螢光定年法作為地質熱定年工具的可行性測試:以中台灣造山帶為例

台灣不僅位於歐亞大陸板塊及菲律賓海板塊的聚合帶上，並且屬於濕熱多雨的副熱帶季風氣候，活躍的構造運動加上每年颱風、梅雨所帶來的強降雨，使得台灣在近年來地表抬升量及砂石輸出量都是世界上最高的區域之一。剝蝕速率是抬升與侵蝕交互作用下的結果，利用各種熱定年工具及冷卻模型來觀察剝蝕速率數百萬年以來的變化，是了解造山活動細部過程的重要方法。台灣山脈抬升的剝蝕速率歷史，過去已經使用不同的熱定年工具進行了很多科學探討，發現遠從六百萬年以來，平均的剝蝕速率可以達到每年3-6 mm，是全世界活動造山帶速率最高的地方之一。然而，對於年輕於五十萬年剝蝕歷史，則因為缺乏合適的工具及材料，仍是一件十分具有挑戰性的研究工作。螢光定年法已經成功為地質學以及考古學的研究工作提供非常多年代，作為熱定年工具時，螢光定年法因為具有非常低的封存溫度，有機會來探討年輕造山帶近地表的剝蝕歷史。因為對石英的螢光訊號有較完整的了解，加上分布普遍而廣泛，所以本研究選擇石英顆粒為定年材料。研究區域以台灣中部山脈區為對象，採集了超過十個標本，其由東到西分布自綠色片岩相的大南澳片岩帶，至葡萄石-綠纖石相與成岩帶的雪山山脈，其中四個來自雪山山脈西側的較低變質度的標本，在各種測試中表現出理想的螢光訊號，適合地質年代的測定之使用。在等效劑量的估計上，相較於多片增加劑量法，單片再生法能提供較高的準確度，是較建議的測試方法，但是單片再生法的螢光訊號較易飽和的問題，也同時限制方法使用的年代上限。本研究將四個來自雪山山脈西側的標本分別利用數學模型建立了冷卻模型，依據冷卻模型的結果，雪山山脈西側在近十萬年具有非常高的平均冷卻速率，而近三萬年的平均冷卻速率更加驚人。這樣的快速的冷卻可能源自於標本由地殼深處往近地表剝蝕的過程中，地溫隨著與地表距離減少而降低。若考慮地溫梯度為每公里50 ˚C，剝蝕速率在過去十萬年的平均約為每年8 mm，而最近三萬年則平均每年約25 mm。

關鍵字

光螢光定年法；低溫熱定年法；石英；台灣造山；剝蝕速率

並列摘要

The island of Taiwan results from the tectonic collision between the Philippine Sea and the Eurasian plates, which has begun from late Miocene and currently an ongoing active tectonic. Over past a few decades, numerous studies have been executed to unravel the history of this active orogeny under different time scales. However, the limited archives for the history in last 0.5 Ma bring out the aspiration on the methodology to work on this timescale. Optically Stimulated Luminescence (OSL)-based dating method has been considered to be a possible geo-thermochronometer because of the low closure temperature and the relevant laboratory techniques have been extensively applied to the geology and archeology for late Quaternary. This study is designed to test the possibility of OSL method in investigating the exhumation history of the young orogeny Taiwan. Quartz is the target material for OSL-thermochronometer in this study. More than 10 bedrock samples were collected from central Taiwan. From east to west they are distributed from Tananao schist belt of greenschist facies to Hsuehshan belt of prehnite-pumpellyite facies and diagenetic zone. Four of them from western Hsuehshan Range are confirmed with accepted luminescence characteristics consistent with ideal quartz used in luminescence dating. For equivalent dose estimation, the SAR protocol is recommended because of relatively high precision. However, its natural signal saturation level caps the maximum dating limit for OSL-thermochronometer. The cooling processes for above four samples were further evaluated by the mathematical model. The results show a high cooling rate of 8 mm/yr in the past 0.1 Ma and even higher in the recent 0.03 Ma.

並列關鍵字

OSL dating method ； Low-temperature thermochronometry ； Quartz ； Taiwan Orogeny ； Exhumation rate

參考文獻

Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London.

Google Scholar

Abramowitz, M., Stegun, I.A., 1964. Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. National Bureau of Standards Applied Mathematics Series 55. United States Printing Office.

Google Scholar

Adamiec, G., and Aitken, M. J., 1998. Dose-rate conversion factors: update. Ancient TL 16, 37-50.Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. 359pp.

Google Scholar

Ankjærgaard, C., Jain, M., Wallinga, J., 2013. Towards Dating Quaternary Sediments Using the Quartz Violet Stimulated Luminescence (VSL) Signal. Quaternary Geochronology 18, 99-109.

Google Scholar

Ankjærgaard, C., Guralnik, B., Porat, N., Heimann, A., Jain, M., Wallinga, J., 2015. Violet stimulated luminescence: geo- or thermochronometer? Radiation Measurements 81, 78-84.

Google Scholar

國際替代計量

螢光定年法作為地質熱定年工具的可行性測試:以中台灣造山帶為例

全文下載

主題瀏覽