[S-IT20] New perspectives on accretion and exhumation processes during oblique plate convergence

[SIT20-06] A new interpretation of the metamorphic core in the Taiwan orogen: A regional-scale, left-lateral shear zone that accommodated highly oblique plate convergence in the Plio-Pleistocen

*Gong-ruei Ho¹, Timothy B Byrne^2,3, Jian-Cheng Lee¹ (1.Academia Sinica, Institute of Earth Sciences, 2.Department of Natural Resources and Environmental Studies, National Dong Hwa University, Hualien, Taiwan, 3.Department of Geosciences, University of Connecticut, Storrs, CT, United States)

Keywords:Tailuko belt, Yuli belt, Left-lateral shearing, Deformation fabrics, Ductile shear zone

The young and presently active Taiwan orogeny provides an excellent tectonic “fossil” for studying orogenic processes. In this study, we propose that structures preserved in the metamorphic core, primarily the Tailuko and Yuli Belts, record oblique, left-lateral shear, which was mainly driven by north to northwestward convergence of the Philippine Sea plate wrt the Eurasian plate. A penetrative foliation, S2, that includes mylonitic and gneissic fabrics, dips moderately WNW and is associated with a sub-horizontal stretching lineation, L2. Observations along 6 transects across the belt reveal hundreds of kinematic indicators including: asymmetric folds, strain fringes around pyrite or magnetite crystals, S-C fabrics, en échelon veins, R and R’ shears, sigmoidal core-and-mantle structures. In cases where the L2 asymmetry could be determined 80-90% yielded a left-lateral sense of shear. A synthesis of available data suggests that a previously unrecognized zone of strike-slip deformation exists in the Tailuko and Yuli Belts, and here we document: 1) the distribution of horizontal shear, 2) the kinematics of deformation, 3) the age of deformation and 4) regional consistency between geologic studies and plate reconstructions. We also present a 3D model and discuss the impact of horizontal shear in the shallow structural level of southern Taiwan (e.g., Taimali River and Hengchun Peninsula areas) where previously published kinematic data argue for CCW block rotations. Finally, we integrate these new field-based observations with previously published kinematic data from the nearly contemporaneous Slate Belt to the west that suggests plate convergence was partitioned with strike-slip motion in the retrowedge (i.e., the Tailuko Belt) and shortening in the prowedge (i.e., the Slate Belt).