Late Quaternary Paleomagnetic Intensity Record in Taipei Basin,Taiwan

The Central Weather Bureau has implemented the Taiwan StrongMotion Instrumentation Program(TSMIP)to collect high-quality instrumentalrecordings of strong earthquake shaking.Deployment of theaccelerographs started in August 1992.By June 1997,629 stations hadbeen deployed.Each operating free-field station includes triaxial accelerometers,a digital recording sub-unit,a power supply and a timing system.Since the first installation in August 1992,this network has recorded 2482earthquakes.The local magnitudes of these earthquakes ranged from 1.7to 7.1 and thousands of high-quality digital accelerograms were collected.The quality of 16-bit data is shown to be much better than the conventionalstrong-motion data(either analog or 12-bit digital recordings).These newrecordings provide an excellent database for making various earthquakeengineering and seismological studies.


INTRODUCTION
When studying the correlation between different stratigraphic sections and analyzing the events of paleo-environment changes, it is very important to have the age control as high resolution as possible. Paleomagnetic analysis, including polarity reversal records and/or secular variation patterns of earth magnetic field, has been broadly considered as a very powerful tool for contributing high resolution, continuous and more precise age data, in addition to paleon tological and isotopic methods. For the last twenty years, paleomagnetic study has been very commonly used (Barton & McElhinny, 1981;Creer, 1974;1977;Creer et al., 1976;1980;198 1;Hyodo et al., 1993;Turner and Thompson, 1981). However, most of the works prior to the 1990s dealt only with the directional variations of the field and not its intensity, especially for the sedimentary rocks. This is due to paleomagnetic intensity being controlled not only by the earth magnetic field but also by the magnetic mineral assemblages contained in the samples.
Also, the mechanism for the acquisition of the remanent magnetization plays another impor tant role. To distinguish these factors to get more accurate paleo-intensity information is some what difficult.
For the volcanic rocks and archeological materials, Thellier and Thellier ( 1959) succeeded in developing a method to analyze the paleo-intensity of the earth's magnetic field based on the activity of thermal remanent magnetization. However, these data support only discrete age dating. To have a continuous time record, it is best to have analysis taken from the sedimentary sequences. However, for the sedimentary samples, the Thellier and Thellier method couldn't be applied because (1) magnetic minerals contained in the samples might be changed when oxidation occurred during heating processes and (2) the detrital remanent magnetization ac quired mainly by the sedimentary rocks did not have the activity property.
To try to investigate an effective way to study the paleomagnetic intensity from sedi ments, Tric et al. (1992) analyzed the intensities of NRM and ARM at different demagnetiza tion steps, and compared the stratigraphic variation patterns of both NRM vs. X and ARM vs. X after 20mT cleaning treatment for five marine cores. They found that the two patterns are very similar. Furthermore, they pointed out that the pattern of the NRM vs. ARM at 20 mT cleaning could monitor the existing discrete paleo-intensity records taken from volcanic rocks very well. Since then, several similar studies have been made in marine and lake sediments (Meynadier et al., 1992;Valet & Meynadier, 1993;Yamazaki & Ioka, 1994) and the intensity secular variation pattern has been extended to about 4 million years before present (Valet & Meynadier, 1993).
The Taipei basin is located at the northwest margin of the Western Foothills. It is known to be formed due to tectonic movement, faulting. Since 1991, to assist development planning in the basin, the Central Geological Survey of the R.0.C. has drilled many long piston cores in order to study the detail geological and structural conditions under Taipei basin. One of them, herein referred to as #WK13, was chosen to do the paleomagnetic study for helping to set u p the precise age intervals of the sedimentary formations in the basin. Because this core was not well oriented in the horizontal direction, the paleo-intensity study of the samples was per formed employing the same method used by Tric et al. (1992). The established paleo-intensity secular variation pattern of this study is expected to be a standard record in Taiwan area for the Teh-Quei Lee 367 late Quaternary time, and it could be applied for future stratigraphic correlation and dating work.

