Late Carboniferous Schlingen in the Gotthard (Central Alps) and their relation to the Variscan evolution Mario Bühler, Roger Zurbriggen, Alfons Berger, Marco Herwegh, Daniela Rubatto EGU21-6547 Schlingen Structures in the Alpine Belt

Fig. 1 Schlingen structures in the Alps. (a) Compilation map of pre-Permian Schlingen containing polycyclic metamorphic basement units in the Alpine belt. (b) Compilation map of described Schlingen structures of the Gotthard nappe, Austroalpine and Southalpine basement units. (c) Simplified block model of a Schlinge, vertical fold with a steep fold axis and steep axial plane. 2 Fig. 1a modified after von Raumer et al. (2013) and Fig. 1b modified after Zurbriggen (2015) Objectives

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Mercolli 1992 Huber 1943 Geological Map of Switzerland 1980

• Are there Schlingen structures in the Unteralptal, Gotthard nappe? • Axial trace of large-scale Schlingen fold • Timing of the Schlingen phase in the Gotthard nappe • Tectonic model for Schlingen formation

3 Methods

Structural and petrological Isotope Dating Tectonic Model analysis - LA-ICP-MS U/Pb - Combine structural, - Structural and lithological dating of zircons petrological and field mapping - Structural key geochronological Study area: 1:5000, 10 km2 samples findings - Thin section microscopy - Palinspatic - Cross-sections reconstruction - …

4 Results: Structural Map of Schlingen Phase Tros-Schlinge

Leucocratic dykes

- Migmatites occur in the vicinity of the axial trace (Tros-Schlinge) - Schlingen folds deform the main foliation (Ravetsch = S2) - All measured structures are steep (foliation, leucocratic dykes, fold axes, lineations, …) - Schlingen folds are displaced by high-strain shear zones - Schlingen structures are deformed by a later-staged superimposed minor deformation event 5 Geochronology: LA-ICP-MS U/Pb Dating

- Zircons were sampled in leucosomes of the Streifengneis that formed parallel to the axial plane of

the Tros-Schlinge (APS) -> location asterisked on slide 5 - The Permo-Carboniferous rims (R2, R3) are clearly discordant on resorbed zircon cores and their low Th/U ratio (< 0.02) is in line with metamorphic overgrowth during high-grade and anatexis. 6 Tectonic Model

Pre-Mesozoic evolution of the Gotthard nappe

(d) Permian Vermigel phase is a temperature dependent deformation event with indicative locally open folds and a weak foliation. (c) Widespread early Permian intrusions (late Carboniferous intrusions, Mercolli et al. 1994) cut Schlingen structures and mark a younger time limit (Schaltegger 1994). (b) The Schlingen phase is considered as a continental-scaled tectonic event associated with collision and transpression between Laurussia and Gondwana, affecting pre-existing subvertical anisotropies within peri- Gondwanan terranes. (a) Ordovician to early-Silurian emplacement of the Streifengneis.

7 Tectonic evolution

Orogenic timetable for pre-Permian evolution of Schlingen containing basements in the Alpine belt. The Schlingen phase (DS or D3) spans over 40 Ma from 340 – 300. Metamorphic facies: l,u: prefixes indicating lower and upper; GF: greenschist facies; AF: facies; GrF: granulite facies; EF: facies. 8 Tectonic evolution Palinspatic reconstruction

(a) Sketch of the situations of the Neo-Variscides during the Schlingen-Phase (see slide 2 for abbreviation). Note the necessary transpressional deformation to reorganize these units. Additionally, Schlingen units are inferred, which are not at the surface today (areas with Schlingen signature, but without a unit). (b) The situation after the transpressional stage of the Schlingen phase. This indicates a strike-slip situation or a more transtensional stage, which is well-known from the Permian. 9 Conclusion I

The investigated kilometre-scale Schlingen fold in the Gotthard nappe is an amphibolite facies regional structure with a steep fold axis and axial plane. The eponymous Schlingen phase expresses a polyphase deformation, which is associated with Variscan crustal-scale shear zones that affected units south of the Eastern Shelf within peri-Gondwana terranes. Characteristic pre-Schlingen structures for such units are pre-existing subvertical anisotropies such as foliations and bandings.

10 Conclusions II

• Schlingen in the Gotthard nappe developed under conditions of water-fluxed partial melting (amphibolite facies conditions, 650° – 700°), which migmatised Ordovician granitoids (Streifengneis) and older migmatites (). • Melt-segregation during the Schlingen phase is variable, but often occurs in fold hinges, fold limbs and along shear zones of Schlingen structures. • Zircon from Schlingen axial plane parallel leucosomes within a Streifengneis body constrain the metamorphic peak of the Schlingen phase to the lower Pennsylvanian (315 ± 4 Ma). Implications: • We propose that the Schlingen developed during oblique collision between Laurussia and Gondwana. • The northern rim of Gondwana became steeply structured by the Cenerian (Zurbriggen 2015) during Ordovician time. In these peri-Gondwanan regions the existing steep structures became folded during the Variscan orogeny witnessed by the formation of Schlingen. • As the formation of Schlingen is associated with the formation of strike-slip zones, the latter coalesced and triggered the formation of the EVSZ (East Variscan Shear Zone) in its eastern part. 11