Tectonic significance of magnetic susceptibility fabrics in Plio-Quaternary mudstones of southwestern foothills, Taiwan

Abstract

Anisotropy of magnetic susceptibility (AMS) was studied in three Plio-Pleistocene turbiditic mudstone sequences accumulated in the foreland basin of southwestern Taiwan. These formations were incorporated in the front units of the collision belt and underwent folding and thrusting during the last 2 Ma. Five types of fabrics were identified from more than 3,000 samples collected in 352 sites, with 251 sites allowing determination of a magnetic lineation. NNE-SSW trends are predominant, minor N-S and NE-SW trends are present. Magnetic lineations are widespread in the lower section where folds are tight, and scarce in the youngest sediments where folds are gentle. The strong correlation between the structural features and the AMS orientations suggests a tectonic origin for most magnetic lineations superimposed on the initial flattening that results from sediment compaction. This is confirmed by tectonic studies based on structural analysis and paleostress tensor reconstructions. The tectonic studies reveal a major WNW-ESE compression, which provide orientations of compressive tectonic regimes consistent for resulting the magnetic lineations. In contrast, the hypothesis of a sedimentary origin can be ruled out in most cases, because the orientations of magnetic lineations and those of depositional fabrics (paleocurrents, sediment supply directions and even slumps) are oblique at a variety of angles. Furthermore, based on magnetostratigraphy, we conclude that this compression culminated about 0.9–1 Ma ago. Earlier minor events, NW-SE and W-E compression, have also been found and we propose that they have occurred in approximately 1 and 2 Ma ago, respectively. Thus, the main cause of AMS trend is thought to be the WNW-ESE Quaternary compression responsible for major folding and thrusting. In addition, the magnetic fabric of tectonic origin is absent, or poorly marked, in formations younger than about 0.9 Ma to the north. However, it is still recognized but decreased after about 0.7 Ma ago to the south. This indicates that the WNW-ESE compression propagated southward between 0.9 and 0.7 Ma ago, consistent with the migration of folding and thrusting during the last Taiwan collision. 527-538