NEW MAGNETOTELLURIC IMAGING OF THE ACTIVE COLLISION ZONE: TAIWAN ISLAND

Taiwan is located in a complex tectonically active region where the Philippine Sea plate is subducting northwestward along the Ryukyu Trench while the Eurasia plate is subducting eastward along the Manila Trench. During the past six years, National Central University (NCU) deployed more than 200 MT stations with MT-2000 instruments, Phoenix Geophysics, uniformly covering the entire Island of Taiwan. This dense MT sites provide one of the high-quality non-seismic information, and, thus, gives us a good opportunity to determine the resistivity structure of the crust and upper mantle structures beneath Taiwan. We have organized the all resistivity data, displayed as depth slices and vertical profiles, to infer the fine 3-D resistivity image beneath Taiwan. The result demonstrates the resistivity patterns of NNE-SSW strike, which is oriented in approximately the same direction as the main geological provinces of Taiwan. Moreover, the resistivity variations to be closely correlated to the change of the surface geology of Taiwan down, at best, to a shallow depth of the upper 20 km. Beneath this depth, there exists a regional zone of sharply-decreasing electrical resistivity (less than 150 ohm-m), which is probably caused by a structural zone with densely interconnected fluid phase. In other wards, the recent Taiwan Orogeny has formed an evident decollement structure in which the upper zone above the depth of 20 km exhibits entirely independent structural style from the lower one. MT observations seem to support the model of thin-skinned Taiwan orogeny. The thin-skinned Taiwan orogeny was supported by the geological mapping of the fold-and-thrust belt in western Taiwan and by exploration seismics, together with some drilling data.