CHARACTERISTICS OF IMBRICATE THRUST SYSTEM IN THE FOOTHILLS BELT, SOUTHWESTERN TAIWAN

In the southwestern Taiwan, the surface geology in the Foothills belt is mainly characterized by imbricate thrust system, which has been formed during the on-going orogeny since the Early Pleistocene. The major thrusts on the surface are parallel with the stratal boundary of the top of the Changchikeng Formation, the youngest formation of the Miocene, implying that the bedding plane in the middle or lower part of the Changchikeng Formation is an important slip surface for the major thrusts in the subsurface. The major thrusts cut through the boundaries between the younger strata to the south. On the other hand, the surface synclinal axes striking between the major thrusts are always terminated on the major thrusts.

We constructed several balanced cross sections across the Foothills belt to infer the subsurface features of the thrust system with its lateral variation in structural feature. The major thrusts and the strata in their hanging wall are steeply dipping toward the east but turn into synclinal structure between the major thrusts. In order to explain how the flat bedding-slip faults are raised into steep attitude, duplex structures are delineated in the cross sections for the interpretation. Most of the duplex structures are composed of horses involving strata older than the Changchikeng Formation, and the number of the horse accounts for displacement of the adjacent major thrust forelandward and tightness of the syncline above the duplex structure. In addition to the slip surface in the middle or lower part of the Changchikeng Formation, there are several blind slip surfaces and some of them extend westward into the coastal plain area. In the southern part of the study area, the duplex structures are not so well developed and thrust displacement decreases as well.

According to the balanced cross sections, the terminated synclinal axes on the major thrusts on the surface mainly reflect the subsurface structure in that the more horses in the duplex or the more displacement of thrust in the footwall of a major thrust would turn a syncline into an anticline on the surface once the cutoff of the anticlinal axis in the footwall shifts to the surface along the major thrust. Therefore, the surface synclinal axes terminated on the thrusts can be the manifestation of lateral variation in the duplex structure in the subsurface.