Shock effects in quartz: Compression versus shear deformation — An example from the Rochechouart impact structure, France
Introduction
Microstructures in target rocks and impact breccias can record shock wave conditions resulting from impact of a large meteorite. Shock waves are strong compression waves that travel with supersonic velocity through the target evolving complex interactions (e.g. Melosh, 1989, Migault, 1998, Nesterenko, 2001, Sharp and DeCarli, 2006). They compress, heat and deform the affected rocks at a very short time of loading (10− 6s to 1 s depending on the size of the projectile). The mean stress of the shock wave-associated stress tensor is generally denoted as shock pressure, which can be up to several tens of GPa. The strain that is accumulated in the target rocks is strongly localised and heterogeneous and is recorded in the microstructures. Shock effects that indicate shock compression include phase transformations, local melting, or even vaporization and the formation of rhombohedral planar deformation features (PDFs) (e.g. Doukhan, 1998, Goltrant et al., 1992a, Goltrant et al., 1992b, Grieve et al., 1996, Langenhorst, 1994, Stöffler, 1984, Stöffler and Langenhorst, 1994). Shock effects observed from natural impact sites also show evidence of shear deformation, e.g. basal PDFs that represent mechanical Brazil twins (Doukhan, 1998, Goltrant et al., 1992a, Goltrant et al., 1992b, Leroux et al., 1994, Leroux and Doukhan, 1996, Trepmann and Spray, 2006), planar fractures (PFs) (Gratz et al., 1988, Gratz et al., 1992), pseudotachylytes (e.g. Martini, 1991, Spray, 1995) and kinked micas (e.g. French et al., 1997). These microstructures indicate shock-induced differential stresses, which are necessary to cause shear deformation. Specific shock effects are sensitive to the magnitude of the shock pressure, as shown in many shock experiment (e.g., Grieve et al., 1996, Langenhorst, 1994, Stöffler and Langenhorst, 1994, Gratz et al., 1988, Gratz et al., 1992, Langenhorst and Deutsch, 1994). However, these experiments are performed on lower loading rates compared to nature and the influence of temperature and of the loading path on the shock effects has to be considered (Gratz et al., 1992, Bowden et al., 2000). Nonetheless, the occurrence and abundance of specific shock effects are useful for a qualitative estimation of relative shock intensities. Quartz is especially useful for providing information on the stress conditions during shock, given the widespread occurrence of this mineral in the Earth's crust and its comprehensive experimental calibration. This study compiles different quartz shock effects indicative of shock-induced compression and shear deformation in samples from the Rochechouart impact structure, France, with the aim to obtain information on the mean stress and deviatoric component of the shock wave-associated stress.
Section snippets
Geological setting
The Rochechouart impact structure in France is located within crystalline basement rocks of the Massif Central (Fig. 1). The structure is eroded and no morphological evidence of a central uplift or a crater wall of the structure is preserved. Based on the distribution of impact breccias and shocked basement rocks, however, the diameter is assumed to be 20–25 km and the centre to be located about 4 km W of the city Rochechouart (Kraut and French, 1971, Lambert, 1977). Impact took place in a very
Analytical methods
Polished thin sections (∼ 30 µm thick) were prepared from over 150 samples collected in the field. The microstructures were examined in thin section with a polarizing microscope. The 3D-orientation of planar features in shocked quartz crystals was measured using a Leitz U-stage mounted on a Leitz optical microscope. The crystallographic orientation and microfabric of quartz were analysed by SEM, using a LEO 1530 instrument with field emission gun, forescatter detector, electron X-ray diffraction
Planar deformation features (PDFs) parallel to rhombohedral planes and mosaicism
Planar deformation features (PDFs) are widespread in shocked quartz from all Rochechouart impact breccias and from basement rocks near the assumed centre of the structure. They appear in the optical microscope as sets of parallel planes decorated by fluid inclusions (Fig. 2, Fig. 3a). PDFs parallel to the ω rhombohedra {101¯3} and π′ {011¯2} rhombohedra are most common in the Rochechouart impact breccias. Less common are PDFs parallel to planes of the {224¯1} form (Fig. 3a), which has been
Rhombohedral PDFs
Planar deformation features (PDFs) parallel to rhombohedral planes are well-known shock effects in quartz (Doukhan, 1998, Goltrant et al., 1992a, Goltrant et al., 1992b, Langenhorst, 1994, Stöffler and Langenhorst, 1994, Leroux et al., 1994, Leroux and Doukhan, 1996, Trepmann and Spray, 2006, Langenhorst and Deutsch, 1994, Goltrant et al., 1991). They are assumed to develop by a transformation into a high-density amorphous phase due to structural instabilities during shock compression at
Brazil twins
Brazil twins in the basal plane have not unequivocally been reported from dynamic high-pressure shock experiments (Langenhorst, 1994, Gratz et al., 1992, Ashworth and Schneider, 1985). They have, however, been experimentally produced at high shear stress experiments at static pressures (McLaren et al., 1967). Though not well constrained, a critical shear stress for twinning is found by these authors to be on the order of 2 GPa. Thus, basal PDFs indicate high differential stresses and
Occurrence of microstructures resulting from shock compression and from shear deformation
Rhombohedral planar deformation features, mosaicism and diaplectic glass are features that are attributed to a transformation of quartz into a high dense amorphous phase and indicate high shock pressures (> 20 GPa). Similar features are also found in static high-pressure experiments at low to nominally no differential stress in a diamond anvil cell (Kingma et al., 1993). In contrast, basal PDFs that represent mechanical Brazil twins, PFs and cataclastic zones indicate shear deformation at high
Summary and conclusion
In quartz fragments from Rochechouart impact breccias, microstructures characteristic for compression and for shear deformation have been investigated.
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Microstructures indicating high shock pressures are:
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Annealed planar deformation features parallel to rhombohedral planes, mostly parallel to {101¯3} and {011¯2} (Fig. 1, Fig. 2).
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Mosaicism with domains of < 200 nm in diameter, which are misoriented as indicated by streaking and multiple spots observed in selected-area electron-diffraction pattern (
Acknowledgements
The author thanks O. Dupuy and C. Marchat for support during field work. The constructive reviews by two anonymous referees are gratefully acknowledged. The work was funded by the German Science Foundation within the scope of the Collaborative Research Centre 526 “Rheology of the Earth — from the upper crust into the subduction zone”.
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