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Book Chapter

Statistical significance of magnetic fabric data in studies of paramagnetic granites

By
E. L. Pueyo
E. L. Pueyo
1
Laboratoire des Mécanismes de Transfert en Géologie, Université Paul-Sabatier, Toulouse, France
2
Geología Estructural, Geodinámica Interna. Universidad de Zaragoza, c/ Pedro Cerbuna 12, 50009-Zaragoza, Spain e-mail: unaim@unizar.es
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M. T. Román-Berdiel
M. T. Román-Berdiel
2
Geología Estructural, Geodinámica Interna. Universidad de Zaragoza, c/ Pedro Cerbuna 12, 50009-Zaragoza, Spain e-mail: unaim@unizar.es
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J. L. Bouchez
J. L. Bouchez
1
Laboratoire des Mécanismes de Transfert en Géologie, Université Paul-Sabatier, Toulouse, France
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A. M. Casas
A. M. Casas
2
Geología Estructural, Geodinámica Interna. Universidad de Zaragoza, c/ Pedro Cerbuna 12, 50009-Zaragoza, Spain e-mail: unaim@unizar.es
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J. C. Larrasoaña
J. C. Larrasoaña
3
Laboratoiro de Paleomagnetismo, ICT ‘Jaume Almera’ CSIC, Barcelona, Spain
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Published:
January 01, 2004

Abstract

The low anisotropies of paramagnetic granites, due to magnetocrystalline anisotropy, require a statistical treatment of the anisotropy of magnetic susceptibility (AMS) data when systematic fabric studies are performed. Absence of statistical information on these data makes evaluation of their quality difficult. The statistical significance of magnetic fabric in granites is evaluated in this paper. Jelínek’s elliptical confidence angles for the three principal susceptibility axes (E13, E12, E23) of a specimen are used as markers of the quality of the AMS data. Comparing these markers at sample-, site- and massif-scale with the mean AMS axes that result from spherical statistical models helps clarify the reliability of the AMS data. This analysis is presented in detail for the plutons of Veiga and Trives (Spain). It is then applied to seven other massifs from the Pyrenees. We propose the following guides: (1) fabrics with E13 between 10° and 20° tend to isotropy; the directional data and the shape parameter should be considered with great care; (2) lineation is not reliable when E12 > 25°, i.e. when Kmax is almost the same as Kint; (3) similarly, foliation is considered as not reliable when E23 > 25°, i.e. Kmin does not easily differentiate from Kint. Errors attached to the mean Kmax and Kmin axes should always be produced, thus allowing further interpretation. In Trives and Veiga, ‘perfect’ triaxiality cannot be automatically assumed since foliation and lineation could be defined simultaneously in only 53% of the cases. Finally, a minimum of three cores (9 specimens) per site would considerably increase the proportion of reliable orientation data.

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Contents

Geological Society, London, Special Publications

Magnetic Fabric: Methods and Applications

F. Martín-Hernández
F. Martín-Hernández
Faculty of Geosciences, Utrecht University, The Netherlands
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C. M. Lüneburg
C. M. Lüneburg
Department of Geology and Geophysics, University of New Orleans, USA
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C. Aubourg
C. Aubourg
Laboratoire de Tectonique, Université de Cergy-Pontoise, France
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M. Jackson
M. Jackson
Institute for Rock Magnetism, University of Minnesota, USA
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Geological Society of London
Volume
238
ISBN electronic:
9781862394865
Publication date:
January 01, 2004

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