Geological Evolution and Subsurface Structure in the TaipeiBasin

Chen-Fong Yu

Abstract

The TaipeiBasin is floored with the deformed Tertiary strata filled with flat-lying late Quaternary fluvial and lacustrine loose sediments.These late Quaternary Sediments are the thickest on the northwestern border(about 700m within the Wuku area).

From the Oligocene until the early Pliocene,the Kuanyin Paleoland composed mainly of the quartzose sedimentary rocks and acidic igneous rocks uplifted in the present Taiwan Strait,which supplied the quartzose clastic materials toward the southeast.The Pliocene-Pleistocene Taiwan Orogeny due to the collision between the Eurasian plate and the Philippine Sea plate raised the northern Taiwan.The Taishan-Chinshan,Hsinchuang-Wuchihshan and Chingmei-Hsichih paleoranges were formed due to the fault activities of the Chinshan-Hsinchuang,Kanchiao and Taipei Faults in the late Pliocene.The pliocene-Pleistocene strataderived from these paleoranges were deposited in the Linkou area.

The andesitic and basaltic magma was intruded and erupted to form the Tatun Volcanoes during the late Pliocene to early Pleistocene.These andesitic and basaltic magma underneaththe Taipei area migrated toward the north and northwest,and the Kuanyin Volcano and a part of the Tatun Volcanoes were erupted during 0.4-0.3Ma.This caused the subsidence and normal faulting along the Shanchiao Fault,the Kungkuan Fault and the other faults of the Taipeiarea,forming the TaipeiBasin around 0.4-0.3Ma.

The geophysical surveys of the Taipei Basin,having an interpretation of the combination study of gravity result with the surface geologic feature of that area is given(Meng and Pan1963),and the seismic reflection surveys, Over the past 10 years, the Central Geological Survey (CGS) has conducted a wide-range of investigations of the Taipei basin by drilling over 30 wells (100 - 700 m). During the same period, we have also scanned the basin area ( 20× 20 km) using over 300 shallow reflection seismic lines. The purpose of this paper is to compile these seismic data, incorporating it with the borehole drilling results, to better describe the Tertiary basement, and the Quaternary layers above the basement including their respective P- and Swave velocities. It is found that 1) the deepest part of the basin basement is probably at the outlet of the TanshuiRiver, i.e., at the NW corner of the basin, 2) the Kanchiao fault forms a separation boundary bisecting the basin into a deep NW part and a shallow SE part, 3) the Sungshan and Chingmei Formations are relatively flat deposits at the top, which implies the existence of a comparatively quiet deposition period since about 100 thousand years ago, 4) the P- and S-wave velocities inside the basin are 1500 - 2200m s-1 and 170 - 880 m s-1, and 3000 m s-1 and 1500 m s-1 for the basement.

References

Chou,J.T.,2004:Geological structure and characteristics of subsurface formations of the Taipei basin in northern Taiwan.Western.Pac.Earth Sci.,4,1-24

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Wang,C.Y.,Y.H.Lee,M.L.Ger,and Y.L.Chen,2004:Investigating subsurface sturctures and P-and S-wave velocities in the Taipei basin.TAO,15,609-628

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Meng,C.Y.and Y.S.Pan,1963:A coordinated interpretation of the TaipeiBasin based on the geophysical,surface and subsurface geological data:MRSO Report.(in Chinese).

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