Possible effects of tectonic shear strain on phyllosilicates: A case study from the Kandersteg area, Helvetic domain, Central Alps, Switzerland

Péter Árkai, Rafael Ferreiro Mählmann, Václav Suchý, Kadosa Balogh, Ivana Sýkorová, Martin Frey

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Abstract

Relatively few and rather controversial data exist on the effects of tectonic shear strain on phyllosilicate properties under conditions of incipient metamorphism. In the present paper, X-ray powder diffractometric illite and chlorite "crystallinity", apparent mean crystallite thickness and lattice strain data are provided from a shale-slate- mylonitized slate sample series. These results are compared with published and new organic petrographic data and microstructural observations. The sample series represents a characteristic vertical section from the Helvetic domain (Central Alps, Switzerland). From top the bottom, this section crosses the boundaries between the Wildhorn nappe, the Gellihorn nappe, and the Doldenhorn nappe. The diagenetic/metamorphic grade increases downwards, as shown by phyllosilicate characteristics: Mainly diagenetic conditions prevail in the Wildhorn, partly diagenetic, partly anchizonal conditions in the Gellihorn, and mostly anchizonal conditions in the studied part of the Doldenhorn nappe. This pattern confirms the previously published trend for the Helvetic domain in general. Compared with the retarded phyllosilicate reaction progress, high grades of coalification, expressed in vitrinite reflectance and graphitization data, are demonstrated throughout the profile. Most probably, this discrepancy can be attributed to differences in kinetic factors influencing metamorphism of phyllosilicates and coalified organic matter during the relatively short-term regional heating. A local, but systematic increase in metamorphic grade in the vicinity of nappe boundaries and also within each nappe, marked by a decrease in "crystallinity" values of clay-size phyllosilicates suggests that high tectonic shear strain enhanced phyllosilicate reaction progress. High tectonic strain near the thrust planes has been well established also by the occurrence of highly coalified organic particles and by the development of graphitized organic matter. In addition to shear-strain, migration of fluids also affected the phyllosilicates, especially in the lower, cataclastic-mylonitic zone of the Gellihorn nappe. K-Ar dates obtained on the 0.1-2 μm illite-muscovite-rich fraction samples indicate incomplete, partial rejuvenation in the low anchizonal part of the Wildhorn nappe and in the low to high anchizonal parts of the Gellihorn nappe, but almost complete resetting in the anchizonal Doldenhorn nappe.

Original languageEnglish
Pages (from-to)273-290
Number of pages18
JournalSchweizerische Mineralogische und Petrographische Mitteilungen
Volume82
Issue number2
Publication statusPublished - Jan 1 2002

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Keywords

  • Central Alps
  • Coal petrographic parameters
  • K-Ar geochronology
  • Low-temperature metamorphism
  • Phyllosilicate "crystallinity"
  • Tectonic strain

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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