Late-stage Deformation and Exhumation of the Blueschist-Facies Yuli Belt in the Taiwan Arc-Continent Collision

The Taiwan arc-continent collision is actively exhuming some of the world's youngest (~5 Ma, Sandmann et al., 2015) blueschist-facies rocks from depths over 50 km. Understanding rapid exhumation requires a knowledge of the geometry and kinematics of the structures that accommodated ascent, as well as the P-T-t during the deformation. Previous studies within the blueschist-facies Yuli Belt suggest three episodes of deformation, generalized as D₁, D₂ and D₃. D₁ and D₂ are considered to have formed as the result of subduction and subsequent collision between the Eurasian passive margin and the Luzon volcanic arc and are represented by fabrics, S₁ and S₂. D₃ is represented by a crenulation cleavage, S₃, a weak mineral lineation that plunges gently NE, and conspicuous, mineralized mode-1 fractures (veins), typically filled with quartz, calcite, albite and/or adularia. S₃ appears to be present throughout the Yuli Belt but is less well developed in the adjacent Mesozoic Tailuko Belt and forms a broad, gently NE plunging antiform. In X-Z sections, asymmetric structures are weakly developed and suggest both top-to-NE and top-to-SW. The veins (n=92) generally strike NW-SE, dip steeply and appear mildly deformed by S₃, suggesting development late in the formation of S₃. Microthermometry of fluid inclusions in euhedral crystals of quartz and adularia suggests entrapment temperatures up to 350°C. These conditions are also interpreted to apply to the late stages of S₃ development, s uggesting D₃ records the transition from ductile to brittle deformation. Ar⁴⁰/Ar³⁹ radiometric dating of adularia from similar veins along strike suggests crystallization ages ranging from 1.5 to 1.1 Ma (Wang, 1998, see Chen et al., 2019). We propose that the relatively low dipping S3 fabric developed as the Yuli Belt was exhumed along a subvertical material path, possibly represented by the left-lateral shear zone recognized in the Tailuko Formation (see Ho et al., this session). In this interpretation, ductile flow in the mantle and lower crust may have induced subvertical shortening in the overlying rocks as they reached the ductile-brittle transition. The S₃ antiform and veins may reflect a change in plate motion from regional-scale strike-slip to more orthogonal convergence 2 to 1 Ma (Wu et al., 2016; Byrne et al., this session).