Active Deformation and Paleostress Analysis in the Pakua Anticline Area of Western Taiwan

Abstract

The active Quaternary deformation of the Pakua anticline area in the foreland belt of western Taiwan has been examined based on field investigation as well as paleostress analysis. The strata exposed in the Pakua area are composed of Quaternary molasse sediments of conglomerate and sand or sandstone (Toukoshan Formation). The seismic reflection profiles show that the main structure of this area is characterized by frontal thrust (the Changhua Fault) and the associated anticline fold (the Pakua anticline). Three principal deformation structures at outcrop scale have been recognized: (1) compressional fracture of pebbles, (2) strike-slip fault, and (3) normal fault. It indicates that the Pakua anticline area has experienced a complex history of tectonics including different stress regimes of contraction, strike-slip and extension. The study of paleostress shows: (1) compression in a WNW direction as deduced from both the fracturing of pebbles and strike-slip faults; and (2) the occurrence of E-W and N-S extension accommodated by normal faulting, especially in the northern part of the Pakua anticline area. The WNW compression, being perpendicular to the trend of the frontal thrust and the regional major folds, represents the regional compressional stress direction in the foreland of Taiwan. The normal faulting in this fold-and-thrust belt suggests that the importance of the force of uplift/gravitation resulted from the release of the frontal thrusting and/or the transtensional stress regime of possible regional transfer faults.