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 the-quartz transition temperature is reached at a mean depth of 24 3 km. The transition temperature is estimated to be 750 25C at this depth, yielding an average thermal gradient of 30 3C/km.