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Tectonophysics 510 (2011) 69–79 Contents lists available at ScienceDirect Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o 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 b c Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany Institut für Geowissenschaften, Geologie, Albert-Ludwigs-Universität, 79085 Freiburg, Germany a r t i c l e i n f o Article history: Received 19 March 2011 Received in revised form 14 June 2011 Accepted 17 June 2011 Available online 28 June 2011 Keywords: Quartz Dauphiné twinning Shock deformation Seismic stress Pseudotachylites EBSD a b s t r a c t Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally a distinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structure positive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {0111}) are 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 quartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentally shocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in South Africa. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosa mylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twinfree. It suggests that twinning was produced by local seismic stresses that caused fracture and frictional 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 It has long been known that quartz undergoes mechanical twinning when exposed to high stresses (Schubnikov, 1930; Schubnikov and Zinserling, 1932). The significance of these twins in deformed quartz aggregates was first investigated by Tullis (1970) and Tullis and Tullis (1972). Mechanical twins occur in many materials (e.g., KlassenNeklyudova, 1964) but Dauphiné twins in quartz are rather special compared, for example, with classical twins in carbonates (e.g. Barber and Wenk, 1979; Pfaff, 1859) or hexagonal metals (e.g. Partridge, 1967; Yoo, 1981). The twin–host relationship for Dauphiné twins is a 180° rotation about the c-axis of trigonal quartz. On the atomic scale, it is achieved by a slight distortion of the structure (Fig. 1), without significant change in macroscopic shape of the quartz crystal. Twinning does not change the orientation of the c-axis or a-axes but reverses positive rhombs such as {1011} and negative rhombs {0111}. This is of profound mechanical importance, as directions normal to positive rhombs are half as stiff as those normal to negative rhombs (e.g., McSkimin et al., 1965). In a compression experiment with a quartz ⁎ Corresponding author. Fax: + 1 510 643 9980. E-mail address: [email protected] (H.-R. Wenk). 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.06.016 aggregate, crystals with normal to negative rhombs parallel to the compression direction will become twinned, resulting in a new orientation with poles of positive rhombs parallel to the compression direction. In situ neutron diffraction experiments indicate that twinning initiates at 50–100 MPa and that activation of twinning is temperaturedependent (Wenk et al., 2006, 2007). Dauphiné twinning is expressed in the bulk preferred orientation of quartz crystals in a rock. If the orientation of c-axes and a-axes is random, but positive and negative rhombs show an inverse pattern with corresponding minima and maxima, then it is likely that this pattern was produced by twinning; but the volume fractions of twins and hosts must be different (e.g. Tullis, 1970). If positive and negative rhombs show the same orientation distribution (i.e., identical pole figures), this could be interpreted as grains that are divided into equal fractions of host and twin domains. It could also be due to a statistical distribution of untwinned grains in one orientation and another orientation related to the first orientation by a 180° rotation about the c-axis. For most metamorphic quartzites pole figures of positive and 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. The actual presence of twins needs to be investigated at the microstructural scale. Contrary to calcite twins, Dauphiné twins cannot 70 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 Fig. 1. Schematic structure of a mechanical Dauphiné twin, produced by shear. [0001] 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 (from Schubnikov and Zinserling, 1932). be detected with a petrographic microscope, due to the coincidence of c-axes between twin and host. Thus, either transmission (e.g., Barber and Wenk, 1991) or scanning