The Cenozoic tectonic evolution of offshore Taiwan

Offshore of Taiwan to the west and north many Paleogene and Neogene basins have been identified by seismic-reflection surveys and subsurface geologic data from drilled wells. A large and widespread unconformity exists in the upper Paleogene section. Paleogene basins are formed mainly by growth faults and subsidence, and are also slightly folded. Except for the Tainan Basin, which was formed by block faulting in the Taiwan Strait and which may be affected by the South China Sea Basin, the Neogene basins are a continuation of Paleogene basins. During late Cretaceous time, a strong orogenic movement folded and uplifted Cretaceous and older rocks throughout the offshore area and the island of Taiwan. After a period of erosion and volcanism, a shallow sea transgressed into the slightly warped areas north of the Penghu Uplift in late Paleocene time, and Cenozoic deposition was begun. A hiatus at the top of the upper Paleocene section indicates that a second movement occurred. Warping and block faulting formed the relatively deeper Eocene basins, which have a very thick argillaceous sequence. Early Oligocene orogenic movement deformed and uplifted the Eocene and older rocks and also stabilized the Paleogene basins to the west. Strong erosion removed almost all Upper Eocene rocks from the basin edges and structurally higher areas. Subsidence resumed again in offshore Taiwan during Late Oligocene time and the Neogene basins began to form. Uplift converted the entire offshore area to a lowland that received subaerial erosions from late Mid-Miocene time onward. The resulting stratigraphic hiatus is very large, ranging from Mid-Miocene to Pliocene or Pleistocene time in the west and gradually diminishing and disappearing eastward. A Plio-Pleistocene orogenic movement on the island of Taiwan strongly folded and thrusted the Cenozoic sequence in the areas from the foothills to the Central Range. Folding and thrusting diminishes in intensity from east to west, and the anticlinal structures along the coast in the offshore areas are very gentle.