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| Titel |
Progressive softening of brittle-ductile transition due to interplay between
chemical and deformation processes |
| VerfasserIn |
Petr Jeřábek, Zita Bukovská, Luiz F. G. Morales |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143322
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| Publikation (Nr.) |
 EGU/EGU2017-7030.pdf |
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| Zusammenfassung |
| The micro-scale shear zones (shear bands) in granitoids from the South Armorican Shear
Zone reflect localization of deformation and progressive weakening in the conditions of
brittle-ductile transition. We studied microstructures in the shear bands with the aim to
establish their P-T conditions and to derive stress and strain rates for specific deformation
mechanisms. The evolving microstructure within shear bands documents switches in
deformation mechanisms related to positive feedbacks between deformation and chemical
processes and imposes mechanical constraints on the evolution of the brittle-ductile
transition in the continental transform fault domains. The metamorphic mineral
assemblage present in the shear bands indicate their formation at 300-350 ˚ C and
100-400 MPa. Focusing on the early development of shear bands, we identified
three stages of shear band evolution. The early stage I associated with initiation of
shear bands occurs via formation of microcracks with possible yielding differential
stress of up to 250 MPa (Diamond and Tarantola, 2015). Stage II is associated
with subgrain rotation recrystallization and dislocation creep in quartz and coeval
dissolution-precipitation creep of microcline. Recrystallized quartz grains in shear bands
show continual increase in size, and decrease in stress and strain rates from 94 MPa to
17-26 MPa (Stipp and Tullis, 2003) and 3.8*10−12 s−1- 1.8*10−14 s−1 (Patterson
and Luan, 1990) associated with deformation partitioning into weaker microcline
layer and shear band widening. The quartz mechanical data allowed us to set some
constrains for coeval dissolution-precipitation of microcline which at our estimated
P-T conditions suggests creep at 17-26 MPa differential stress and 3.8*10−13 s−1
strain rate. Stage III is characterized by localized slip along interconnected white
mica bands accommodated by dislocation creep at strain rate 3.8*10−12 s−1 and
stress 9.36 MPa (Mares and Kronenberg, 1993). The studied example documents a
competition between shear zone widening and narrowing mechanisms, i.e. distributed and
localized deformation, depending on the specific mineral phase and deformation
mechanism active in each moment of the shear zone evolution. In addition, our
mechanical data point to dynamic evolution of the studied brittle-ductile transition
characterized by major weakening to strengths ∼10 MPa. Such non-steady-state evolution
may be common in crustal shear zones especially when phase transformations are
involved.
References:
Diamond, L. W., and A. Tarantola (2015), Interpretation of fluid inclusions in quartz
deformed by weak ductile shearing: Reconstruction of differential stress magnitudes and
pre-deformation fluid properties, Earth Planet. Sci. Lett., 417, 107–119.
Mares, V. M., and A. K. Kronenberg (1993), Experimental deformation of muscovite, J.
Struct. Geol., 15(9), 1061–1075.
Paterson, M. S., and F. C. Luan (1990), Quartzite rheology under geological conditions,
Geol. Soc. London, Spec. Publ., 54(1), 299–307.
Stipp, M., and J. Tullis (2003), The recrystallized grain size piezometer for quartz,
Geophys. Res. Lett., 30(21), 1–5. |
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