electron microscopy (e.g., Lloyd, 2000, 2004; Trimby et al., 1998) is required to image them. The electron backscatter diffraction (EBSD) technique is most suitable and has been first applied by Heidelbach et al. (2000) to map twin boundaries in metamorphic quartz. This is the method which we will apply in this study. There is no doubt that Dauphiné twins in quartz can be produced under tectonic conditions, just as calcite twins form in metamorphic marbles. It has also been established that twins develop during the β–α phase transformation (Van Tendeloo et al., 1976). Here, we are focusing on quartz in rocks that were subjected to high local dynamic stresses, such as in shock experiments, meteorite impacts and seismic events. 2. Methods From rock slabs 30 μm thick petrographic thin sections were prepared and subsequently polished. First a 3 μm diamond polish was applied for roughly 2 h, then a 1/4 μm diamond polish for half an hour, and finally the sample was polished for 5 min by hand with colloidal silica. No coating was applied to the sample. 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 μm in 1 μm steps. Such a fine 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 same Bragg angle, though with different intensity. The intensity differences rely on the crystal structure, and hereby further confusion 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 and electron diffraction the following intensity relationships exists between positive and negative rhombs: 1011 N 0111, 1012 b 0112, 2011 b 0211, 2022 N 0222, 1013 N 0113. Unfortunately, some later descriptions of the quartz crystal structure have not followed this convention (e.g., discussion by Heaney et al., 1994, p 8) which is critical for an unequivocal definition of crystal orientation, as well as regarding physical properties such as elasticity. Before entering a quartz structure into EBSD indexing software it is necessary to carefully check reflectors. Mostly EBSD systems do not discriminate intensity and, thus, only the more intense rhombohedral reflections should be used for indexing. Fig. 2 shows two diffraction patterns which are related by Dauphiné twinning. Note that most lines are identical. They define the geometry of the hexagonal unit cell. A few lines are different in intensity and one is indicated by arrows. If image quality is low, there is a fair probability that automatic indexing chooses the wrong orientation, resulting in individual spots which are related by the twin orientation. We describe the procedure in some detail for a quartz crystal from the Vredefort impact site which will be described later in more detail. Fig. 3a shows an optical micrograph of a grain with parallel deformation lamellae that was selected in a thin section. The SEM image with backscattered electron (BE) contrast (collected with the forward 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. 3. Quartz crystal from stishovite-bearing quartzite near Weltevrede farm in theVredefort dome. (a) Optical micrograph showing deformation lamellae, crossed polarizers. (b) SEM forward scattering image, of which an area was selected for a detailed EBSD scan (green outline). (c) EBSD scan in 0.5 μm steps with a map of Euler angle ϕ2. Note that some Dauphiné twin boundaries (red) are parallel to deformation lamellae. The corresponding gray-shade scale is shown to the lower right and applies to all subsequent orientation maps as well. (d) Histogram of misorientation versus ϕ2 with a sharp peak at 60°, corresponding to the 60° rotation about the c-axis which relates twin and host. (e) Pole figures of twin and host with a single c-axis maximum (determined by Euler angles ϕ1 Φ) and 103 as well as 013 pole figures. The diffraction peak intensities (arrows, pole densities in multiples of a random distribution—m.r.d.) correspond to the volume fractions of host and twin in the image (c). Equal area projection. morphology (with some holes) and contrast variations that are partially due to the presence of Dauphiné twins. BE contrast depends mainly on atomic number and crystal orientation. On this image a region was selected for a detailed scan (green square). Fig. 3c shows a corresponding orientation map of angle ϕ2. Based on indexing of diffraction patterns, orientations of crystals relative to sample coordinates are defined with three Euler angles ϕ1,Φ, ϕ2 (in Bunge notation, Bunge, 1965). Angles ϕ1 and Φ define the