Investigating deep structures in the northwestern Taipei basin

Chih-Yo Chien

Abstract

Taipei basin is located in the northern Taiwan. It’s a younger tectonic basin. Over the past ten years, the Central Geological Survey (CGS) has done numerous investigations by the borehole data. Our group has also used shallow reflection seismic combined with the borehole data, describing the subsurface of shallow Quaternary strata in the basin. We also got preliminary understanding of the Tertiary basement below the Quaternary strata. Taipei basin is a half-graben basin and the basement is deeper at the northwestern region. The seismic profiles obtained by using the EWG-Ⅲ impact source can’t control the deeper basement obviously in the past. So this investigation used the newly-bought sweep source “MiniVibe” to obtain the one-second successive profile with several kilometers in length. The sweep source can make large energy, wide frequency, and superior penetration. Therefore, we can explore the basement depth and individual strata efficiently.

The survey lines are concentrated on the northwestern basin, included four short and three long survey lines. The GuanDu line is about 2km in length and obtained by MiniVibe. Utilizing KT-1 and the angular unconformity on the profile, we can decide the basement depth. The depth matched with the profile obtained by the EWG-Ⅲ in the past, and the Tertiary strata below the basement can be recognized obviously in new profile. At WuKu-LuChow region, four short profiles surveyed by the EWG-Ⅲ. The signal of the profiles is distinct and we can decide several Quaternary strata. Owing to the surrounding is spacious we also surveyed by MiniVibe and obtained a high-quality successive profile (SuHon) with 5km in length. It shows the basement depth and structure distinctly. In addition, the long profile (HuaChung, 2km in length) of HuaChungBridge shows distinct unconformity and Tertiary structure.

This investigation obtains the following results: 1) confirming the deepest basement in the Taipei basin is not over 700m. (The basement depth is 679m in WK-1.) The basement depth at Wuku-LuChow region keeps up about 600m in a wide range. 2) SanChiao fault is a normal fault and the hanging wall shows distinct drag caused by the basin depression. The dip angle of SanChiao fault is about 50-60 degree. 3) The dip angles of Quaternary strata indicate the faulting was active in the early formation, and becoming inactive in the past 10Ma. 4) The surface terrain reflects the variation of subsurface structure. This indicates the basin is still active. 5) The lithological characters of diverse strata below the basement influence the basement depth, and the KanChiao fault on the basin is also control the basement relief.