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| Titel |
A database of plagioclase crystal preferred orientations (CPO) and microstructures – implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks |
| VerfasserIn |
T. Satsukawa, B. Ildefonse, D. Mainprice, L. F. G. Morales, K. Michibayashi, F. Barou |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1869-9510
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| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 4, no. 2 ; Nr. 4, no. 2 (2013-12-18), S.511-542 |
| Datensatznummer |
250084948
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| Publikation (Nr.) |
 copernicus.org/se-4-511-2013.pdf |
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| Zusammenfassung |
| This study presents a unique database of 172 plagioclase Crystallographic
Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO
characteristics as a function of the deformation regime (magmatic or
crystal-plastic) are outlined and discussed. The studied samples are
dominantly from slow- and fast-spread present-day ocean crust, as well as
from the Oman ophiolite. Plagioclase is the dominant mineral phase in the
studied samples. Plagioclase CPOs are grouped into three main categories:
Axial-B, a strong point alignment of (010) with a girdle distribution of
[100]; Axial-A, a strong point maximum concentration of [100] with parallel
girdle distributions of (010) and (001); and P-type, point maxima of [100],
(010), and (001). A majority of CPO patterns are Axial-B and P-type, in
samples showing either magmatic or crystal-plastic deformation textures.
Axial-A CPOs are less common; they represent 21% of the samples deformed
by crystal-plastic flow. Although fabric strength (ODF J index) does not
show any consistent variation as a function of the CPO patterns, there is a
significant difference in the relationship between the ODF and pole figures
J indices; the magmatic type microstructures have high (010) pole figures
J indices, which increase linearly with ODF J index, whereas the high
[100] pole figures J indices of plastically deformed samples vary in a more
scattered manner with ODF J index. The multistage nature of plastic
deformation superposed on a magmatic structure compared with magmatic flow,
and the large number of possible slip-systems in plagioclase probably account
for these differences. Calculated seismic properties (P wave and S wave
velocities and anisotropies) of plagioclase aggregates show that anisotropy
(up to 12% for P wave and 14% for S wave) tends to increase as
a function of ODF J index. In comparison with the olivine 1998 CPO
database, the magnitude of P wave anisotropy for a given J index is much
less than olivine, whereas it is similar for S wave anisotropy. Despite a
large variation of fabric patterns and geodynamic setting, seismic properties
of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a
small difference in the aggregate elastic symmetry, with magmatic
microstructures having higher orthorhombic and hexagonal components, whereas
plastic deformation microstructures have a slightly higher monoclinic
component, possibly correlated with predominant monoclinic simple shear flow
in plastically deformed samples. Overall, plots for CPO strength (ODF
J index), pole figure strength, CPO symmetry and seismic anisotropy show
significant scattering. This could be related to sampling statistics,
although our database is a factor of ten higher than the olivine database of
1998, or it could be related to the low symmetry (triclinic) structure of
plagioclase resulting in the addition of degrees of freedom in the processes
creating the CPOs. |
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