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Lithostratigraphy and U-Pb zircon dating in the overturned limb of the Siviez-Mischabel nappe: a new key for Middle Penninic nappe geometry.


Detailed field work and zircon analysis have improved the knowledge of the lithostratigraphy at the base of the Siviez-Mischabel nappe in the Mattertal (St-Niklaus-Törbel area). They confirm the existence of an overturned limb and clarify the structure of the St-Niklaus syncline. The following formations can be observed:

  • Polymetamorphic gneisses; composed of paragneisses, amphibolites and micaschists (Bielen Unit, pre-Ordovician).

  • Fine-grained, greyish quartzite and graywacke with kerogen-rich horizons (Törbel Formation, presumed Carboniferous).

  • Green or white micaschists characterized by brown carbonate spots associated with white conglomeratic quartzites (Moosalp Formation, Early Permian).

  • Massive, green or white, fine grained, microconglomeratic or conglomeratic quartzites with characteristic pink quartz pebbles (Bruneggjoch Formation, Late Permian-Early Triassic).

This coherent overturned sequence can be observed from the St-Niklaus area to the Moosalp pass to the north. Detailed mapping revealed that the St-Niklaus syncline is symmetrical and connects the overturned limb of the Siviez-Mischabel nappe to the normal series of the Upper Stalden zone. U-Pb zircon geochronology on magmatic and detrital zircons allowed constraining ages of these formations. Detrital zircons display ages ranging from 2900 ± 50 to 520 ± 4 Ma in the Törbel Formation, and from 514 ± 6 to 292 ± 9 Ma in the Moosalp Formation. In addition, the Permian Randa orthogneiss is intrusive into the polymetamorphic gneisses and into the Permo-Carboniferous metasediments of the overturned limb of the Siviez-Mischabel nappe. This revision clarified also the lithostratigraphy of the nearby and subjacent Lower Stalden zone composed of an overturned limb with Permo-Carboniferous formations. This has critical implications for the tectonic setting of the nappes in the region, speaking for few recumbent folds with well preserved normal and overturned limbs instead of a succession of imbricate thrust sheets in a normal stratigraphic position.

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Correspondence to Jean-Luc Epard.

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Edtitorial handling: Stefan Bucher

Manuscript received March 15, 2007. Revision accepted January 9, 2008

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Genier, F., Epard, JL., Bussy, F. et al. Lithostratigraphy and U-Pb zircon dating in the overturned limb of the Siviez-Mischabel nappe: a new key for Middle Penninic nappe geometry.. Swiss J. Geosci. 101, 431–452 (2008).

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