T68 Convergent Margin Systems

Sunday, 15 October 2023
1:30 PM - 5:30 PM
326

DYNAMICS OF A CRUSTAL-SCALE SHEAR ZONE IN THE CENTRAL EUROPEAN ALPS

Paper number: T68-15
Program: Technical Sessions
Time: 4:40 PM – 4:55 PM

Authors

Abstract

Simple shear causes stretching and rotation of deformable bodies preferentially along the shear direction. Hence, constrictional structures are typical and sometimes diagnostic of crustal shear zones where deformation is non-coaxial. In such a context, the deformational evolution may lead to superimposed structures and the loss of the lateral continuity of the more competent lithological units, challenging the large-scale geological mapping, and potentially resulting in misleading regional interpretations. Here, we present new geological maps and profiles (scale 1:10’000) along the crustal shear zone at the base of the largest high-pressure unit of the Central European Alps (Adula-Maggia Nappe(s)).

Overall, geological data show that the lithological contacts are horizontal or dip gently E-SE, parallel to the penetrative foliation developed at metamorphic amphibolite-facies conditions. On the foliation plane, the mineral and stretching lineation is oriented from NNW-SSE to N-S independently of the orientation of the schistosity. Shear indicators show a top-to-N shearing. However, within this general trend, up to several km-long gneissic bodies (mostly orthogneisses) deflects the foliation steeply to the E or the W. The deflection of the foliation depicts large-scale prolate ellipsoids, which are elongated parallel to the mineral and stretching lineation. Around these prolate bodies, folds have axes parallel to the lineation and form concentric- or Ω-shapes typical of sheath folds. In addition, Ω-folds have been mapped over 30 km along the main shear zone, building tectonic windows.

We conclude that the most complete explanation for these complex structural patterns is the progressive constrictional shear regime acting at upper amphibolite facies until ca. 31 Ma, as shown by U-Pb zircon dating on syn-tectonic migmatites along the shear zone. In the footwall of the shear zone, the penetrative character of foliation and lineation at local peak temperature conditions suggests that the thermal imprint of the nappe emplacement was responsible for the regional Barrovian metamorphism. Therefore, we do not invoke a regional polyphase deformation and several thermal pulses to explain the emplacement of the Adula-Maggia nappe and the regional metamorphism of the root of the Central European Alps.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-392493

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