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Mechanical Twinning in Quartz: Shock Experiments, Impact, Pseudotachylites and Fault Breccias

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Mechanical Twinning in Quartz: Shock Experiments, Impact, Pseudotachylites and Fault Breccias Tectonophysics 510 (2011) 69–79 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Mechanical twinning in quartz: Shock experiments, impact, pseudotachylites and fault breccias Hans-Rudolf Wenk a,⁎, Christoph Janssen b, Thomas Kenkmann c, Georg Dresen b a Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA b GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany c Institut für Geowissenschaften, Geologie, Albert-Ludwigs-Universität, 79085 Freiburg, Germany article info abstract Article history: Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in Received 19 March 2011 deformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally a Received in revised form 14 June 2011 distinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structure Accepted 17 June 2011 positive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {0111}) are Available online 28 June 2011 much stiffer than positive rhombs (e.g. {1011}). Here, we focus on the role of mechanical Dauphiné twinning under stress as a cause of this difference and illustrate with EBSD measurements ubiquitous twinning in Keywords: Quartz quartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentally Dauphiné twinning shocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in South Shock deformation Africa. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosa Seismic stress mylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twin- Pseudotachylites free. It suggests that twinning was produced by local seismic stresses that caused fracture and frictional EBSD melting on fault surfaces. Quartz-bearing breccias from the SAFOD (San Andreas Fault Observatory at Depth) drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the San Andreas Fault at Parkfield. From these observations it appears that Dauphiné twin microstructures can be diagnostic of high local and transient stresses. © 2011 Elsevier B.V. All rights reserved. 1. Introduction aggregate, crystals with normal to negative rhombs parallel to the compression direction will become twinned, resulting in a new It has long been known that quartz undergoes mechanical twinning orientation with poles of positive rhombs parallel to the compression when exposed to high stresses (Schubnikov, 1930; Schubnikov and direction. In situ neutron diffraction experiments indicate that twinning Zinserling, 1932). The significance of these twins in deformed quartz initiates at 50–100 MPa and that activation of twinning is temperature- aggregates was first investigated by Tullis (1970) and Tullis and Tullis dependent (Wenk et al., 2006, 2007). (1972). Mechanical twins occur in many materials (e.g., Klassen- Dauphiné twinning is expressed in the bulk preferred orientation Neklyudova, 1964) but Dauphiné twins in quartz are rather special of quartz crystals in a rock. If the orientation of c-axes and a-axes is compared, for example, with classical twins in carbonates (e.g. Barber random, but positive and negative rhombs show an inverse pattern and Wenk, 1979; Pfaff, 1859) or hexagonal metals (e.g. Partridge, 1967; with corresponding minima and maxima, then it is likely that this Yoo, 1981). The twin–host relationship for Dauphiné twins is a 180° pattern was produced by twinning; but the volume fractions of twins rotation about the c-axis of trigonal quartz. On the atomic scale, it is and hosts must be different (e.g. Tullis, 1970). If positive and negative achieved by a slight distortion of the structure (Fig. 1), without rhombs show the same orientation distribution (i.e., identical pole significant change in macroscopic shape of the quartz crystal. Twinning figures), this could be interpreted as grains that are divided into equal does not change the orientation of the c-axis or a-axes but reverses fractions of host and twin domains. It could also be due to a statistical positive rhombs such as {1011} and negative rhombs {0111}. This is of distribution of untwinned grains in one orientation and another profound mechanical importance, as directions normal to positive orientation related to the first orientation by a 180° rotation about the rhombs are half as stiff as those normal to negative rhombs (e.g., c-axis. For most metamorphic quartzites pole figures of positive and McSkimin et al., 1965). In a compression experiment with a quartz negative rhombs are distinctly different (e.g., Baker and Wenk, 1972; Pehl and Wenk, 2005; Wenk et al., 2009, 2010). This precludes a large fraction of twins in individual grains. ⁎ Corresponding author. Fax: +1 510 643 9980. The actual presence of twins needs to be investigated at the E-mail address: [email protected] (H.