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| Titel |
Meteoric water-rock interaction and clay-gouge formation during higher temperature brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria) |
| VerfasserIn |
Neil Mancktelow, Horst Zwingmann, Marion Campani, Bernhard Fügenschuh  , Andreas Mulch |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250102952
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| Publikation (Nr.) |
 EGU/EGU2015-2353.pdf |
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| Zusammenfassung |
| The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, with a normal,
down-to-west sense of movement. It is marked by a narrow zone of cataclasis and, in three
sampled locations, clay-rich fault gouges. The clay mineral composition of these gouges is
dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or
mixed layer illite/smectite detected. Smectite is limited to the northern samples from the
Stephansbrücke location, whereas chlorite is present in all samples. New growth of 2M1
polytype illite in the finest size fractions indicates temperatures > 200-250Ë C and therefore
fault gouge development at depths and temperatures close to the ductile-brittle transition in
quartz rich rocks (~280-300Ë C). Hydrogen stable isotope (δD) analyses show
that gouge formation was associated with the influx of meteoric water, which was
strongly focused within the fault zone itself, without significant interchange with the
adjacent footwall and hanging wall rocks. K-Ar ages from the different sample grain
size fractions (< 0.1 to 6-10 μm and “whole rock gouge”) show a wide spread,
from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although
the ranges overlap, ages from the northern Stephansbrücke samples are generally
older (115-36 Ma) than those from the south near Matrei (55-12 Ma), possibly
reflecting increasing regional metamorphic temperatures to the south. The well-defined
linear relationship between apparent age and hydrogen stable isotope (δD) values
establishes a direct correlation between rejuvenation of the K-Ar system and increased
interaction with meteoric water. The dependence of both apparent age and δD on
grain size also indicates that radiogenic and stable isotope exchange was controlled
by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith
muscovite during cataclastic faulting, or both. The results demonstrate the advantages of
combining radiogenic and stable isotope analysis in interpretation of K-Ar ages from
clay fault gouges. The combined approach was necessary to establish the crucial
influence on apparent K-Ar ages of meteoric water influx focused on the fault zone and
its interaction with the clay-size-fraction grains localized within the fault core. |
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