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 Wes t 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 Hu atung Basin are Early Cretaceous. These old ages are consistent with Early Cretaceous ages determined on radiolarian assemblages from Lanyu Island (Lu zon Are). We have performed magnetic anomalies modeling for an Early Cretac eous oceanic crust. Our results are in good agreement with new Ar/Ar ages d eterminations. The best fit is indeed obtained with an opening of the Huatu ng Basin during the Early Cretaceous from 131 to 119 Ma, with a half spread ing rate varying between 25 and 30 mm/yr. The spreading center appears to b e located south of the actual basin. The abnormal depth (5500 m instead of 5900 m) and thickness (similar to 12 km instead of 6 km) of the crust benea th the basin indicate that there was probably an excess supply of magma dur ing its formation. We propose that the basin is a fragment of the former 'p roto-south China Sea' or possibly the 'New Guinea Basin' that has been trap ped by the Philippine Sea Plate. (C) 2000 Elsevier Science B.V. All rights reserved.