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

Anisotropy of magnetic susceptibility (AMS) was studied in three Plio-Pleis tocene turbiditic mudstone sequences accumulated in the foreland basin of s outhwestern Taiwan. These formations were incorporated in the front units o f 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 coll ected in 352 sites, with 251 sites allowing determination of a magnetic lin eation. NNE-SSW trends are predominant, minor N-S and NE-SW trends are pres ent. Magnetic lineations are widespread in the lower section where folds ar e tight, and scarce in the youngest sediments where folds are gentle. The s trong 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 confirme d by tectonic studies based on structural analysis and paleostress tensor r econstructions. The tectonic studies reveal a major WNW-ESE compression, wh ich provide orientations of compressive tectonic regimes consistent for res ulting the magnetic lineations. In contrast, the hypothesis of a sedimentar y origin can be ruled out in most cases, because the orientations of magnet ic lineations and those of depositional fabrics (paleocurrents, sediment su pply directions and even slumps) are oblique at a variety of angles. Furthe rmore, based on magnetostratigraphy, we conclude that this compression culm inated 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 approximate ly 1 and 2 Ma ago, respectively. Thus, the main cause of AMS trend is thoug ht to be the WNW-ESE Quaternary compression responsible for major folding a nd 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. H owever, it is still recognized but decreased after about 0.7 Ma ago to the south. This indicates that the WNW-ESE compression propagated southward bet ween 0.9 and 0.7 Ma ago, consistent with the migration of folding and thrus ting during the last Taiwan collision.