Seismic evidence for the α‐β quartz transition beneath Taiwan from Vp/Vs tomography

Abstract

Knowledge of the rock types and pressure‐temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation, erosion and earthquakes. Seismic‐wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the P‐T conditions at which they exist. But laboratory‐measured ratios of velocities of P to S waves (Vp/Vs) have been shown to be effective. Results of 3‐D Vp and Vp/Vs tomographic imaging based on dense seismic arrays in the highly seismic environment of Taiwan provides the first detailed Vp/Vs structures of the orogen. The sharp reduction in the observed Vp/Vs ratio in the felsic core of the mountain belts implies that theα‐β quartz transition temperature is reached at a mean depth of 24 ± 3 km. The transition temperature is estimated to be 750 ± 25°C at this depth, yielding an average thermal gradient of 30 ± 3°C/km.