dc.description.abstract | The Sani fault is a low angle thrust fault distributed in the Maioli-Taichung area, northwestern Taiwan. The fault trends in a northward direction from Fengyuan, across the Tachiahsi stream, the Houli terrace, the TaAnhsi stream into the Miaoli hills. After this, the fault trace changes to the east near the town of Sani and stops near the town of Tahu. The previous studies point out that the Sani fault is a low angle overthrust fault which slides along the bottom of the Miocene Tungkeng formation by more than 10 km. The old structures thrust above the young structures which forms a tongue-shape intrusion, cutting through the southern flank of the Chuhuangkeng anticline. The Sani fault is covered by a thick laterite and gravel layer on the Houli terrace. Hence, it is recognized as an old fault, and may not be active now. However recently, the Central Geological Survey (CGS) bases on an outcrop along the Tachiahsi stream, claims that the fault may be quite young. The problem of the activity of the Sani fault seems still unsolved.
The purpose of this study is to re-examine the Sani fault and to determine the degree of its activity. The method of the shallow seismic reflection is used for the fault detection. Besides finding accurate surface positions of the fault, we also carefully examine the patterns of faulting in order to understand its characteristics. More than 10 seismic lines are deployed form the north to the south across the fault. The quality of the obtained seismic profiles is considerably good. We can easily identify the structures dipping to the east with the Tungkeng Formation thrusting over the young Toukoshan Formation. The following conclusions can be drawn:
1.The hanging wall of the Sani fault is the Tungkeng formation which dips to the east by 20o-25o. The footwall is the Toukoshan formation which is relatively flat.
2.There exist several secondary fractures in the Tungkeng formation, which sometimes are large and may extend to the surface. However, the thrusting dose not affect the Toukoshan formation much. No obvious fractures in the Toukoshan formation are found.
3.The behaviors of the Sani fault can be explained by the theory of thin-skinned thrust. The sliding fault plane is along the bottom of the Tungkeng formation.
4.According to the drilling data and the geophysical results, we find that some front parts of the Tungkeng formation have been eroded and replaced by terrace deposits. This strongly implies that the fault may have certain ages and could not be active at the present time.
5.The fault line is found to move to the east by several hundred meters on some deep deposited river bank. It also indicates that the SanI fault has no activity for a long time. The possibility of fault’s reactivity could be quite low. | en_US |