orientation of the c-axis and angle ϕ2 the rotation of a-axes around the crystal c-axis and is thus sensitive to Dauphiné twinning. Scan data with orientations, confidence index, which is a measure of pattern identification, and image quality were then exported from the TSL-OIM software to BEARTEX (Wenk et al., 1998) for mapping and identification of Dauphiné twin boundaries with the routine MAPTEX. Dauphiné boundaries must satisfy two conditions: c-axes across the boundary are the same, thus Euler angles ϕ1 and Φ are identical (In our processing we allow a ±2° variation) and the rotation around c defined by Euler angle ϕ2 is 60° (180°) (also here we allow for a 60° ± 2° variation). If both conditions are satisfied, twin boundaries are plotted as red lines on the maps. We can compile misorientation statistics between each cell on the map and surrounding cells and represent them on a histogram (Fig. 3d). For a single crystal all misorientations are close to zero with small deviations due to subgrain misorientations. For quartz, there is almost always a peak at 60°, which is partly due to the presence of Dauphiné twins but can also be produced by misindexing. To some extent misindexing can be minimized by rejecting orientations with low confidence index or low image quality, but we need to keep in mind that the confidence index is based only on positions of the diffraction bands, not their intensity (Fig. 2)—and some ambiguity remains. The map (Fig. 3c) clearly shows two orientations (different gray shades). These orientations are represented in pole figures, which display two orientations related to Dauphiné twinning (Fig. 3e). The angle ϕ2 is most critical for identifying Dauphiné twins, as the two 72 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 domains are related by a ϕ2 = 60° (180°–120°) angle. The poles of (1013) for the host correspond to the poles of (0113) of the twin. The pole density for the host is higher (43 multiples of a random distribution, m.r.d.) than that of the twin (25 m.r.d.), indicating that about 60% of the surface of Fig. 3c is host (dark) and 40% twin (light). A few grains were scanned in each sample to establish consistency. Not all the data can be shown. Clearly, what is presented here is not a statistical result and we have not even attempted to quantify twin fractions in grains or relationships between twins and grain orientation, but the results obtained so far appeared to us convincing enough to support our conclusions. For two samples, a granitic breccia from the Nördlinger Ries and a metamorphic quartzite from the Bergell Alps, we also display pole figures to illustrate bulk preferred orientation, especially the difference between positive and negative rhombs. Textures have been measured on 1 cm cubes by time-of-flight neutron diffraction with the HIPPO diffractometer at Los Alamos (Wenk et al., 2010). 3. Results 3.1. Experimentally Shocked Sandstone The first sample that was investigated is a porous sandstone (Seeberger Sandstein, from Gotha, Germany, Seidel, 1992) that underwent shock-loading. Cratering experiments were performed at the twostage light-gas acceleration facility at Fraunhofer Ernst-Mach-Institute in Efringen-Kirchen, Germany, as part of the MEMIN program (Multidisciplinary Experimental and Modeling Impact Research Network, Kenkmann et al., 2011). MEMIN focuses on impact cratering experiments in geological materials to comprehensively understand details of the cratering process through in situ measurements, extensive postimpact analysis, and numerical modeling. The investigated sample stems from a calibration test shot in which a 1 cm steel projectile weighing 4.1 g was accelerated horizontally to ~4500 m s − 1 and impacted onto the flat, vertical surface of a 40 cm cube of Seeberger sandstone. The kinetic energy of the experiment was 41.5 kJ and the expected peak shock pressures were 50–55 GPa at the contact of the projectile with the target. The crater volume was ~620 cm3. However, impact-induced fractures reached the edges and completely disjointed the target cube. The 20 × 15 × 2 cm sample investigated here contains the crater floor. Some material around the projectile impact was partly pulverized and ejected. As the shock pressure rapidly decays in porous targets with increasing distance from the point of impact, the assumed shock pressure in the sample is estimated to be of the order of a few