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

Abstract

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 specifi c 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 fi ner-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 infl uence of the crystallographic orientation of quartz on the orientation distribution of planar microstructures appears to superimpose an infl uence of the orientation of the impact-related stress fi eld. 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-modifi cation state. Dauphiné twinning is proposed to be mainly due to a reversion to α-quartz during cooling. These fi ndings imply that the uplifted, preheated target rocks have locally been shock-heated to the α-β transition temperature.

IMPACT-RELATED Ca-METASOMATISM IN CRYSTALLINE TARGET-ROCKS FROM THE CHARLEVOIX STRUCTURE, QUEBEC, CANADA

“…Microthermometric data from fluid inclusions aligned in PDFs in shocked quartz from Charlevoix reveal evidence for the circulation of a hot aqueous fluid at initial temperatures of >500 C and at atmospheric pressure (Pagel & Poty 1975). This is consistent with the development of Dauphiné twins associated with PDFs and the crystallographic orientation of associated planar microstructures, which together indicate that temperatures must have attained at least 573 C shortly after impact (Trepmann & Spray 2005). CL photomicrograph showing large, blue luminescent, shocked (PDF-bearing) quartz in Ca-rich zone from location 2.…”

“…Grains containing PDFs parallel to {102} rhombohedra were not observed. Using the calibration scheme of Robertson & Grieve (1977), this abundance of PDF features indicates that these rocks experienced shock pressures of ~9 GPa (Robertson 1975, Trepmann & Spray 2005.…”

“…5d). This abundance of characteristic sets of PDF features implies shock pressures of ~13 GPa (Robertson 1975, Robertson & Grieve 1977, Trepmann & Spray 2005.…”

In situ TEM observations of plastic deformation in quartz crystals

“…In structural geology, quartz twinning has been used to determine the deformation history of rocks. This is particularly significant in highly stressed conditions, such as meteorite impacts (Trepmann and Spray 2005) and earthquake ruptures (Wenk et al 2011), where twins have been identified with electron microscopy. In polycrystalline quartz aggregates, twinning has also been induced experimentally (Tullis 1970;Tullis and Tullis 1972;Wenk et al 2006Wenk et al , 2007.…”

Coupled effects of impact and orogeny: Is the marine Lockne crater, Sweden, pristine?

“…There are a few examples of terrestrial impact craters that were affected but not destroyed by orogeny: Sudbury, Canada, (e.g., Riller 2005), Charlevoix, Canada (e.g., Trepmann and Spray 2005), Gardnos, Norway (French et al 1997), and Lockne, Sweden, (e.g., Lindström et al 1996;Sturkell 1998a;Lindström et al 2005a). Here, we focus on the Lockne crater, which is situated at the eastern border of the Caledonian orogen in central Sweden (Fig.…”