-R. Wenk). microstructural scale. Contrary to calcite twins, Dauphiné twins cannot 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.06.016 70 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 The thin section was first investigated with a petrographic microscope to identify regions of interest. Then selected regions were studied in a Zeiss EVO MA10 scanning electron microscope (SEM) at 25 kV, 100 μA beam current, 5 nA I Probe current, 10 Pa variable pressure vacuum to avoid charging, and a working distance of 18– 25 mm. The sample surface was tilted 70˚ relative to the horizontal. Diffraction patterns were recorded with a Digiview IV high resolution digital camera. Data collection and pattern indexing was performed with the TSL-OIM software. Images with 1024×1024 resolution were binned 2×2 or 4×4. Scans were performed over rectangular regions of 200–300 μmin1μmsteps.Suchafine step size is necessary to resolve the twin boundaries satisfactorily. A scan usually took about 24 h. Indexing of trigonal quartz is not trivial. Identification of the trigonal orientation relies on intensity differences between diffractions of positive and negative rhombs. Contrary to calcite, where rhombohedral space group symmetry causes systematic extinctions for unambiguous indexing, in quartz both positive and negative rhombs diffract at the Fig. 1. Schematic structure of a mechanical Dauphiné twin, produced by shear. [0001] same Bragg angle, though with different intensity. The intensity projection. Twin plane is horizontal and sense of shear is indicated. Only Si atoms are shown with gray shades for different z-coordinates. Trigonal distortion is exaggerated differences rely on the crystal structure, and hereby further confusion (from Schubnikov and Zinserling, 1932). may occur for quartz. Traditionally {1011} is the morphologically dominant rhomb (e.g., Frondel, 1962; Goldschmidt, 1897; Hauy, 1801). This setting for the unit cell was used by Gibbs (1926) for the description of the crystal structure in space group P3121, and it follows that for X-ray be detected with a petrographic microscope, due to the coincidence and electron diffraction the following intensity relationships exists of c-axes between twin and host. Thus, either transmission (e.g., between positive and negative rhombs: 1011N0111, 1012b0112, Barber and Wenk, 1991) or scanning electron microscopy (e.g., Lloyd, 2011b0211, 2022N0222, 1013N0113. Unfortunately, some later de- 2000, 2004; Trimby et al., 1998)isrequiredtoimagethem.The scriptions of the quartz crystal structure have not followed this electron backscatter diffraction (EBSD) technique is most suitable convention (e.g., discussion by Heaney et al., 1994, p 8) which is critical and has been first applied by Heidelbach et al. (2000) to map twin for an unequivocal definition of crystal orientation, as well as regarding boundaries in metamorphic quartz. This is the method which we will physical properties such as elasticity. apply in this study. Before entering a quartz structure into EBSD indexing software it is There is no doubt that Dauphiné twins in quartz can be produced necessary to carefully check reflectors. Mostly EBSD systems do not under tectonic conditions, just as calcite twins form in metamorphic discriminate intensity and, thus, only the more intense rhombohedral marbles. It has also been established that twins develop during the reflections should be used for indexing. Fig. 2 shows two diffraction β–α phase transformation (Van Tendeloo et al., 1976). Here, we are patterns which are related by Dauphiné twinning. Note that most focusing on quartz in rocks that were subjected to high local dynamic lines are identical. They define the geometry of the hexagonal unit stresses, such as in shock experiments, meteorite impacts and seismic cell. A few lines are different in intensity and one is indicated by events. arrows. If image quality is low, there is a fair probability that auto- matic indexing chooses the wrong orientation, resulting in individual 2. Methods spots which are related by the twin orientation. We describe the procedure in some detail for a quartz crystal from From rock slabs 30 μm thick petrographic thin sections were the Vredefort impact site which will be described later in more detail. prepared and subsequently polished. First a 3 μm diamond polish was Fig. 3a shows an optical micrograph of a grain with parallel deformation applied for roughly 2 h, then a 1/4 μm diamond polish for half an hour, lamellae that was selected in a thin section. The SEM image with and finally the sample was polished for 5 min by hand with colloidal backscattered electron (BE) contrast (collected with the forward silica. No coating was applied to the sample. scattering detector on the tilted sample) (Fig. 3b) displays surface Fig. 2. EBSD diffraction patterns of quartz from two domains related by Dauphiné twinning. Note that only some bands are different. Arrows point towards corresponding band of trigonal reflections with different intensity. H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 71 Fig.
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