以臺灣中部新近系地層碎屑鋯石紀錄來探討前陸隆起演化

Abstract

臺灣褶皺逆衝造山帶形成於新生代，是歐亞板塊與菲律賓海板塊聚合碰撞之結果，且於造山帶荷重不斷增加下，逆衝褶皺帶前緣撓曲，形成前陸隆起及前陸盆地，積累沉積物於前陸盆地中。儘管目前已有一些研究針對前陸隆起開始的時間進行探討，結果仍眾說紛紜。因此為更加了解前陸隆起始於何時，本研究利用較連續完整之苗栗後龍溪剖面，採集北寮砂岩至錦水頁岩樣本，進行碎屑鋯石之鈾-鉛定年分析，並同時對比明德水庫與大安溪兩個中部剖面之碎屑鋯石研究。結果顯示，臺灣前陸隆起應是開始於南莊層(晚中新世)，啟動時間又以後龍溪剖面稍早於明德水庫剖面。此外，本研究發現，三剖面之碎屑鋯石年代頻譜紀錄會因構造、古地形與沉積環境影響，使在相同岩性地層下，呈現出相異年代頻譜變化。此案例雖於澳洲西部亦可見，但其樣本之間距離相對於臺灣較遠，顯見臺灣短距離之古地形變化明顯。另外，本研究亦由後龍溪剖面關刀山砂岩段與錦水頁岩樣本中，發現一特殊約670 Ma峰值，該峰值僅見於武夷山地區，然而其於關刀山砂岩段中可見，可能是因為該層沉積時位處低水位期，使閩江向下切蝕武夷山地區沉積物並將其運送堆積。而錦水頁岩樣本偵測到該峰值，可能是前陸隆起西移，使沉積物物源多元的結果。此外，透過碎屑鋯石年代頻譜紀錄，也反映出週期性的海水面升降變化，例如：後龍溪剖面南港層年輕碎屑鋯石(< 300 Ma)比例呈現週期性變化，比例以海進時期的打鹿頁岩為最高，可能是受到海水面上升影響，僅能以近物源供應為主所致，這一點和明德水庫剖面相同。顯見地層中的碎屑鋯石組成，不但能靈敏反映構造、海平面升降變化，更可藉此獲知前陸隆起演化訊息，增進對臺灣中部前陸隆起歷史之了解。Taiwan fold-and-thrust orogenic belt was generated as a consequence of oblique convergence between the Eurasian plate and the Philippine Sea plate in Cenozoic. Since loads of orogeny gradually increase, lithosphere flexures as well as forms foreland basins and forebulge. Despite many studies have been conducted, the exact starting time of the forebulge still remains debated. In order to better understand the initiation of the forebulge, samples collected from Nankang Formation to Chinshui Shale in Houlong River profile, Miaoli, are used to carry out detrital zircon U-Pb dating. With well-documented geologic profiles in Miaoli and sufficiently sensitive indicator of detrital zircons, it is possible to demonstrate the starting time of the forebulge during the deposition of Nanchuang Formation as it shows higher young detrital zircon proportions (< 300 Ma) with larger detrital zircon grains. While Houlong River profile recorded earlier starting time than Minder Reservoir profile. In addition, results from the three profiles show variations in detrital zircon age distributions even though they are classified as the same lithostratigraphic unit. It is probably caused by changing in tectonic settings, paleotopography, and sedimentary environments. In addition to Taiwan, there is a similar case found in Western Australia, with much longer distance between the samples than in Taiwan. It may imply dramatic topography variations in paleo-Central Taiwan.In this study, additionally, distinctive peaks around 668 and 671 Ma can be detected in samples of Kuantaoshan Sandstone and Chinshui Shale from Houlong River profile, which can only clearly be found in the Wuyishan terrain (about 670 Ma). From the unmixing model (Kuantaoshan Sandstone), it shows relatively higher contributions of Minjiang River (59 %) and more Wuyishan terrain deposits (14 %) than in other samples after uplifting of forebulge (Tungkeng Formation). The unique peak detected in Kuantaoshan Sandstone is probably the result of deposition during sea-level lowstands (regression), and thus Minjiang River could bring specific sediments to Houlong River profile. Whereas in sample of Chinshui Shale, the appearance of this age peak is probably due to west moving of forebulge, carrying multi-sources of sediments like Wuyishan terrain for deposition. Moreover, periodic transgression-regression cycles may affect on detrital zircon age distributions as well. Take Nankang Formation for example, young detrital zircon proportions (< 300 Ma) show regular changes from Peiliao Sandstone to Kuanyinshan Sandstone. The highest young detrital zircon proportions (about 37 % in Houlong River profile and 50 % in Minder Reservoir profile) in Talu Shale imply regression during this time, with mainly nearby sources supplied. In summary, analyzing detrital zircon U-Pb age distributions can not only reflect sea-level fluctuations, source-to-sink relationship, but also is sensitive to basin evolution history and paleogeographic changes,which allows to improve our understanding of tectonic history in Central Taiwan.