Tomographic imaging of the convergent zone in Eastern Taiwan — A subducting forearc sliver revealed?

Abstract

This paper investigates crustal features of the plate boundary in eastern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. We found two crustal anomalies: (1) two prominent high-velocity/high Poisson's ratio anomalies in the mid- to lower crust beneath the offshore area; (2) several volumes of relatively high-velocity/high Poisson's ratio rocks in the upper- to mid-crust beneath the Central Range. The former is interpreted as the Luzon arc and forearc blocks from east to west. The latter is interpreted as uplift material from the oceanic crust scraped from the Luzon forearc, which forms the core of the Central Range. The features of northward-narrowing in dimension and deepening in depth of the forearc block are revealed in the velocity solution, implying a subducting Luzon forearc fragment in eastern Taiwan. The values of Poisson's ratios and earthquake activity in the northern domain are generally larger than that in the southern domain, suggesting that the crustal accretion process has not been completed in the northern domain of the study area. Our resulting velocity and Poisson's ratio models provide some insight into the tectonic processes presently operating in eastern Taiwan. It's also suggested that the subducting Luzon forarc acts as a backstop against which accreted rocks are thrust, and this contact may have seismogenic potential.