Preliminary result of magnetotelluric soundings in the fold-thrust belt of Taiwan and possible detection of dehydration

Until the magnetotelluric (MT) soundings were conducted in the last two years, there was no systematic view of the deep electric conductivity in the fold-thrust belt of the Taiwan orogen, being a typical example of arc-continental collision. Present in this paper are the initial results and preliminary interpretation of the MT survey in Taiwan. The determinant response of the impedance tensor was calculated for inversion and Occam's inversion which could generate smooth models was mainly used for interpreting MT data at each site. The existence of a conductive zone with depths of 10-20 km beneath the Island of Taiwan is undoubtedly the most important feature found in this study. The depths are well correlated with the inferred depth of dehydration reactions of Suppe (1981) and the top of the aseismic lower crust observed by Wang et al. (1994). Thus, the authors believe that the cause of this conductive zone is probably the fluids released from dehydration reactions. Additionally, close to the Lishan fault the MT data show strong distortions which may be the result of induction in more complex three-dimensional structures. Further multi-dimensional analysis of the data is required in the future for delineating a more realistic image of the subsurface heterogenous structure.