GPa. An optical examination of the thin section reveals rounded and slightly flattened grains of quartz with interstitial phyllosilicates (Fig. 4a). There are no obvious deformation features such as planar deformation lamellae in quartz. EBSD scans were performed on several grains from about 4 cm beneath the central crater floor and on grains immediately on the crater floor. Grains from the subsurface are Fig. 4. Seeberger sandstone, experimentally shock-deformed within the MEMIN project. (a) Microstructure viewed with a petrographic microscope, crossed polarizers. (b-d) EBSD maps of Euler angle ϕ2, 1 μm steps. Dauphiné twin boundaries are red. In material distant (20 mm) from the point of projectile impact, grains are largely uniform (b). Some boundaries coinciding with grain boundaries may be artifacts of misindexing. (c, d) Near the point of impact grains display Dauphiné twinning, particularly near grain boundaries and at edges of grains. White regions could not be indexed or were rejected because of poor confidence index, or because they represent other phases between quartz grains. H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 uniform with only occasional twin boundaries (Fig. 4b). Grains in the impact region clearly show an abundance of twins, particularly at the margins of grains and on surfaces where two quartz grains are in direct contact (Fig. 4c,d). Some grains are more intensively twinned than others, which could be related to the relation between shock wave direction and crystal orientation. 3.2. Quartz Deformed During Meteorite Impact Based on preferred orientation patterns of quartz it was suggested that shock stresses may have caused Dauphiné twinning in stishovitebearing quartzites from the Vredefort impact site in South Africa (Wenk et al., 2005). This was recently confirmed by EBSD measurements (Chen et al., 2011). Here, we reinvestigate microstructures in the same sample from Weltevrede farm northeast of Parys, about 30 km northeast of the impact center (Gibson et al., 1997). Dauphiné twins occur in grains that are oriented with the c-axis in the plane of the section (Figs. 3a, 5a) and the c-axis perpendicular to the thin section (Fig. 5b). Figs. 3a and 5a display deformation lamellae oriented at high angles to the c-axis. Orientation mapping clearly shows abundant twins with dominant twin boundaries that are parallel to the deformation lamellae (Figs. 3c, 5c). This suggests a close relationship between the two microstructures. The lamellae are subparallel to {1013} or {0113} (Fig. 3e, Trepmann and Spray, 2005; Vernooij and Langenhorst, 2005). Dauphiné twinning resolves this puzzling relationship with equivalent positive and negative rhombs: a boundary that is parallel to a positive rhomb in the host is also parallel to the adjacent negative rhomb in the twin. In grains viewed along the c-axis (Fig. 5b) no such relationship is evident. Also, there are abundant twins here (Fig. 5d). The domains are larger and less regular, though some boundaries display approximate 120° / 60° angles (arrow). 73 A second natural impact sample investigated originates from the Ries impact structure (sample 2008-02385 from the Museum für Naturkunde, Berlin). The “granite breccia” from W of the town of Schmähingen consists of coherent granitic fragments in a more finegrained matrix of the same material (Von Engelhardt, 1974). The fragments are rotated; we analyzed the microstructure of one of the fragments (our label R8). In the thin section we can see elongated quartz grains (Fig. 6a). Texture analysis with the HIPPO neutron timeof-flight diffractometer establishes a strong preferred orientation of c-axes (Fig. 6c), with a c-axis maximum exceeding 6 multiples of a random distribution and distinct differences between positive rhombs {1011} and negative rhombs {0111}. The c-axis pattern is clearly not caused by impact and the rock represents a foliated gneiss rather than a granite and has undergone extensive ductile deformation. However, the orientation of the rhombs may be related to the impact. Note that in pole figures of the positive rhomb {1011} subsidiary concentrations corresponding to maxima of the negative rhomb {0111}, are indicated by arrows (Fig. 6c). This is likely evidence for reorientation by partial twinning. Quartz grains in this Ries sample do not display visible deformation lamellae (Fig. 6a), suggesting lower stresses than in the case of Vredefort. EBSD maps nevertheless display abundant twinning (Fig. 6b). 