An analysis of the subsurface fault systems with audio-magnetotelluric surveys in the western Ilan Plain of NE Taiwan

Abstract

We used the audio magnetotelluric (AMT) method to delineate subsurface resistivity structures associated with the western Ilan Plain, which is emerged from the orogenic belt in northeastern Taiwan and is thought to be influenced by the opening Okinawa Trough. The most recent extension phase of the Okinawa Trough starting from approximately 0.1 Ma involved ENE- and WSW-trending normal faults off the Ilan Plain. Nevertheless, the region was originally a thrust-and-fold belt related to the subduction of the Philippine Sea Plate under the Eurasia Plate. Our study focuses on how the later opening of the Okinawa Trough has affected this previously compressional region. From the results of the AMT surveys, we found that the area features a series of N-S trending fracture zones in addition to the E-W normal fault systems. Comparing the AMT data with the data from seismic reflection profiles, we found that the normal fault systems are truncated by young strike-slip faults and modified the relief of the rock basement in a recent stage of formation. The western part of the study area is dominated by left-lateral strike-slip faults while the eastern part is dominated by right-lateral strike-slip faults. The findings drawn from our AMT resistivity images as well as from the reinterpreted seismic profiles constitute new evidence that the extrusion regime may have gradually taken over the regional stress state and reset the regional stress-strain conditions that was one dominated by the Okinawa extension in the western and southern part of the Ilan plain.