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

Abstract

The Lichi Mélange 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 3.0 Ma are discerned in the Lichi Mélange. These units are obviously older than the remnant forearc sequences in the east (3.4–1.2 Ma), supporting the interpretation that the Lichi Mélange arose from the shearing of the lower forearc basin sequences. The stratigraphy also suggests that the older forearc strata (<8.5–3.4 Ma) were deformed and uplifted as a bathymetric high similar to the Huatung Ridge in the western North Luzon Trough by arcward back thrusting before 3.4 Ma, while sedimentation continued in the remnant forearc basin in the east during 3.4–1.2 Ma. 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 <8.5–6.4 Ma forearc sediments were sourced from both the volcanic arc and the accretionary prism, indicating the emergence of the accretionary prism within <8.5–6.4 Ma. 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 6.4 Ma forearc sediments were mainly derived from the accretionary prism, reflecting its constant uplift and large‐scale exposure.

Geochemical and Geochronological Constraints on the Origin and Emplacement of the East Taiwan Ophiolite

“…The western flank of the central southern Coastal Range features the late Miocene to Pliocene Lichi Mélange (Barrier & Muller, ; Chang, , ; W. H. Chen et al, ; Chi et al, ; Huang et al, , , ). The Lichi Mélange, which is exposed extensively along the eastern side of the Longitudinal Valley, contains abundant exotic blocks of various sedimentary rocks, volcanic arc units, and mafic and ultramafic bodies.…”

“…Also, the blocks of sheared sedimentary units are older than the remnant forearc sequences (e.g., Huang et al, ). Seismic profiles across the North Luzon Trough forearc basin off southeastern Taiwan, structural data, vitrinite reflectance data, and Nd isotope compositions all indicate that the Lichi Mélange is not coeval with Takangkou Formation (W. H. Chen et al, ; Huang et al, ). The third model for the Liche Mélange interprets it as generated by shearing of the retro wedge thrust system of the collision involving lower forearc sequences.…”

“…The third model for the Liche Mélange interprets it as generated by shearing of the retro wedge thrust system of the collision involving lower forearc sequences. The mélange continues southward to the Huatung Ridge off southeastern Taiwan and is therefore part of the deformed North Luzon Trough forearc sequences (Chang et al, , ; W. H. Chen et al, ; Huang et al, , ).…”

“…The latter model is based on seismic reflection profiles and along‐strike tectonic reconstructions that depict the ETO as a dismembered SSZ‐type ophiolitic block originating as forearc basement, from the Huatung basin or from the Philippine Sea Plate. The mélange blocks would then be fragments of the western edge of the Philippine Sea Plate that is being emplaced by arc‐ward back‐thrusting during arc‐continent collision (Chang et al, , ; W. H. Chen et al, ; Huang et al, , ).…”

“…Yet until now, there is no confirmation that ophiolitic rocks in the accretionary prism match with those of the ETO. Later studies demonstrate that these stratified sedimentary rock occurrences are limited in their lateral extent, and that they represent exotic blocks of sedimentary broken formation, that were likely forearc basement incorporated into the mélange by two tectonic thrusting events during arc‐continent collision (Chang et al, , ; W. H. Chen et al, ; Huang et al, , ). The retrowedge evolution model is proposed to reconcile conflicting interpretations of the olistostrome model and the tectonic collision model (Chi et al, ; Malavieille et al, ).…”