運用多重熱定年法探討雅魯藏布江支流之碎屑物源

Abstract

利用沉積物的組成與年代特性，我們可以推論其源區歷史以及集水區的地表地形作用。在此研究中我們使用一序列由低溫至高溫的熱定年學方法(核飛跡定年、氩氩定年、鈾鉛定年)，選用適當之標的礦物(鋯石、磷灰石、鉀長石)，針對現生河砂沉積物進行分析研究，由結果的熱年代頻譜分布圖我們可以探討集水區的地表作用強度以及分布差異。 本研究選定西藏南部雅魯藏布江的兩條大支流(拉薩河以及尼洋河)進行探討，在這兩條支流的河口所取樣的河砂標本，其鋯石、磷灰石核飛跡年代、鉀長石氩氩年代皆有明顯差異:在拉薩河流域有非常比例的年輕族群，而尼洋河流域並無明顯的年輕年代訊號。由鋯石的核飛跡-鈾鉛雙重定年法結果顯示，年輕的核飛跡年代主要指示在拉薩河流域內有強烈的剝蝕事件發生。藉由上游與下游沉積物氩氩年代的對比，我們確認了主要的沉積物來源應為拉薩河流域中跨過念青唐古拉山脈前緣(即古露張裂帶谷地)的區域。

Tibetan plateau is one of the most phenomenal orogens in the world. The spectacular landscape provides the opportunity to understand the fundamental mechanisms of mountain building process from varied disciplines such as geomorphology, geochemistry, and geophysics. Like many other orogenic belts around the world, the expedition into the plateau can be hampered by the inaccessibility of intended outcrops. These rough terrains, however, are often the most crucial outcrops to reveal the tectonic picture of the regime. Alternatively, sediments collected from the downstream of selected watershed can reflect a synthetic picture and provide integrated information in a catchment scale. With appropriate strategies and targets, we can therefore establish a comprehensive understanding toward the aimed tributaries and distinguish the veiled governing forces. In this study, we used multiple thermo-chronometers to detect the provenance of modern sediments from two tributaries of Yarlung-Tsangpo River, southeast Tibet. Results from zircon fission track (ZFT), apatite fissiontrack (AFT), Ar-Ar single grain analysis on K-feldspar and U/Pb-ZFT double dating all indicate the occurrence of grains with young thermal ages prevailing in the Lhasa River. This remarkable young population is not significantly detected in the Nyang River, another tributary east of Lhasa River. The discrepancy of age population between the two catchments suggests that the fundamental surface process must be different. Zircon U/Pb and fission track double dating suggests that the young age component represent the recent exhumation episode in Lhasa River. Comparisons between downstream, upstream sediments and in situ rock samples inside Lhasa River explicate that the provenance of the young grains is related to the major structure, Gulu Rifting belt. The high percentage of these young grains suggests a focused denudation in a restricted area of the Lhasa River, mostly along the Nyainqentanglha range.