Stratigraphy and provenance of forearc sequences in the Lichi Melange, Coastal Range: Geological records of the active Taiwan arc-continent collision

摘要

The Lichi Melange in the Coastal Range, eastern Taiwan, is considered as sheared forearc sequences. However, the age of these sequences is still uncertain, and their significance for the arc-continent collision has been overlooked. Based on field surveys and micropaleontological analysis, four independent biostratigraphic units ranging from <8.5 to 3.0Ma are discerned in the Lichi Melange. These units are obviously older than the remnant forearc sequences in the east (3.4-1.2Ma), supporting the interpretation that the Lichi Melange arose from the shearing of the lower forearc basin sequences. The stratigraphy also suggests that the older forearc strata (<8.5-3.4Ma) were deformed and uplifted as a bathymetric high similar to the Huatung Ridge in the western North Luzon Trough by arcward back thrusting before 3.4Ma, while sedimentation continued in the remnant forearc basin in the east during 3.4-1.2Ma. The older forearc strata possess vitrinite reflectance values even lower than those of the remnant forearc sequences, which also supports that they were uplifted by back thrusting and therefore did not experience significant burial. Neodymium isotope analysis shows that the <8.5-6.4Ma forearc sediments were sourced from both the volcanic arc and the accretionary prism, indicating the emergence of the accretionary prism within <8.5-6.4Ma. It might have resulted from the underplating of thinned continental crust and provides a good time constraint for the onset of arc-continent collision. The post 6.4Ma forearc sediments were mainly derived from the accretionary prism, reflecting its constant uplift and large-scale exposure.