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In this study, we used the electrical resistivity method for imaging subsurface structures of lake-bottom sediment in Da-Hu Lake in Ilan, Taiwan. Floating passive electrodes were used for the surveys in the study area. We deployed eighteen survey lines across the lake, and executed the survey with the Schlumberger and Wenner array. There are two geological cores, DH-7A and DH-7B, located at different locations in Da-Hu Lake. The cores were carefully collected for the purpose of the sediment and dating analyses, as well as for the geochemical testing, which can be useful to climate-change studies covering the past 10,000 years. The inverted images clarify the spatial relationships between the sediment and basement structures. The sediments above the resistive rock basement consisted of the sand sediments (from 45 to 60 Ω m) and mud sediments (less than 30 Ω m). The results of our study show that a sandy layer comprising slate debris with a resistivity of 45 to 60 Ω m is located 3 to 8 m under the water's surface in the eastern part of the lake, and the outcomes confirm the findings from the DH-7A core data. The resistivity spatial distribution suggests that the lake's sand layer originally came from a region east of the lake. We also found a sharp linear resistivity structure in the western part of the lake. The structure's strike is consistent with observations from the normal fault's surface outcrops near the western part of the lake. Consequently, we infer that this structure line might be a stretch of the normal fault and that the two cores are located at the hanging wall as well. In addition, the results show that we can use the electrical resistivity imaging method with floating passive electrodes to investigate the lake-bottom in detail.

We present the results of determinations of the ellipsoids of magnetic susceptibility anisotropy and of the paleomagnetic directions for 17 sedimentary rock sites of the northeastern collision belt of Taiwan. The results of magnetic fabric determinations indicate that magnetic lineations have developed in most sites. Two major groups of lineations have been reconstructed: the trends are distributed around the NNE-SSW direction for the first one and around the ENE-WSW direction for the second one. Comparing these directions with the results of tectonic analysis of fault-slip data in the same area (Angelier et al., 1990), the first group of magnetic lineation is found to be perpendicular to the maximum compression axis σ1 of the compressional event number 3 (direction 113). This direction of major compression corresponds to the present-day convergent direction of the Philippine Sea plate relative to the Eurasian plate. The second group of magnetic lineations is perpendicular to another axis σ1 of a major compressional event number 4 (direction 150). Because no relationship between magnetic lineations and paleocurrent distributions has been found, we consider that the magnetic fabric in our samples principally results from the influence of tectonic paleostress. Our paleomagnetic results indicated reversed polarities in sampling sites. The declinations range from 190° to 220°, indicating that a clockwise rotation of at least 15° had occurred in the northeastern part of Taiwan. This supports previous results principally based on microtectonic analyses that led Angelier et al. (1986, 1990) to propose that a clockwise rotation of approximately 20° had occurred in the northern Taiwan belt. This clockwise rotation is probably related to the N-S opening of the Okinawa Trough as well as to the SE-NW compression induced by the collision between the northwestern tip of the Philippine Sea plate and the Eurasian plate in northern Taiwan.

Detailed studies of fracture patterns and paleostress reconstructions based on analyses of fault-slip data sets allow identification of four main patterns of compressional paleostress orientations in the Cenozoic rocks of northern Taiwan. These patterns correspond to five successive tectonic events dominated by (1) N-S, (2) WSW-ENE, (3) WNW-ESE, (4) NW-SE and (5) WNW-ESE directions of compression. The first two events are characterized by relatively minor structures in the foreland fold-and-thrust belt; the first event probably reflects major left-lateral shear along NNE-SSW trends which occurred in the adjacent eastern metamorphic belt during the middle-late Miocene. These first two events do not affect the Plio-Pleistocene in the localities studied. In contrast, the last three events created the major compressional structures of the foreland belt during the Plio-Pleistocene collision in Taiwan. Preliminary studies suggest that the magnetic fabric of the rocks has been significantly influenced by compressional deformation along WNW-ESE and NW-SE trends, especially in the Slate Belt, thus confirming the importance of these last three events. On the basis of paleomagnetic work in progress the northernmost segment of the Taiwan Belt has been affected by a clockwise rotation of about 20°. Taking into account the distribution of successive compressional paleostress patterns, we conclude that of the 55° angle of the sharply curved belt of northern Taiwan, about 20° probably correspond to actual clockwise rotation during the Plio-Pleistocene whereas the remaining 35° reflect the initial shape of the curved Plio-Quaternary belt. The paleostress history of northern Taiwan is examined in terms of geodynamic evolution and kinematic reconstructions. The Plio-Quaternary development of the collision between the Luzon Arc and Eurasia followed a late Miocene predominantly transform motion from south to north of the Philippine Sea Plate relative to Eurasia. The succession of Plio-Pleistocene paleostress patterns in northern Taiwan is found to be consistent with the longitudinal shift (from northeast to southwest) and the transverse migration (from the inner to the outer zones) of compressional phenomena related to the evolution of oblique collision. Some major changes in compressional trends are also accounted for by variations in the kinematics of plate convergence: the most recent charge comprises a counterclockwise change in the direction of Philippine Sea Plate-Eurasia convergence.