3.3. Quartz Associated with Pseudotachylites Having established that Dauphiné twins are common in shockdeformed materials, we wanted to explore whether stresses generated during seismic events could induce similar features. Obvious rocks to investigate are pseudotachylites that are thought to have originated by frictional melting during seismic rupture (Sibson, 1975). We explored twinning in quartz-bearing rocks from the Santa Rosa mylonite zone in Fig. 5. Vredefort quartzite from Weltevrede farm. (a,c): Grain viewed perpendicular to the c-axis shows deformation lamellae. (b,d): Grain viewed parallel to c-axis. (a,b) Optical photomicrographs with crossed polarizers. (c,d) EBSD maps of Euler angle ϕ2, 1 μm steps. Dauphiné twin boundaries shown in red. Note the clear morphologic relationship between deformation lamellae (a) and twin boundaries (c). Arrow in (d) points to an approximate 120˚ angle at a twin boundary. 74 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 Fig. 6. Quartz in granitic breccia R8 from Schmähingen, Nördlinger Ries. (a) Optical photomicrograph (plane polarized light) illustrating deformed quartz grains. (b) EBSD map of a grain, illustrating a few Dauphiné twin boundaries. (c) Pole figures measured by neutron diffraction, displaying very strong preferred orientation with a pattern close to that of a single crystal. Arrows in (1011) pole figure point at subsidiary maxima, corresponding to principal maxima of the negative rhomb (0111). Equal area projection. Linear contour levels in multiples of a random distribution. Southern California. Here, there is a gradient from ductile mylonites at depth to brittle deformation above (Wenk, 1998). Pseudotachylites are observed in the vicinity of the brittle–ductile transition zone (Wenk et al., 2000). In largely ductile mylonites, texture analysis reveals a strong difference between positive and negative rhombs (Pehl and Wenk, 2005). Thus, twins cannot be prevalent. Two examples of moderately deformed granite from the ductile zone obtained in upper Palm Canyon are shown in Fig. 7. Typically there are large grains of quartz with undulatory extinction, as in PC 89 that was investigated previously for residual strain (Kunz et al., 2009) (Fig. 7a). With increasing deformation quartz recrystallizes, first along grain boundaries as in PC 88 (Fig. 7b), and ultimately throughout a sample. EBSD scans of ductilely deformed quartz in both rocks indicate that grains are largely uniform and devoid of twins (Fig. 7c,d), although there are slight variations in orientation, visible as variations in gray shades that are indicative of subgrain formation. This is very different from observations made on quartz from the vicinity of pseudotachylite veins, as shown by two examples: one is a fragmented quartz grain directly adjacent to a pseudotachylite vein in PC 825 from the contiguous pseudotachylite zone exposed at elevation 1900 ft E of Martinez Mountain (Fig. 8a), and the second one is a large quartz grain from about 2 cm from the pseudotachylite in PC 738c from Deep Canyon SE of Black Hill (at 3000 ft) (Fig. 8b). Both grains are fractured, with misplaced and slightly rotated fragments. EBSD maps reveal a profusion of twin domain structures (Fig. 8 c,d), in many ways similar to the Vredefort quartzite (Figs. 3c, 5c). 3.4. Brecciated Quartz from the SAFOD Drill Hole Another sample that was analyzed is a fractured core sample recovered from 3141 m depth (measured along the borehole) of the phase 3 borehole of the San Andreas Fault Observatory at Depth ICDP project (SAFOD; Hole E, Run 1, Section 6). This brecciated sample was taken from a sequence of arkosic sandstone, which belongs to the Salinian Block (Springer et al., 2009). In thin section, the matrix is predominantly composed of coarse- to very coarse, subrounded to subangular quartz (36 wt.%), plagioclase (22%) and microcline (17%) (Janssen et al., 2011). The sample position was 16 m from the presently inactive (non-creeping) ‘geological’ San Andreas Fault that forms the eastern limit of the Salinian Block and 50 m from the active fault trace (southwest deforming zone/SDZ). Large quartz grains are often fractured with displaced fragments in a fine-grained cataclastic matrix (Fig. 9a,b). Some of these quartz clusters have been analyzed in detail. A very interesting pattern emerges: large crystals are generally uniform, without much twinning, but corners and edges, as well as small fragments are pervasively twinned (Fig. 9b,c). Stresses which caused fragmentation were high enough to induce local twinning. 