Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

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Date
2021Type
- Journal Article
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Cited 12 times in
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Cited 12 times in
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Abstract
The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From ca.\;26\,Ma onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (ca. 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50∘ already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000474265Publication status
publishedExternal links
Journal / series
Swiss Journal of GeosciencesVolume
Pages / Article No.
Publisher
SpringerSubject
Aar Massif; Compressional deformation and exhumation; Relative timing of peak-metamorphism and deformation; Raman thermometry; Structural evolution; Steep reverse faults; Inherited normal faultsOrganisational unit
02707 - Fachgruppe Georessourcen Schweiz
03946 - Schönbächler, Maria / Schönbächler, Maria
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Citations
Cited 12 times in
Web of Science
Cited 12 times in
Scopus
ETH Bibliography
yes
Altmetrics