Structure and tectonic evolution of Taiwan

Abstract

Although Taiwan is one link in the chain of island arcs in the West Pacific, the concave side of the Taiwan arc faces toward the Pacific, opposite all the other arcs of this chain. Structurally, Taiwan is a typical mountain range, which was formed from a preexisting geosyncline of Tertiary age. The pre-Tertiary history of Taiwan is little known, as most of the older rocks are strongly deformed and meta- morphosed. Because of a few Permian fossils, they are believed to be Paleozoic in age. The Tertiary geosyncline formed probably during early Paleocene or latest Cretaceous (?), and a thick pile of monotonous argillaceous rocks was then deposited. Later the axis of this sedimentary trough migrated gTadually toward the west, and thick alter- nating beds of sandstone and shale were deposited. In the Early Pleistocene, the oro- genic paroxysm took place, and the Tertiary beds were folded into a mountain range. The Eastern Coastal Range is quite different from the rest of Taiwan and has its own peculiar structure and history. There was strong volcanism in that region dur- ing early Miocene time, and all later sediments seem to have been deposited in a highly unstable basin. Orogenic disturbance occurred there also in Early Pleistocene time, and the rocks were transported to their present position by gravitational gliding. Tectonically, Taiwan can be divided roughly into infrastructure, transition zone, and suprastructure, proceeding from the west side of the subduction zone in the Longitudinal Valley westward. The pre-Tertiary crystalline rocks belong to the in- frastructure and are characterized by flow folding. The Paleogene sequences belong to the transition zone, where shear folding predominates. The Neogene sequences belong to the suprastructure, and the folding is concentric or flexural-slip. Structurally, the Eastern Coastal Range is also suprastructure, but it is an independent tectomc province and can not be grouped with the province west of the subduction zone. The direction of curvature, the tectonic break of the Longitudinal Valley, Bouguer gravity anomaly data, seismic data, volcanic activity, and submarine topography all seem to indicate that there is an east-dipping subduction zone (or Benioff zone) along the Longitudinal Valley. Within the framework of plate tectonics. Taiwan is an ex- cellent example of a continent-arc collision structure, and the tectonic evolution of Taiwan is controlled by the underthrusting mechanism of the continental plate under the oceanic plate along the subduction zone of the Longitudinal Valley.