4. Discussion Mechanical twinning in quartz was discovered in experimentally stressed single crystals (Schubnikov, 1930; Schubnikov and Zinserling, 1932). Localized twins were produced by mechanical action with steel pins or spheres. Later, stresses during growth were used to prevent growth twins in piezoelectric material or to remove existing twins by applying a torque at elevated temperature (Thomas and Wooster, 1951; Wooster et al., 1947). At ambient temperature twinning occurs at average stresses around 500 MPa (Bertagnolli et al., 1979) and is pervasive at 1 GPa (Tullis and Tullis, 1972). At elevated temperatures and pressures twinning is already initiated at stresses of 50–200 MPa (Wenk et al., 2006, 2007). But twins are induced at local stress concentrations (Schubnikov, 1930) that are likely much higher than H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 75 Fig. 7. Moderately deformed granite from Palm Canyon in the Santa Rosa mylonite zone in Southern California. (a,c): Sample PC 89. (b,d): Sample PC 88. (a,b) Optical photomicrographs taken with crossed polarizers. (c,d) EBSD maps of Euler angle ϕ2. Dauphiné twin boundaries are shown in red. (a) The large quartz grain shows undulatory extinction due to moderate plastic deformation. (b) In this sample, quartz has become recrystallized along grain boundaries. (c,d) In both samples division into subgrains (variation in gray shades) is noted, but Dauphiné twins are largely absent. average stresses. The experimentally shocked sandstone was subjected to stresses up to 50–55 GPa. (Kenkmann et al., 2011). However, the coherent material studied here, contrary to ejected fragments, represents much lower shock stresses, presumably 3–5 GPa. Typical features in quartz, experimentally shocked below 8– 10 GPa, are planar shock lamellae (e.g., Gratz et al., 1988, 1992; Stöffler and Langenhorst, 1994). Such lamellar structures are not restricted to shock deformation but are also produced in conventional deformation experiments at elevated pressures and temperatures (e.g. Vernooij and Langenhorst, 2005) and observed in metamorphic rocks. As we have shown in Fig. 5c, these lamellae are closely linked to Dauphiné twins, at least in Vredefort quartzite. The morphology, with twin boundaries parallel to the rhombohedral lamellae, explains why the lamellae are either of {1013} or {0113} orientations. In this sample there is no evidence for amorphous zones or high dislocation densities along lamellar boundaries, but obviously the EBSD resolution is not at the nanometer scale. The close geometric relationship between lamellae and twin boundaries suggests that the two features were produced simultaneously. Dauphiné twinning is pervasive in quartz rocks subjected to meteorite impact. Apart from Vredefort and Ries, it has been documented for the Charlevoix impact structure in Canada (Trepmann and Spray, 2005) and the Rochechouart impact in France (Trepmann, 2008). Shock pressures for their production are above the Hugoniot elastic limit and have been estimated at N3–8 GPa (Stöffler and Langenhorst, 1994). This is consistent with the partial conversion of quartz to stishovite in the Vredefort quartzite (Martini, 1978) which also suggests pressures N7 GPa. While twinning in quartz subjected to meteorite impact is well established, it has been surprising to find similar microstructures in rocks exposed to seismic stresses. For this a survey through the brittle– ductile transition in the Santa Rosa mylonite zone in Southern California has been most revealing. Quartz in granitic rocks from the ductile deformation zone, also associated with sillimanite and cordieritebearing paragneisses and tremolite-bearing marbles, shows signs of plastic deformation such as undulatory extinction, grain flattening and recrystallization. This implies high temperatures and low stresses. In such rocks Dauphiné twins are largely absent. This situation changes as one passes the brittle–ductile transition zone, with a distinct zone of pseudotachylites that can be attributed to ancient seismic activity. If one accepts the hypothesis that these enigmatic rocks are produced by frictional melting on ruptured fault surfaces (Sibson, 1975), then