台湾南部恒春半岛中新世增生楔碎屑锆石U-Pb年代学及其地质意义

恒春半岛中新世增生楔沉积岩相、遗迹化石均表明其牡丹层主体部分和乐水层均为深海沉积环境,沉积相为中扇至下扇亚相。里龙山层属于大陆棚至大陆斜坡的浅海环境,沉积相为补给水道和上扇亚相。锆石U-Pb定年物源分析结果显示,里龙山层与牡丹层源自福建武夷山的闽江水系,而乐水层砂岩与九龙江河口砂质沉积物的锆石年谱相似,说明乐水层可能源自九龙江水系。结合野外沉积地层观察等,认为恒春半岛晚中新统是南海北坡深水扇的一部分,因晚中新世南极冰盖迅速扩张(12~5 Ma),导致全球海水面大降,海岸线往坡折带外移约200 m,有利于深水扇的发育,然后随着南海海洋地壳向东俯冲卷入恒春半岛-恒春海脊增生楔内。

DETRITUS ZIRCON U-PB GEOCHRONOLOGY AND ITS GEOLOGICAL SIGNIFICANCE IN MIOCENE ACCRETIONARY PRISM OF HENGCHUN PENINSULA,SOUTHERN TAIWAN

The Miocene accretionary prism in the Hengchun Peninsula, southern Taiwan, consists of three formations. The Mutan and Loshui Formations are deposited in a middle-lower deep-water fan and the Lilungshan Formation deposited in a shallow-water environment. U-Pb age data show that the age patterns of zircon grains of the Lilungshan and Mutan Formations are similar with those of the Minjiang estuary sands, while the age pattern of the Loshui Formation is identical to the Jiulongjiang estuary sands by absence of 400~600 Ma zircon grains and less 700~1 000 Ma and 1 600~2 000 Ma zircon grains. During the Late Miocene, the global sea-level fell significantly due to a dramatic expansion of the Antarctic ice sheet. The coast line of the China continent shifted eastward and a large part of the present East China Sea-Taiwan Strait was exposed. Consequently, large rivers like the Minjiang River in SE China could transport debris flow deposits southeastward to the upper slope of the Chinese continental margin. These turbidite sequences were then deformed as the accretionary prism in the Hengchun Peninsula, while the South China Sea subducted in the Late Miocene time.