From convergent plate margin to arc-continent collision: formation of the Kenting Mélange, southern Taiwan

The Kenting Mélange on the Hengchun Peninsula, Taiwan, formed through tectonic shearing of subduction complex lithologies, probably within the plate boundary subduction channel between the Eurasian and Philippine Sea plates, with further deformation and exhumation in the Pliocene-Pleistocene during arc-continent collision. Field relations reveal a structural gradation from normal stratified turbidite sequence (Mutan Formation) through broken formation to highly sheared Kenting Mélange containing allochthonous polygenic blocks. This gradation is consistent with an increase of average vitrinite reflection values from ~ 0.72% in the Mutan Formation through ~ 0.93% in the broken formation to ~ 0.99% in the mélange, suggesting temperatures of at least 140 °C during formation of the Kenting Mélange. Zircons from gabbro in the Kenting Mélange are dated as 25.46 ± 0.18 Ma, which together with geochemical data constrains the source to South China Sea oceanic lithosphere. In combination with the field relationships, vitrinite reflectance values, microfossil stratigraphy and offshore geophysical data from S and SE Taiwan, we propose that the Kenting Mélange initially formed at the subduction plate boundary from offscraped trench deposits. Minor Plio-Pleistocene microfossils (< 5%) occur within the mélange in proximity to slope basin of equivalent age and were likely sheared into the mélange during out-of-sequence thrusting associated with active arc-continent collision, which in the Hengchun Peninsula commenced after 6.5 Ma.