stresses had to exceed the shear strength of the rocks. Local stress concentrations had to be higher than the shear strength of quartz to produce extensive failure. Quartz is one of the strongest components of gneisses. There is abundant twinning in quartz associated with 76 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 Fig. 8. Quartz associated with pseudotachylite from the Santa Rosa mylonite zone. (a,c): Sample PC 825, Martinez Mountain. (b,d): Sample PC 738c, Deep Canyon. (a,b) Photomicrographs taken with crossed polarizers, illustrating fractured quartz grains. (c,d) EBSD maps of Euler angle ϕ2, 1 μm steps. Dauphiné twins (boundaries are red) occur pervasively. pseudotachylites. If frictional melting occurred during rupture, then at least locally temperatures were elevated, facilitating twinning. There is no evidence that local conversion to hexagonal β-quartz was achieved (650 °C at 1 GPa). The pseudotachylites studied here are of tectonic/ seismic origin but it should be mentioned that they also occur associated with impacts (Reimold, 1995). Quartz in brecciated fault rocks from the San Andreas Fault at depth also shows twinning, and it is likely that also in this case it was caused by seismic stresses. These are likely local stresses during an earthquake, which is not inconsistent with the current creep mode of the fault at Parkfield, with very low average stresses (Hickman and Zoback, 2004; Townend and Zoback, 2004). SAFOD stresses based on dislocation densities, twinning microstructures and preserved residual stresses in calcite were estimated at 100–200 MPa (Rybacki et al., 2011), but some of these microstructures were attributed to creep. Also in the SAFOD sample analyzed here, fragmention of quartz is evidence of local stress exceeding the strength of quartz. What is the strength of quartz? The compressive strength of quartz is very high (1–5 GPa, Griggs et al., 1960; Kimberley et al., 2010), but the tensile and shear strengths are two orders of magnitude lower (30–50 MPa, Ball and Payne, 1976). Thus, in brecciated and fragmented quartz such stresses must have been exceeded. This corresponds to stresses that can induce Dauphiné twinning at low temperature and we should again keep in mind that twins nucleate at local stress concentrations which are likely much higher than average applied stresses. The experiment of Schubnikov (1930) demonstrated that twinning occurs well below fracture. The relative ease by which twinning occurs, raises the possibility that some twins could be artifacts and have been produced during sampling, e.g. extracting specimens with a geological hammer. We do not think that this was the case for the specimens described here. MEMIN and SAFOD samples were carefully cut with a microsaw. And while pseudotachylite and mylonites were collected with a hammer in the field, they were later also cut and internal portions were used. So far bulk texture analyses of quartzites always show very systematic orientation distributions that can be followed over large distances (e.g. Pehl and Wenk, 2005). If sample extraction would alter crystal orientations, this would not be the case. Nevertheless we wanted to mention this possibility and should be aware of conceivable artifacts. Twinning has been documented in metamorphic rocks (e.g. Heidelbach et al., 2000; Trimby et al., 1998) where it is, however, not dominant. Particularly at higher metamorphic grade, pole figures always display a strong difference between positive and negative rhombs, which precludes pervasive twinning. This is also illustrated for a greenschist-grade muscovite quartzite from the Bergell Alps in Fig. 10. The microstructure consists of slightly flattened grains, separated by muscovite flakes (Fig. 10a). Neutron diffraction pole H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 77 Fig. 9. SAFOD breccia 1B. Two regions of this sample are shown (a,c) and (b,d), both with fractured quartz clasts. (a,b) Photomicrographs taken with crossed polarizers. (c,d) EBSD maps of Euler angle ϕ2, 1 μm steps. Dauphiné twin boundaries are red. Twinning is concentrated in the outer portions of the grains. White areas correspond to fine-grained matrix that is largely feldspar. figures measured on a 1 cm cube show a bimodal c-axis distribution and a very distinct difference between positive and negative rhombs (Fig. 10c, Wenk et al., 2010). Positive rhombs {1011} have a