Two Contrasting Kinematic Styles of Active Folding Above Thrust Ramps, Western Taiwan

Abstract

Two adjacent active thrust ramps in western Taiwan show contrasting hanging-wall structural geometries that suggest different kinematics, although they involve the same stratigraphic section and basal detachment. The Chelungpu thrust shows a classic fault-bend folding geometry, which predicts folding by kink-band migration without limb rotation, whereas the hanging wall of the Changhua thrust shows the characteristic geometry of a shear fault-bend folding, which predicts a progressive limb rotation with minor kink-band migration. We test the kinematic predictions of classical and shear fault-bend folding theories by analyzing deformed flights of terraces and coseismic displacements in the 7.6 moment magnitude scale Chi-Chi earthquake. In particular, differences in terrace uplift across active axial surfaces are used to show that the assumptions of classical fault-bend folding are closely approximated, including constant fault-parallel displacement, implying conservation of bed length, and hanging-wall uplift rates that are proportional to the sine of the fault dip. This provides a basis for precise determination of total fault slip because the formation of each terrace, combined with terrace dating, gives long-term fault-slip rates for the Chelungpu thrust system. Even the coseismic displacements of 3 to 9 m (10 to 29 ft) in the Chi-Chi earthquake are approximately fault parallel but have additional transient components that are averaged out over the time scale of terrace deformation, which represents 10–100 large earthquakes. In contrast, terrace deformation in the hanging wall of the Changhua thrust ramp shows progressive limb rotation, as predicted from its shear fault-bend folding geometry, which combined with terrace dating allows an estimation of the long-term fault-slip rate of 21 mm/yr (0.83 in./yr) over the last 31 ka. A combined shortening rate of 37 mm/yr (1.46 in./yr) is obtained for this part of the western Taiwan thrust belt, which is about 45% of the total plate-tectonic shortening rate across Taiwan. The Changhua shear fault-bend fold ramp is in the early stages of its development with only 1.7 km (1.06 mi) total displacement, whereas the Chelungpu classical fault-bend fold ramp in the same stratigraphy has nearly an order of magnitude more displacement (14 km [8.7 mi]). We suggest that shear fault-bend folding may be favored mechanically at low displacement, whereas classical fault-bend folding would be favored at large displacement.