Evidence for Early Cretaceous oceanic crust trapped in the Philippine Sea Plate

The Huatung Basin is a small oceanic basin located east of Taiwan. Previous age estimates from magnetic lineation studies indicated an Eocene age for the basin, and formation from the Central Basin Spreading Center of the West Philippine Basin during the last phase of spreading in Middle Eocene. New Ar/Ar ages obtained on gabbros dredged on oceanic basement highs of the Huatung Basin are Early Cretaceous. These old ages are consistent with Early Cretaceous ages determined on radiolarian assemblages from Lanyu Island (Luzon Arc). We have performed magnetic anomalies modeling for an Early Cretaceous oceanic crust. Our results are in good agreement with new Ar/Ar ages determinations. The best fit is indeed obtained with an opening of the Huatung Basin during the Early Cretaceous from 131 to 119 Ma, with a half spreading rate varying between 25 and 30 mm/yr. The spreading center appears to be located south of the actual basin. The abnormal depth (5500 m instead of 5900 m) and thickness (∼12 km instead of 6 km) of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation. We propose that the basin is a fragment of the former ‘proto-south China Sea’ or possibly the ‘New Guinea Basin’ that has been trapped by the Philippine Sea Plate.