dc.description.abstract | 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 (24.6N) 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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