GEOMETRY AND KINEMATICS OF THRUST FRONT IN FOOTHILLS BELT, SW TAIWAN

The Tachienshan-Chukou thrust is the major geological structure in the thrust front of the fold-and-thrust belt, with vergence toward the west, in southwestern Taiwan. The thickness of the Upper Miocene and Lower Pliocene in the hanging wall of the thrust is as twice as that in the footwall. It has been believed that the subsurface part of the thrust is highly related to a major earthquake happened in 1998. The northern extending part of the thrust is the disastrous 921 Earthquake fault happened in 1999. In front of the thrust, the frontal thrust, the Chiuchiungkeng fault with less magnitude of displacement, runs parallel with the Tachienshan-Chukou thrust. Below both the thrusts are regarded as the Tachienshan-Chukou thrust system. In the foreland area of the foothills belt stands the pre-orogenic basement massif, which is characterized mainly by several pre-existing normal faults. Several balanced cross-sections across the entire foothills belt were constructed by using surface geological data, as well as seismic and drilled well data, to describe the lateral variation in subsurface geometry and to analyze the kinematics of the thrust system. The subsurface geometry of the Tachienshan-Chukou thrust varies from the north to the south, depending on the magnitude of the displacement along the frontal thrust. In the northern part, greater displacement along the frontal thrust formed the lower angle Tachienshan thrust and wider hanging wall syncline. The regional of the top of the Lower Pliocene across the major thrust indicates that there exists a normal fault coincident with a frontal ramp behind the syncline. In the southern part where displacement along the frontal thrust decreases and even almost disappears the Chukou thrust appears a high angle fault. The variations in structural features along strike of the thrust system suggests that during its propagation toward the foreland the Tachienshan-Chukou thrust originally was a low angle thrust before it met a high angle normal fault which then was reactivated and used as the frontal ramp of the thrust. In the next stage, the frontal thrust developed from the middle of the high angle thrust and its displacement gradient influenced the final subsurface geometry of the major thrust.