Active collision in eastern Taiwan: The Coastal Range

Abstract

A detailed analysis of Late Cenozoic faulting has been carried out in the Coastal Range of eastern Taiwan. 4000 measurements have been collected in 60 sites where faults affect formations that range from upper Miocene to Pleistocene and scattered Quaternary deposits. This large data set enables us to accurately determine paleostress axes in the area. During the major tectonic event (the Pleistocene Coastal Range orogeny), the direction of the maximum compressional stress σ 1 has remained constant. 25% of reconstructed σ 1 axes trend approximately 120; 90% trend between 090 and 140. Taking into account the internal deformation in the Coastal Range during folding and thrusting, these reconstructed directions of compression are remarkably homogeneous. Present faulting in the Longitudinal Valley is consistent with the paleostress pattern recorded in the adjacent Coastal Range formations. The Longitudinal Valley fault is a thrust, with a minor left-lateral strike-slip component (20% of the total movement). This quantitative estimate of motion is compatible with reconstructed Pleistocene paleostress and more general reconstructions of recent plate motion in the vicinity of Taiwan. The analysis of the focal mechanisms of earthquakes provides similar results. Finally, the analysis of the Coastal Range faults systems related to the collision of the volcanic Luzon arc (including the Luzon trough) with the Central Range of Taiwan provides a consistent picture of the distribution of stress patterns and deformations. The strikes of folds and thrusts in the Coastal Range are mainly controlled by the direction of convergent plate motion (125), whereas the overall trend of the Coastal Range is similar to that of the Central Range (020). The obliquity of convergent motion has resulted in the complex en échelon structure of the Coastal Range. The relative plate motion is not the sole factor controlling the development of a collision chain such as the Coastal Range; the existence of major pre-existing structures also plays an important role.