Structure and motion of the southwestern Taiwan Fold and Thrust Belt

Surface and limited seismic data have been used to construct new cross sect ions 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 1 0 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 th is area. Available data indicate the frontal structure may be composed of i ncipient reactivated normal faults in the north and triangle zones in the s outh. Published leveling data show that, whatever the structure, it is grow ing. From north of Chiayi to the south the salient nature of the structures of t he Foothills province include: 1) a considerable amount of pre-Miocene stra ta are involved in the deformation. The involvement of pre-Miocene decrease s to the south, but the depth of the basal detachment does not change due t o the thickening of the Miocene and younger section; 2) the involvement of pre-existing normal faults and consequent 'basement' highs affecting the tr ajectory of thrust ramps abruptly ends south of Chiayi, separated by buried transverse faults from, 3) the thick foreland synorogenic stratigraphic se quence rides passively above the duplex wedge. Current seismicity appears t o have no correlation to the interpreted locations of faults, whether thrus t, normal or strike-slip. Preliminary GPS data from this portion of the fold and thrust belt indicate s, as first shown by Yu and Chen (1994), that structures are generally movi ng westward with respect to the Chinese craton. However, our data show abru pt changes in horizontal velocity, not all of which are associated with map ped faults. There is a region of high horizontal velocity and strain rate t o the east of the Chukou-Lunhou fault. On a finer scale, the velocities can vary greatly over distances of as little as 10 km. Some thrust sheets appe ar to be rotating counterclockwise around a vertical axis whereas others ap pear to be moving uniformly westward. Horizontal velocities are uniformly l ow in the Coastal Plain although they increase from west to east into the r egion of folding and blind thrusting in the footwall of the Chukou-Lunhou f ault. The eastward increasing of shortening velocity in the Foothills Belt implies that a common detachment exists at depth or thrusts in the east of the Chukou-Lunhou fault are also active or both.