GEOLOGICAL SETTINGS
The studied core #WK13 was drilled in the Wukou area at the western side of the Taipei basin ( Figure 1). It has a length of about 86.4 meters distributed from a depth of 10 m to 96.4 m below surface. Stratigraphically, the whole core belongs to the youngest geological unit of the Taipei basin, the Songshan Formation. The detailed lithology of this core is shown in Fig. 1. The morphology of Taipei basin and its surrounding area (modified from Tseng, 1998). The close circle shows the site locality of the studied core.
Square in the index map shows the location of Taipei basin. Figure 2. It was found that this core is mainly composed of mud, mud interbedded with silt and silt layers. Only a few sand layers appeared at the top part of this core. Sediments were gener ally the late Quaternary embouchure and/or fresh water lacustrine facies deposits.
Several levels of this core have been dated by the C-14 method (Liu et al., 1994). The results indicated that the top and the bottom of the core have C-14 ages of 3,430 ±60 yr.B.P. and 21,070±130 yr.B.P. The other important age data are 9,010± 110 yr.B.P., 9,880±80 yr.B.P., 10180± 150 yr.B.P., 14,860± 100 yr.B.P. and 17,380±70 yr.B.P. dated at the depths of about 50 m, 61 m, 64 m, 68 m and 81 m, respectively. These C-14 ages could be used as the control for the paleo-intensity study of the earth's magnetic field in this paper.

SAMPLING AND LABORATORY ANALYSIS
Continuous sampling was carri ed out on the #WK13 core by using 2.0 cm X 2.0 cm x 1.6 cm plastic boxes. The detailed stratigraphical height of each sample was carefully recorded. More than 4,000 samples were obtained. One seventh of them have been analyzed and the results arepresented herein. The stratigraphical heights of the samples studied are distributed from 44.3 m to 96.4 min depth. The distance between two chosen adjacent samples is gener- For these samples studied, measurements of low field magnetic susceptibility were first carried out on a Bartington MS2 susceptibility instrument in its low frequency mode. Then, NRM measurements and the demagnetization treatments of samples were made on the hori zontal-type cryogenic magnetometer (2G Enterprise Company, model SRM 755) which is equipped with an AF demagnetizer and has been installed in a magnetic shielding room at the Institute of Earth Sciences, Academia Sinica in Taiwan. The demagnetization treatments are stepwise from 0 mT to 100 mT in increments of 5 mT. After these processes, the acquisition of ARM for each sample was built up by applying a stable magnetic field of 1 Gauss under a peak field of I 00 mT AF demagnetization at the same time. This ARM acquisition was also carried out using the 755 SRM cryogenic magnetometer system. Finally, the samples' ARMs were measured and demagnetized again using exactly the same procedures as for the NRMs.
Stratigraphical variations of NRM and ARM before demagnetization treatment were first set up and compared mutually in order to find out whether ARM could simulate NRM. Then, variation patterns of NRM/ X and ARM/ X at different demagnetization levels were analyzed.
These are used to find the influence of magnetic mineralogy especially the grain size of mag-.
netic minerals and also to determine the optimum stage for paleo-intensity analysis in this study. In addition, to confirm whether the chosen step is suitable, the demagnetization plots on the Zijderveld diagram (Zijderveld, 1967) were analyzed. Although the core studied is not well oriented, the directional variation pattern during demagnetization should still help for such an analysis. Finally, the ratio of NRM vs. ARM at the chosen level was calculated for all the samples studied. The results obtained are compared to the other records and take the exist ing C-14 ages into consideration in order to build up a paleo-intensity secular variation pattern during the late Quaternary in Taiwan. Figure 3 shows the stratigraphic variation patterns of X , NRM and ARM intensities before AF demagnetization treatments of the samples studied. The patterns generally look very alike. This phenomenon seems to suggest that the ARM pattern could be used to simulate the acquisition of NRM. The acquisition of remanent magnetization (RM) of rock and sedi ment samples is not only controlled by earth's magnetic field but also by kinds, grain size and amount of magnetic minerals contained in the samples and also depend on the environments during and after the acquisition process. Therefore, using ARM and NRM to simulate the paleointensity pattern of paleomagnetic field at the area studied, discrimination of these fac tors should be done.

