宜蘭平原南緣山區之電性構造;The geoelectrical structure of southern margin mountain area of the Ilan Plain

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题名:	宜蘭平原南緣山區之電性構造;The geoelectrical structure of southern margin mountain area of the Ilan Plain
作者:	賴楷軒;Lai,Kai-xuan
贡献者:	地球科學學系
关键词:	大地電磁法;電性構造;電阻率;相位張量;magnetotelluric;geoelectrical structure;resistivity;phase tensor
日期:	2014-06-24
上传时间:	2014-08-11 18:22:07 (UTC+8)
出版者:	國立中央大學
摘要:	宜蘭平原位在臺灣東北方，其淺部充滿來自內陸之沖積物，下方受到沖繩海槽的張裂活動影響，東南邊則有菲律賓海板塊與歐亞大陸板塊碰撞機制形成之琉球海溝，宜蘭平原南緣(24.62N)之菲律賓海板塊已隱沒至深度約50公里。為進一步了解宜蘭平原南緣山區之構造，本研究利用大地電磁法(MT)，收集範圍涵蓋宜蘭縣蘇澳鎮、冬山鄉與大同鄉(約121.66E~ 121.86E)等東西一線上之大地電磁資料，總計14個測站，剖線總長約20公里，藉以探討宜蘭平原南緣山區之電性構造。走向分析得出測區構造走向為N75E，與地質背景相符，維度分析發現臺灣東北部屬於三維電性構造，研判與海水效應及臺灣東北方之海底火山岩漿庫、隱沒帶等三維構造有密切關係。為滿足MT二維逆推條件，除了旋轉各測站至剖面之主軸外，同時降低TE資料的權重，以獲致最佳解析之二維剖面，深度可達30公里。宜蘭平原南緣山區之二維電性剖面大致呈現深度7公里下屬低電阻率構造，包括四處電阻率異常，由淺而深依序為R1、C1、C2與C3：(1)剖面東側深度10至15公里之低電阻率異常(C1)，其主要成因為高孔隙相互連通之含鹽度流體與富集黏土礦物；(2)深度15至20公里的低電阻帶(C2)，從其低震帶研判認為此區域與含鹽度流體與富集黏土礦物有關聯；(3)深度20公里以下的西側低電阻率異常(C3)，對應高VP/VS比值、低震區，此異常區之岩層可能蘊藏豐富的孔隙流體(0.43~1.26 %)，推估此異常區除了原先富集於此的地層水之外，亦含有來自下部地殼因變質作用釋出的結晶水及板塊隱沒碰撞所釋放的殼內流體釋，流體的存在將造成礦物熔點下降，恐促使岩石礦物開始發生部份熔融現象，因此，呈現C3低電阻率；(4)深度7公里之寒溪以東的高電阻率物質(R1)，根據低磁力異常現象，排除火成岩之可能性，推斷其可能為變質岩，由於其位在脊樑山脈北段，屬變質岩層區，變質岩孔隙率低，較為緻密，電性反映上呈現高電阻率，比對高VP、高VS與低VP/ VS比值，研判該高電阻率異常與中新世中期的厚層板岩有密切關係。 ;The Ilan plain located in the northeast of Taiwan is filled with alluvial from the inland, and overlaid the activity rifting Okinawa Trough; to the southeast side of the Ilan plain there is Ryukyu trench caused by collision between the Philippine Sea plate and the Eurasian plate. The subducting Philippine Sea plate at Ilan plain (24.6N) is located at a depth of ~50 km. In order to understand the Ilan plain in details, especially the southern margin part of the plain, magnetotelluric method (MT) was employed to study the electrical structures by deploying a profile from east of Suao township to west of Chingshui geothermal areas. A number of 14 wide-banded (0.03 < period < 3,000 s) MT stations were collected, forming a profile of a length of ~20 km. Phase tensor analysis and induction arrows plots show the MT data set close to the 2-D dimensionality except stations close to the east side of the profile to be 3-D structure as they are close to the sea shore. The NLCG 2-D MT inversion algorithm of Rodi and Mackie (2001) was used to generate models of more than 30 km depth, by down weighting the TE mode data during the inversion so as to avoide the 3-D effect. There are four resistivity anomalies in the optimum inverted MT model, R1, C1, C2 and C3, from shallow to depth accordingly: (1) A low resistivity anomaly (C1) at depth from 10 to 15 km at the eastern side of the profile is observed, the main cause of the low resistivity anomaly is contributed to the interconnected salty water and filled with clay mineral. (2) Beneath C1, there is another low resistivity anomaly (C2) at depth about 15-20 km. C2 and C1 are quite close in space, thus, the main cause of the both could be the interconnected salty water and filled with clay mineral also. (3) The low resistivity anomaly (C3) at depth about 20 km at the west side of the profile is observed. Based on the information of high VP/ VS and low seismicity, it may conclude that where may be rich in pore fluids (about 0.43~1.26% as calculated from Archie’s law). As the collision between the Philippine Sea plate and the Eurasian plate, the lower crust of the subducting Philippine Sea plate prograde metamorphism, water from the dehydration via buoyancy would migrate and deposit to be as C3. (4) At shallow depth less than 7 km, a high resistivity anomaly is observed. Correlated with the magnetic anomaly to be of low, it is reasonable to rule out the possibility of igneous intrusions. Thus, it may suggest that this high resistivity anomalies may be metamorphic rocks based on the profile is located in the Backbone Range of northern Taiwan. Metamorphic rocks are dense and low porosity, response for high resistivity, high VP, high VS, and low VP/ VS infered that it may be dry, and dense thick layer of slate.
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