Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin in Switzerland are well studied, yet they lack robust geochronologic and geochemical analysis of detrital zircon for provenance tracing purposes. Here, we present detrital zircon U–Pb ages coupled with rare-earth and trace element geochemistry to provide insights into the sedimentary provenance and to elucidate the tectonic activity of the central Alpine Orogen from the late Eocene to mid Miocene. Between 35 and 22.5 ± 1 Ma, the detrital zircon U–Pb age signatures are dominated by age groups of 300–370, 380–490, and 500–710 Ma, with minor Proterozoic age contributions. In contrast, from 21 Ma to ∼ 13.5 Ma (youngest preserved sediments), the detrital zircon U–Pb age signatures were dominated by a 252–300 Ma age group, with a secondary abundance of the 380–490 Ma age group and only minor contributions of the 500–710 Ma age group. The Eo-Oligocene provenance signatures are consistent with interpretations that initial basin deposition primarily recorded unroofing of the Austroalpine orogenic lid and lesser contributions from underlying Penninic units (including the Lepontine dome), containing reworked detritus from Variscan, Caledonian–Sardic, Cadomian, and Pan-African orogenic cycles. In contrast, the dominant 252–300 Ma age group from early Miocene foreland deposits is indicative of the exhumation of Variscan-aged crystalline rocks from the Lepontine dome basement units. Noticeable is the lack of Alpine-aged detrital zircon in all samples with the exception of one late Eocene sample, which reflects Alpine volcanism linked to incipient continent–continent collision. In addition, detrital zircon rare-earth and trace element data, coupled with zircon morphology and U∕Th ratios, point to primarily igneous and rare metamorphic sources. The observed switch from Austroalpine to Penninic detrital provenance in the Molasse Basin at ∼ 21 Ma appears to mark the onset of synorogenic extension of the Central Alps. Synorogenic extension accommodated by the Simplon fault zone promoted updoming and exhumation the Penninic crystalline core of the Alpine Orogen. The lack of Alpine detrital zircon U–Pb ages in all Oligo-Miocene strata corroborate the interpretations that between ∼ 25 and 15 Ma, the exposed bedrock in the Lepontine dome comprised greenschist-facies rocks only, where temperatures were too low for allowing zircon rims to grow, and that the Molasse Basin drainage network did not access the prominent Alpine-age Periadriatic intrusions located in the area surrounding the Periadriatic Line.

中文翻译：

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瑞士莫拉塞盆地碎屑锆石记录的中部阿尔卑斯山的构造发掘

对瑞士北部高山莫拉斯盆地的始新世至中新世沉积地层进行了深入研究，但它们缺乏用于碎屑锆石的稳健的年代学和地球化学分析以进行物源追踪。在这里，我们介绍碎屑锆石U–Pb年龄以及稀土和微量元素地球化学，以提供有关沉积物源的见识，并阐明始新世晚期至中新世中期的中部高山造山带的构造活动。在35至22.5  ±  1 Ma之间，碎屑锆石U–Pb年龄特征主要是300–370、380–490和500–710 Ma年龄组，元古代的贡献较小。与此相反，从21马云〜 13.5 Ma（保存得最年轻的沉积物）中，碎屑锆石的U–Pb年龄特征主要是252–300 Ma年龄组，次要丰度是380–490 Ma年龄组，只有较小的贡献是500–710 Ma年龄。组。始新世始新世的特征与这样的解释一致，即盆地初始沉积主要记录了奥山高山造山带盖的顶盖，而底层的Penninic单元（包括Lepontine穹顶）的贡献较小，其中包含Variscan，Caledonian-Sardic，Cadomian和Pan的碎屑。 -非洲造山循环。相反，中新世前陆早期沉积的主要年龄为252-300 Ma，这表明从Lepontine穹顶基底单元中挖出了瓦里斯坎年龄的结晶岩。值得注意的是，除了一个始新世末期样品外，所有样品中都缺乏阿尔卑斯年龄的碎屑锆石，这反映了与大陆与大陆初始碰撞有关的高山火山作用。此外，碎屑锆石稀土和痕量元素数据，以及锆石的形态和U ∕ Th比率主要指向火成岩和稀有的变质源。在约 21 Ma观察到的莫拉西盆地从奥山地松向Penninic碎屑物源的转变似乎标志着中部阿尔卑斯山的协同增生开始。Simplon断层带所容纳的同步成因扩展促进了上山造山带的Penninic晶核的上覆和掘出。在所有中新世地层中都缺乏阿尔卑斯碎屑锆石的U–Pb年龄，这证实了〜 在25和15 Ma时，Lepontine圆顶中裸露的基岩仅包括绿片岩相岩石，温度太低而无法使锆石边缘生长，并且Molasse盆地的排水网络无法进入位于该地区的显着的阿尔卑斯山时代的斑岩时代侵入体。 eria线周围的区域。