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Planar microstructures and Dauphiné twins in shocked quartz from the Charlevoix impact structure, Canada

By
Claudia A. Trepmann
Claudia A. Trepmann
Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick, E3B 5A3, Canada
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John G. Spray
John G. Spray
Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick, E3B 5A3, Canada
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Published:
January 01, 2005

Shocked quartz from the Charlevoix impact structure has been investigated by optical and scanning electron microscopy, combined with electron backscatter diffraction techniques. The apparent shock pressure recorded by specific sets of planar deformation features (PDFs) in quartz shows a systematic variation with distance (0–10 km) from the center of the structure from ∼5–20 GPa. The occurrence of basal PDFs at distances of ∼2–10 km from the center of the structure indicates a high deviatoric stress component of the shock wave–associated stress tensor. Grain size effects and a greater mineralogical heterogeneity are proposed to be the main cause for slightly lower shock pressures recorded by PDFs in finer-grained granitic gneisses in the southeastern part of the structure, compared to coarse-grained charnockitic gneisses to the northwest at similar distances from the center of the structure. The influence of the crystallographic orientation of quartz on the orientation distribution of planar microstructures appears to superimpose an influence of the orientation of the impact-related stress field. Based on the appearance of Dauphiné twins that are associated with PDFs and the occurrence of PDFs with orientations that correspond to positive and negative rhombohedra, quartz is suspected to have locally been in the β-modification state. Dauphiné twinning is proposed to be mainly due to a reversion to α-quartz during cooling. These findings imply that the uplifted, preheated target rocks have locally been shock-heated to the α-β transition temperature.

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Contents

GSA Special Papers

Large Meteorite Impacts III

Thomas Kenkmann
Thomas Kenkmann
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Friedrich Hörz
Friedrich Hörz
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Alex Deutsch
Alex Deutsch
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Geological Society of America
Volume
384
ISBN print:
9780813723846
Publication date:
January 01, 2005

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