之基盤深度及構造演化; Investigating the basement depth and tectonic evolution in Ilan Plain by seismic reflection method

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Title:	利用反射震測探討宜蘭平原之基盤深度及構造演化;Investigating the basement depth and tectonic evolution in Ilan Plain by seismic reflection method
Authors:	石政為;Cheng-Wei Shih
Contributors:	地球物理研究所
Keywords:	台灣東北部;反射震測;宜蘭平原;Ilan Plane;seismic reflection method;northeast Taiwan
Date:	2011-08-18
Issue Date:	2012-01-05 14:13:19 (UTC+8)
Abstract:	台灣東北部宜蘭平原底下的基盤面及地層構造是探討該地區地質構造重要的依據。根據近幾年來的地震觀測及GPS大地變形紀錄，反映出宜蘭平原南段受板塊擠壓並產生塊體順時針旋轉變形，此變形顯然與構造活動有關，但因平原上覆蓋大量沉積物，且並未發現斷層出露，地表下的詳細情形仍未能得知。本研究利用反射震測全面測繪宜蘭平原全區的地下構造，包括地層及斷層，並推測其可能的演化。本研究以「震盪震源車」進行高解析度反射震測。測線共可分為兩類：其一為南北向長達20公里的大剖面，用來和1976年中油江新春的震測結果做比對；其二為十數條分散各處的短測線，作為地層及構造的控制點。1976年的震測為3重合，測站間距20公尺，本研究則為30重合，4公尺間距，解析度提高許多，對基盤深度判斷更準確，且可看到許多精細的淺部構造。結果顯示基盤形貌與江新春（1976）測繪的差異頗大，大部分區域都淺約100?200公尺，北段（蘭陽溪以北）有平行山緣的大斜面，南段則深度淺於600公尺且大致平緩。基盤以上的第四紀地層最下部為礫石層，基盤面呈現破碎不平整，礫石層以上為砂泥層，二者界面即為江新春之A反射面，地層平整且均往最深處彎曲，代表平原形成過程，仍不斷受壓而向下撓曲，形成槽谷供砂石堆積。最上面400公尺為海進時期以來的堆積，砂泥層大致水平，僅微彎。震測剖面上出現五條東西向斷層（由南而北）：三星斷層、濁水斷層、坑四斷層、宜蘭斷層、鵠子斷層（即磨碧潭山斷層），其中僅有北端的鵠子斷層為正斷層。南端的三星斷層錯動切穿到達地表為一橫移斷層，且與現階段的線形地震活動一致，可能為活動斷層。蘭陽溪南側的濁水斷層為界限斷層，分隔中央山脈及雪山山脈地質區，為一橫移斷層拉張型花狀構造。另外，坑四斷層及宜蘭斷層為明顯的逆斷層，且以基盤下凹處為中心南北對稱，但兩者都只切穿到A反射面，未往上延伸，代表前段沈積時期的激烈壓縮作用。由此我們推論，現今宜蘭平原的形成係因中央山脈系統北移，受到雪山山脈系統的阻擋，壓縮宜蘭平原北段地下的雪山系統的基盤面，使之撓曲，南段的中央山脈系統亦朝向東脫逸，造成拉張型的平移斷層。因此，本研究認為現階段宜蘭平原受沖繩海槽弧後擴張的影響並不明顯，其形成仍以壓縮為主，僅局部受到拉張作用的影響。 Ilan plain is located at the northeastern Taiwan and is considered as a delta fan formed as a back-extension of Okinawa Trough. Although many faults were identified in the surrounding mountain, their features are unclear under the plain due to thick sediment deposits. In this study, we use the high-resolution seismic reflection method to depict the subsurface structure under the Ilan plain and to infer its possible tectonic evolution. A ‘Minivibe’ source and a 240-channel system are used for the survey. One big NS profile and several distributed short lines are obtained. The results are compared with CPC’s (Chiang, 1976) seismic work. The seismic profiles in 1976 are 3 folds and the receiver interval 20 m. However, this study used 30 folds and 4 m interval. It is expected to reveal much detailed structure features. The results are: 1) the shape of the Tertiary basement is quite different with Chiang’s result. It is shallower by 100~200m in most areas. The basement in the northern part (north of the Lanyang river) has a big slope trending in the NE direction, and in the southern part, it is shallower than 600m and much flat. 2) a gravel layer deposited above the basement with irregular basement bottom. Above it, there are several sand-mud layers which are relatively level. It is found that the layers bend in phase with the basement, which implies a down-warping process may prevail when these layers are deposited. 3) five faults are recognized on the seismic profiles. Among them, the south-most one, the Sansing fault, cuts through all deposit layers and is though as an active fault. The location of this tear fault also agrees with the present earthquake activity in the southern Ilan plain. The Zhuoshui fault could be a boundary between the Shueishan mountain and Central mounain geology provinces. The other two faults, the Kansu and Ilan faults, have apparent reverse fault type which can be related to the compression arising from the plate movement in the area.
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