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| Titel |
Why calcite can be stronger than quartz |
| VerfasserIn |
N. S. Mancktelow, G. Pennacchioni |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250025061
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| Zusammenfassung |
| Calcite and quartz are two of the most common minerals in the continental crust and it is
therefore not surprising that these minerals have been extensively studied since the very
beginning of laboratory rock mechanics experiments. Extending such laboratory data to
geological deformation rates around 10-14 s-1 requires an extrapolation of more than 7
orders of magnitude, with correspondingly large uncertainties. Extrapolation is based on the
assumption that flow parameters are constant with changing conditions and that parameters
not included in the flow law have a negligible influence on the creep properties. The validity
of this extrapolation can only be tested, at least semi-quantitatively, by comparison with
naturally deformed rocks. Observations generally indicate that quartz is significantly stronger
than calcite in natural rocks, with quartz forming porphyroclasts in calcite marble
mylonites, or with quartzite layers being folded or boudinaged within a weaker calcite
marble matrix. However, in the Neves area (Tauern Window, Eastern Alps), shearing
of Alpine coarse grained quartz-calcite veins under hydrous amphibolite facies
conditions (ca. 550Ë C) produced quartz mylonites containing asymmetric cm-scale
single crystal calcite porphyroclasts. Under these conditions, coarse calcite is clearly
stronger than the surrounding polycrystalline, dynamically recrystallized, quartz
matrix. The important parameter controlling this difference in observed natural
behaviour is the grain size of the calcite. Although there is considerable variation,
uncertainty and even contradiction in the published experimental results, we show that
extrapolation of laboratory creep data on calcite single crystals and coarse marbles,
together with the corresponding data for wet quartzites, is indeed consistent with these
natural observations. Extrapolation indicates an inversion in the relative strength
of coarse calcite and quartz at a strain rate around 10-11 s-1, corresponding to a
differential flow stress of ca. 50 MPa. At lower strain rates and stresses, wet quartz
should be weaker than coarse calcite crystals. Field evidence (flow of quartz-rich
layers even for orientations with very low resolved shear stress) and the preserved
microstructure (lack of recrystallized or bulged twins in the calcite porphyroclasts) in the
Neves area are also consistent with flow stresses of less than ca. 50 MPa. These low
values during deformation under water-rich amphibolite facies conditions are in
marked contrast to the much higher differential stresses reported for the flow (and
fracture) of quartz-rich rocks under dry conditions in the middle to lower crust. |
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