Lithospheric bulge in the West Taiwan Basin

ABSTRACT We use well data to investigate the timing and the origin of the lithospheric bulge in the West Taiwan Basin. The possibility that the subsidence patterns observed since Middle‐Upper Miocene are simply related to the flexural response of the Chinese continental margin to loading is examined by the reconstructions of the West Taiwan Basin evolution using two‐dimensional geometric and numerical flexural modelling of a purely elastic plate. Reconstructions of the forebulge and basin evolution since Middle Miocene are finally discussed in terms of plate strength and geological context. The results are finally placed in the framework of the geodynamic setting of the Philippines Sea Plate/Eurasia convergence in order to provide new insights on the early stage of the Taiwan arc‐continent collision. Modelling suggests that the initiation of the flexure in the West Taiwan Basin occurred at ca . 12.5–8.6 Ma. A good fit is obtained for T e of 10–20 km, consistent with earlier studies. During 5–6 m year −1 the growth of the bulge was static and associated with increasing plate curvature. Then, at 3–4 Ma the bulge migrated forelandward within the West Taiwan Basin in relation to the migration of the load and the increase in plate curvature. The passage of the forebulge into an inherited weaker portion of the Chinese margin produced an increase in plate curvature and renewed extension, leading to enhancement of the bulge uplift and to its localization for a prolonged period of time. Taking into account the age of the flexure initiation and plate convergence rates, we infer that the load might not be related to the arc‐continent collision. We conclude that a Middle Miocene obduction, already proposed by some authors, may explain the deflection of the Chinese margin at that time. It is not before 3–4 Ma that the bulge and the load propagated forelandward in association with the development of the Taiwan arc‐continent collision.