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題 名 | Preliminary Paleomagnetic Study on Lake Sediments at Yuanshan, Ilan, Northeastern Taiwan=臺灣東北部宜蘭員山地區湖泊沈積物的初步古地磁研究 |
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作 者 | 李德貴; | 書刊名 | 中國地質學會會刊 |
卷 期 | 36:4 1993.10[民82.10] |
頁 次 | 頁419-436 |
分類號 | 338.96 |
關鍵詞 | 古地磁; 宜蘭; 東北部; 員山地區; 湖泊沈積物; 臺灣; |
語 文 | 英文(English) |
中文摘要 | 本文將介紹臺灣第一個分析自湖泊沉積物的古地磁結果。共 分析三百零三個取自宜蘭員山地區的岩樣,相鄰兩岩樣之距離約 二至五公分,加溫去磁法被使用來研究岩樣的特性殘磁方向。 自然殘磁強度在所採剖面由底部往上四百五十公分以後似乎 有階段性減弱之現象,而磁感率與變化卻顯示輕微的上升,這結 果顯示自然殘磁強度之衰減應與岩樣中所含磁性礦物無關,而可 能指示湖泊沉積期間地磁場曾發生類似磁極轉置的異常事件。此 外,古磁偏角和視磁極位置之經度隨地層高度變化更指出,在大 約六百二十公分處有急劇的改變,似乎建議地磁場之異常現象極 可能發生於這時候。 假如這記錄真代表地磁場之異常事件,則配合同一地區的花 粉研究(劉,個人聯繫),這事件很可能與約二十九萬五千年前之 BIWAⅡ事件相當。此外在此事件發生前的平均古磁偏角約為20° ,意味著此湖泊沉積物沉積之後 ,本地區曾發生約20°的順時針 旋轉現象,這旋轉現象相當強烈的證明與臺灣附近沖繩海槽之開 裂有關,並支持海槽是只向南單一方向開裂。此外,本研究剖面 下半部之古磁偏角和磁傾角變化型態證明地磁場古長期變化( paLEO-SECULAR VARIATION)之存在,同時似乎也支持此剖面頂部記錄 了一個地磁場之異常事件(geo-magnetic excursion)。 |
英文摘要 | This paper will present the results of the first paleomagnetic study on lake sediments in Taiwan. Three hundred and three samples were collected at Yuanshan, Ilan. The distance between two adjacent samples was about 2-5 cm, and the analysis of their characteristic directions was performed by thermal demagnetization. Intensities of the natural remanent magnetization (NRM) seem to decrease stepwisely at the stratigraphic height about 450 cm above the bottom of the sampling sequence. However, variations of the correspondent magnetic susceptibilities showed a slight increases. This indicates that the decreasing paleointensities of NRM must not be due to the different magnetic minerals contained in our samples. Instead, it might be an indicator of the existence of an excursion of geomagnetic field during the deposition in this lake. In addition, from the variation of paleo-declinations and VGP (Virtual Geomagnetic Pole) longitudes, it was found that an abrupt change appeared at the stratigraphical height of about 620 cm. This might also support the existence of a geomagnetic excursion during the deposition of lake sediments. If the records really pointed to an excursion of the earth magnetic field, and if the parallel analyzed pollen data is taken into consideration for the same area, this excursion might be assigned as the BIWA Ⅱ event with the age of about 295 kybp. In addition, the mean paleo-declination which took place before the so-called excursion is about 20 °.This might indicate a clockwise rotation of about 20 ° occurred here after the deposition of the sediments. Such a rotation phenomenon is strongly related to the opening of the Okinawa Trough near Taiwan area and supports the view that the opening was unidirectional only toward south. In addition, the variation patterns of paleo-declination and inclination at the lower part of this record did prove the existence of the paleo-secular variation of the geomagnetic field and seems to support the existence of an excursion at the top part of it. |
本系統中英文摘要資訊取自各篇刊載內容。