Surface and limited seismic data have been used to construct new cross sections across the southwestern Taiwan Fold and Thrust Belt. South of Chiayi, the best fit to the data is achieved if the detachment lies at a depth of 10 to 12 km, stepping up to about 6 km to the west. The Chukou fault and its extension to the south (Lunhou fault) are not the frontal structures in this area. Available data indicate the frontal structure may be composed of incipient reactivated normal faults in the north and triangle zones in the south. Published leveling data show that, whatever the structure, it is growing.