RESULTS
It is weII known that strength of magnetic susceptibility generally reflects most of the effects due to kinds and amounts of magnetic minerals. To eliminate such effects, NRM and ARM should be normalized by magnetic susceptibility X . The stratigraphical variation pat terns of NRM/ X and ARM/ X are shown in Figure4 for different demagnetization levels. The similarity between NRM/ X and ARM/ X before AF demagnetization (0 mT treatment) indi cates that the acquisition of ARM is probably under the same grain size distribution condition ! 100 as that of NRM. Furthermore, it could also be found from Figure 4 that the NRM/ X and ARM/ X patterns almost exceed 95% identity after 20 mT AF demagnetization. Thus, the comparison of NRM and ARM at 20 mT demagnetization step is considered to be the most suitable step for studying the paleo-intensities recorded in the #WK13 core. It supports the proposition made by Tric et al. (1992) for the paleo-intensity study of the earth's magnetic field on sediments.
To confirm the suggestion, demagnetization plots of the NRM shown on the Zijderveld diagrams were studied ( Figure 5). They clearly indicate that the secondary component seems to have been almost completely cleaned out before or at 20 mT. So, the variation pattern of NRM vs. ARM at the 20 mT AF demagnetization level ( Figure 6) is thought to be capable of representing the proposed secular variation pattern of paleo-intensity of the earth's magnetic field during 9,000 to 21,000 yr.B.P. at the area studied. In this figure, four high value peak areas appearing close to 60 m, 70 m, 80 m and 90 m could be clearly identified. These peaks could certainly be used as indicators in stratigraphical correlation.

DISCUSSIONS
Whether a paleomagnetic secular variation record represents only a local pattern or a regional or even a global event, it should be compared with the other data under the same time base. In Taiwan and East Asia, no continuous paleomagnetic secular variation pattern of very high resolution has been reported before, except for one from a lacustrine sediment core drilled from Taihu area, China by Wang et al. (1997). The variation patterns of the two records, from core #WK13 and Taihu, are shown in Figure 7.
From this figure, if the C-14 dated ages of both cores are considered, three of the four major high peak areas of the #WK13 record appear to correlate with the peaks in the Taihu record. These appeared at depths of around 60 m, 70 m and 80 m for core #WK 13 and at .5 m and 13.5 m fot the Taihu core. Under such a comparison , the C-14 age dated at 68 m for core #WK13 seems inconsistent with that at 11.5 m for the Taihu core. From the previous magnetic susceptibility ( X) analysis of the Taihu core (Wang et al., 1996), it was pointed out that an extremely high X was found below a depth of about 12 m. It has been proposed that this anomaly is due to the last glacial event . Generally, it was proposed that last glacial period ended at about 14,000yr.B.P. Thus, i t is doubtful whether the age of core #WK13 at 68 mi s as old as obtained fr om C-14, 14,8 60± 100 yr.B.P. This indicates that the material used for C-14 dating at the depth of about 68 meters might have been reworked. However, this should be further investigated.
In addition to the sedimentary record, the first high peak area having an assigned age of about 10,000 yr.B.P. also seems to be supported by the volcanic records analyzed by Tanaka (1990) in Japan. Thus , the correspondent ages of the four high intensity peaks of core #WK13 could be proposed as about 10,000, 13,500, 17,500 and 21,000 years before present of the uncalibrated C-14 age, respectively. Because the record in this study is similar to those from both the Taihu area and the volcanic record from Japan, it might be at least a regional pattern representing the paleo-intensity secular variation pattern of the earth's magnetic field during the last 9,000 to 21,000 years in East Asia. Teh-Quei Lee 375 6. SUMMARIES This paper shows a paleomagnetic intensity record in Taiwan. The results show that after 20 mT alternating field demagnetization treatments on both NRM and ARM of samples, the stratigraphical variation patterns of NRM/ X and ARM/ X are very similar. This implies that the NRM vs ARM ratio at this demagnetization stage could represent the paleo-intensity secu lar variation of the geomagnetic field in the area studied for the time period between 9,000 yr.B.P. and 21,000 yr.B.P.
The four major high peak areas found in core #WKl 3 of this study occurring at around 60 m, 70 m, 80 m and 90 mi n depth were assigned to have the ages of about 10,000, 13,500, 17,500 and 21,000 years before present, respectively. These age assignments are under a cor relation between the record obtained in this study and the record collected from Taihu area, China (Wang et al., 1997). Also, all the C-14 dated ages for the two cores are taken into account. In addition, the youngest peak, with an assigned age of about 10,000 yr.B .P. seems to also be supported by the volcanic records from Japan (Tanaka, 1990). So, primarily, this study establishes a continuous paleo-intensity secular variation record of the earth's magnetic field in East Asia for the last 9,000 to 21,000 years.