||The Taiwan island is located in the convergent zone between the Eurasian continental plate and the Philippine sea plate. Due to the plate collision, there rise a lot of high mountains on the island. The main mountain chains include the Central Range and the Hsuehshan Range. However, in this compression environment, there exist some basins in the southern portion of the Hsuehshan range. Several researches have been conducted to interpret the causes of this basin group. Due to lack of drilling and geophysical data, many ambiguities still exist about the structure and the development processes of these basins, especially the largest one, the Puli basin.|
In this study, we use the high-resolution shallow seismic reflection to explore the Puli basin. The targets are to figure out the forms of the Tertiary basement and the Quaternary formations of the basin, and basing on these to investigate the mechanisms which may generate the basin.
After combining many observations, we obtain the following conclusions: (1) The deepest basement in the Puli basin dose not exceed 600m, and is deeper in the western portion. The top 100m of the basin sediment is dominant with the gravel and the deeper part with the sand and the mud. (2) The velocity of the P wave in the Puli basin is 2200m/s, and the S wave, 950m/s. The velocity of P wave in the basement is 4300m/s, S wave is 2000m/s. Because of plenty of gravels, the velocity of S wave is relatively high, which induces strong groundrolls and significantly interferes the seismic field work. (3) The basement rock belongs to the Paileng Formation. It is revealed to have 2 anticlines and 1 syncline with the wavelength about 4km and the height about 1km. (4) The sediment layers within the basin are not flat, but with apparent dipping angles. Furthermore, the layers tilt more in deeper places, which implies continuous compression keeps occurring during the sedimentation processes. The upper gravel layer is nearly horizontal, which may show a stronger erosion control by the surface drainage system. (5) There are not apparent faults discovered in the basin. (6) The mechanism of down warping may be dominant in the early stage of the basin, however, in the later stage it is controlled by the river erosion.
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