Fission-Track Dating of Detrital Zircon from the Linkou Tableland, Northern Taiwan
Date Issued
2007
Date
2007
Author(s)
Shen, Tzu-Tsen
DOI
zh-TW
Abstract
In order to understand the depositional ages and the provenance of the sedimentary rocks of the Linkou Tableland in northern Taiwan, a total of 10 core samples were collected from the borehole drilled on the Linkou Tableland for fission-track dating (FTD) of zircon grains. The grain ages obtained range from 0.9 ± 0.6 to 245.6 ± 201.7 (1σ) Ma. Based on previous FT studies on northern Taiwan, the author showed that those ages clearly reflect the FT ages characteristics of the provenance of the Linkou Formation, i.e., volcanic terrain (less than 4 Ma), northern Hsuehshan Range (4 ~ 90 Ma) and Western Foothills (＞90 Ma).
The precision for the young ages is not good due to the scarcity of fission tracks. The youngest grain age of the sample collected from the deepest part of the borehole is 1.2 ± 1.8 (1σ) Ma, implying that the depositional age for the level of 130m below the ground surface is very likely ≦ 1.2 Ma. On the other hand, the zircon FT age from near the ground surface is 0.9 ± 0.6 (1σ) Ma. Therefore, the maximum depositional age of the uppermost 100m-thick gravel strata can be estimated at 1.2 ± 1.8 ~ 0.9 ± 0.6 (1σ) Ma. It is also inferred that the onset of the subsidence of the Taipei Basin due to the Shantzechiao normal fault is very probably around 0.9 ± 0.6 Ma, when the ancient Hsintein Stream and the Keelung River stopped to feed sediments to the Linkou fan delta.
The zircon grains which have FT ages between 4.1 ± 0.9 and 90.8 ± 18.4 (1σ) Ma are considered to be derived from the northern Hsuehshan Range. The grains which have ages of ＜~6 Ma are regarded as totally reset due to the Penglai Orogeny. Lag time was defined as the difference between the time of closure in the source region and time of deposition in the basin which is represented the processes of the exhumation. By subtracting the maximum depositional age of the Linkou Formation (~1.2 ± 1.8 (1σ) Ma) from each totally reset age, the lag time of the northern Hsuehshan Range was estimated at 5~3 m.y. With the assumption of a thermal gradient 30 ℃/km and a closure temperature of 240℃ for zircon FT system, the cooling and exhumation rates during the period of 6 ~ 1 Ma are calculated as 81.0 ~ 46.2 ℃/m.y. and 2.7 ~ 1.5 mm/yr, respectively. This result is basically consistent with that obtained from previous apatite FT study.
The oldest component of the zircon FT ages ranges from 91.5 ± 1.1 to 245.6 ± 201.7 (1σ) Ma with three best-fit peak ages between 110 and 160 Ma, obviously implying that unreset zircon grains of the Western Foothills were most likely derived from the Jurassic－Cretaceous rocks in Southeast China. According to the lag time concept, we calculate the cooling and exhumation rate during the period of ~160 to ~7 Ma as 2.8 ~ 2.2 ℃/m.y. and 0.10 ~ 0.07 mm/yr, respectively. It also implies that the source rocks have experienced rapid cooling during the early higher temperature stage and slow cooling during lower temperature period.
The ratio of the number of zircon gains from the Western Foothills versus that of the Hsuehshan Range is about 1:3, which is similar to the proportion of the drainage area of the present Hsintein Stream flowing on the two different geological belts. This is also implies that the drainage area and exposured strata of the ancient (~1.2 Ma) and present Hsintein Stream do not change significantly.
Subjects
林口台地
鋯石
核飛跡定年法
沉積物來源區
剝蝕作用
Linkou Tableland
detrital zircon
fission-track dating
provenance
exhumation
Type
thesis