Lithostratigraphy and U-Pb Zircon Dating in the Overturned Limb of the Siviez-Mischabel Nappe: a New Key for Middle Penninic Nappe Geometry
1661-8726/08/020431-22 Swiss J. Geosci. 101 (2008) 431–452 DOI 10.1007/s00015-008-1261-5 Birkhäuser Verlag, Basel, 2008 Lithostratigraphy and U-Pb zircon dating in the overturned limb of the Siviez-Mischabel nappe: a new key for Middle Penninic nappe geometry FLORIAN GENIER1, JEAN-LUC EPARD 1, FRANÇOIS BUSSY 2 & TOMAS MAGNA2 Key words: alps, Middle Penninic, Siviez-Mischabel nappe, Permo-Carboniferous, Randa orthogneiss, zircon typology, U-Pb geochronology ABSTRACT Detailed field work and zircon analysis have improved the knowledge of the This coherent overturned sequence can be observed from the St-Niklaus area to lithostratigraphy at the base of the Siviez-Mischabel nappe in the Mattertal the Moosalp pass to the north. Detailed mapping revealed that the St-Niklaus (St-Niklaus-Törbel area). They confirm the existence of an overturned limb syncline is symmetrical and connects the overturned limb of the Siviez-Mischa- and clarify the structure of the St-Niklaus syncline. The following formations bel nappe to the normal series of the Upper Stalden zone. U-Pb zircon geo- can be observed: chronology on magmatic and detrital zircons allowed constraining ages of these formations. Detrital zircons display ages ranging from 2900 ± 50 to 520 ± 4 Ma • Polymetamorphic gneisses; composed of paragneisses, amphibolites and in the Törbel Formation, and from 514 ± 6 to 292 ± 9 Ma in the Moosalp Forma- micaschists (Bielen Unit, pre-Ordovician). tion. In addition, the Permian Randa orthogneiss is intrusive into the polymeta- • Fine-grained, greyish quartzite and graywacke with kerogen-rich hori- morphic gneisses and into the Permo-Carboniferous metasediments of the zons (Törbel Formation, presumed Carboniferous).
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