A Neotectonic Model of Taiwan, with a Focus on the Longitudinal Valley Suture

Abstract

The disastrous effects of the 1999 Chi-Chi earthquake in Taiwan demonstrated an urgent need for better knowledge of the island's potential earthquake sources as well as their neotectonic context. Toward this end, we have utilized digital elevation models of the island to prepare a neotectonic map of Taiwan and proposed a neotectonic model for the orogen. Taiwan's numerous active faults and folds reveal that the active orogen is a tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin. The collision and suturing in the southern part of the orogen and the post-collisional collapse and extension in the island's northern and northeastern flanks have produced eleven distinct neotectonic domains. Each domain is defined by a distinctive suite of active structures. In most of the domains, the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid-7 range. In order to further understand the suturing processes, we have focused the second part of our investigation on the Longitudinal Valley suture in eastern Taiwan. The earthquakes of November 1951 within this suture constitute one of the most destructive seismic episodes in Taiwan's history. The surface ruptures of the earthquakes consist of three distinct sections, two of which are along segments of the Longitudinal Valley fault. From fluvial terraces along the Hsiukuluan River, we have reconstructed a shallow listric geometry for the Longitudinal Valley fault. On the other hand, many uplifted lateritic fluvial terraces along the eastern flank of the Central Range indicate the presence of a west-dipping Central Range reverse fault. We believe the majority of the horizontal shortening across the Longitudinal Valley suture is accommodated by the slip on the Longitudinal Valley fault. The remaining horizontal convergence may be absorbed by a combination of slip on the Central Range fault and subsidence of the Longitudinal Valley floor. The along-strike difference in geometry of the two major faults along the Longitudinal Valley is likely the manifestation of the northward maturation of the suturing of the Luzon volcanic arc to the Central Range continental sliver.