Yuli Belt in the eastern Taiwan orogen: a part of suture zone separating Eurasian and Philippine Sea plates

The Taiwan orogen formed as a consequence of the oblique subduction of the Eurasian continental margin below the Luzon volcanic arc of the Philippine Sea Plate since the late Miocene. The Yuli Belt of the eastern Taiwan Central Range, exposed in the retro-wedge of the fold-and-thrust belt, hosts slivers of a heterogeneous unit of blueschist-facies rocks that are among the youngest blueschist units worldwide. However, the palaeogeographic provenance of this unit is still debated. This is due to the fact that numerous structural aspects, including the kinematics of the Yuli Belt’s tectonic contacts with adjacent units, are improperly understood.

Our studies form part of an ongoing reinvestigation of the tectonic evolution of the Yuli Belt. A revised geological map of the Yuli Belt was generated, incorporating own structural data from several river transects. Fieldwork and microstructural analyses suggest that the Yuli Belt was polyphasely deformed. Based on newly constructed cross sections we suspect that the blueschist-facies units were tectonically emplaced along thrusts on top of a mostly greenschist-facies metasedimentary unit that locally exhibits characteristics of a mélange. Later, both blueschist-facies and metasedimentary units were tightly folded, likely during the emplacement of the Yuli Belt onto the westerly adjacent Eurasia-derived Tailuko Belt along the so-called Shoufeng Fault. Lithological and fabric transitions across this fault are gradual, suggesting that the juxtaposition of Yuli and Tailuko Belts occurred during an early W-directed transport direction before becoming refolded during later E-vergent backfolding. Peak metamorphic temperatures in the greenschist-facies metasediments, estimated by Raman spectroscopic analyses of carbonaceous material (RSCM), reveal systematic spatial variations across the Yuli belt, supporting the idea of an allochthonous nature of the blueschist units on top of the lower grade metasedimentary unit.

The incorporation of published geochronological and whole-rock geochemical data and their combination with own paleogeographic reconstructions led us to fundamentally reinterpret the structural position of the Yuli Belt. We suggest that the blueschist-facies unit most likely represents a mid-Miocene fragment of oceanic crust and mantle issued in the South China Sea before having been subducted, exhumed and ‘sandwiched’ between the (Eurasia-derived) Tailuko Belt and the easterly adjacent Coastal Ranges derived from the Philippine Sea plate. The Yuli Belt should hence be considered to contain the suture between the Eurasian and the Philippine Sea plates.