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Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: Methodology and regional aspects

By
Bernd Leiss
Bernd Leiss
Geowissenschaftliches Zentrum der Universität Göttingen (GZG), Goldschmidstraße 3-5, 37077 Göttingen, Germany
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Heike R. Gröger
Heike R. Gröger
Geowissenschaftliches Zentrum der Universität Göttingen (GZG), Goldschmidstraße 3-5, 37077 Göttingen, GermanyGeologisch-Paläontologisches Institut, Bernoullistrasse 32, 4056 Basel, Switzerland
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Klaus Ullemeyer
Klaus Ullemeyer
Geologisches Institut, Universität Freiburg, Albertstraße 23-B, 79104 Freiburg, Germany
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Hermann Lebit
Hermann Lebit
Department of Geology, 340 Kell Hall, Georgia State University, Atlanta, USA
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Published:
January 01, 2002

Abstract

Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals a-axes distributions scattering along a great circle approximating the foliation plane as well as a-axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.

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Contents

Geological Society, London, Special Publications

Deformation Mechanisms, Rheology and Tectonics: Current Status and Future Perspectives

S. de Meer
S. de Meer
Utrecht University, The Netherlands
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M. R. Drury
M. R. Drury
Utrecht University, The Netherlands
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J. H. P. de Bresser
J. H. P. de Bresser
Utrecht University, The Netherlands
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G. M. Pennock
G. M. Pennock
Utrecht University, The Netherlands
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Geological Society of London
Volume
200
ISBN electronic:
9781862396081
Publication date:
January 01, 2002

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