A wide distributed high strain shear zone in the metamorphic core of the Central Range, Taiwan: A possible Plio-Pleistocene transform fault

The arc-continent collision in Taiwan is traditionally interpreted as a relatively continuous process with an invariant plate convergence vector for at least the last 6-5 Mya (Suppe, 1984; Teng, 1996; Beyssac et al., 2006). This steady convergence and the obliquity between the subducting continental margin and the plate boundary predict a propagating collision tectonic setting with a space-time equivalence along the developing orogen. More recently, detailed low-temperature geochronologic studies (Hsu et al., 2016; Lee et al., 2015) and plate reconstructions (Wu et al., 2016) suggest that the plate convergence changed from highly oblique to nearly orthogonal in the last 2 to 1 Ma. Oblique convergence, driven primarily by the northward motion of the Philippine Sea Plate wrt Eurasian, predicts a significant component of left-lateral, strike-slip shearing along the north-trending plate boundary. A synthesis of available data helps constrain this previously unrecognized zone of strike-slip deformation and shows that: 1) the distribution of horizontal shear throughout the Tailuko Belt, 2) the kinematics of deformation, 3) the age of deformation and 4) regional consistency between geologic studies and plate reconstructions. 1) the Tailuko Belt, which ranges from 30-km in width and extends the 200-km length of the orogen, is characterized by a history subhorizontal shearing. Evidence for high strain includes: mylonitic fabrics, rotated fold axes, sheath folds and rotated stretching lineations. Horizontal shear may even be recorded at shallow structural levels in southern Taiwan by block rotations; 2) S-C fabrics, strain shadows in green and black schists, mylonitic gneisses and sigmoidal structures consistently indicate left-lateral shearing; and 3) ⁴⁰Ar/³⁹Ar ages of metamorphic biotite from an ultra-mylonite in the Tachoshui gneiss indicates the deformation occurred from 3.0 to 4.1 Ma (Wang et al., 1998). We therefore propose that the Tailuko Belt records significant strike-slip motion related to northward motion of the the Philippine Sea plate 4 to 3 Ma. This major zone of strike-slip deformation apparently became inactive as plate motion changed and around 1~2 Ma.