Rotations in the northeastern collision belt of Taiwan: Preliminary results from paleomagnetism

Abstract

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 σ 1 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 σ 1 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.