maximum perpendicular to the schistosity plane s (horizontal). Below (Fig. 10d) are EBSD pole figures measured on a small surface segment. Positive rhombs show a clear maximum perpendicular to the schistosity plane. This requires that there could not be equal fractions of twins and host in each grain. The EBSD map (Fig. 10b) shows some twinning but twins comprise only minor fractions, particularly at grain boundaries, and at boundaries between quartz and muscovite. Some grains are more profusely twinned than others, suggesting orientation control. Indeed, in the EBSD pole figures (Fig. 10d) twin orientations can be clearly identified (arrows). Statistically, over a large volume, this is less evident (Fig. 10c). The significance of these twins in metamorphic rocks is not clear: are they growth twins forming during recrystallization under stress? Is the difference between positive and negative rhombs due to slip or twinning? If twinning has occurred at metamorphic conditions, why has it not gone to completion? The morphology of twins in this metamorphic quartzite is different from that in the stressed samples, where a clear tendency exists for 120° angles of boundaries and twin concentrations at grain corners. The role of twinning in metamorphic rocks will be the subject of a future study. Dauphiné twinning is not really a paleopiezometer because it does not provide information about exact stress magnitudes (Tullis, 1980). Initiation of twinning is highly orientation dependent (Schubnikov and Zinserling, 1932). It also depends on temperature and, at intermediate temperatures, nucleation of twins has been observed at 50–100 MPa (e.g. Wenk et al., 2006, 2007). Nucleation of twins is followed by propagation of twin boundaries and, in an equilibrium situation, twinning should go to completion, which may be the reason why quartz twin boundaries are rare in higher grade metamorphic rocks. But high localized and transient stresses appear to impose characteristic twin domain structures in quartz of impactites and pseudotachylites, linking seismic events to meteorite impacts. This investigation, mainly on natural rocks, cannot determine magnitudes of local stresses to induce twinning and this should be approached with modern methods such as nanoindentation (Wang et al., 2005) or in situ electron microscopy (e.g. Ye et al., 2010). Similarly, deformation experiments, combined with EBSD analyses should be conducted to determine the influence of crystal orientation relative to an applied stress. With such additional data one may be able to better constrain stress magnitudes. 5. Conclusions The investigation documents abundant Dauphiné twinning in quartz subjected to intense dynamic stresses (N100–200 MPa). In natural rocks such conditions occur during meteorite impacts and seismic rupture, with high local stress concentrations. This investigation has focused on a few grains in a few samples. Thus, our general conclusions need to be corroborated in the future. For example, is twinning common in brittle rocks, such as fault gouges? Is twinning observed during spallation such as in deep mines? We would expect a high twin density in quartz fragments ejected during impact. And a systematic investigation of twinning should be conducted in metamorphic rocks. Interestingly, most texture studies of quartz still emphasize pole figures of c-axes and a-axes, even though modern 78 H.-R. Wenk et al. / Tectonophysics 510 (2011) 69–79 Fig. 10. Metamorphic muscovite quartzite Brg 980 from the Bergell Alps. (a) Optical photomicrograph taken with crossed polarizers; schistosity plane is near horizontal. (b) EBSD map of a small area with some Dauphine twins. White areas are muscovite. (c) Neutron diffraction analysis of a 1 cm cube illustrating a rather complex texture with large difference between positive and negative rhombs. (d) EBSD pole figure of a small surface area from (b). Two orientations related by Dauphiné twinning are identified by arrows. Pole figures are equal area projection, linear contours in multiples of a random distribution, s is the schistosity plane (horizontal). diffraction techniques allow to distinguish between positive and negative rhombs, and this trigonal orientation of quartz crystals appears to be very relevant. Finally, twinning should depend on crystal orientation. Are some orientations more twinned than others? 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