Tectonophysics 510 (2011) 69–79

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Mechanical twinning in quartz: Shock experiments, impact, pseudotachylites andfault 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, USAb GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germanyc Institut für Geowissenschaften, Geologie, Albert-Ludwigs-Universität, 79085 Freiburg, Germany

⁎ Corresponding author. Fax: +1 510 643 9980.E-mail address: wenk@berkeley.edu (H.-R. Wenk).

0040-1951/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tecto.2011.06.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 March 2011Received in revised form 14 June 2011Accepted 17 June 2011Available online 28 June 2011

Keywords:QuartzDauphiné twinningShock deformationSeismic stressPseudotachylitesEBSD

Increasing use of diffraction methods to study preferred orientation of minerals has established that quartz indeformed rocks not only displays characteristic c-axis orientation patterns, but that there is also generally adistinct difference in the orientation of positive and negative rhombs. In the trigonal quartz crystal structurepositive and negative rhombs are structurally different, and particularly negative rhombs (e.g. {0111}) aremuch stiffer than positive rhombs (e.g. {1011}). Here, we focus on the role of mechanical Dauphiné twinningunder stress as a cause of this difference and illustrate with EBSD measurements ubiquitous twinning inquartz-bearing rocks subjected to high stresses. Characteristic twinning is observed in experimentallyshocked sandstones and stishovite-bearing quartzites from the Vredefort meteorite impact site in SouthAfrica. Similar twinning is documented for quartz associated with pseudotachylites from the Santa Rosamylonite zone in Southern California, whereas quartz in underlying ductile mylonites are more or less twin-free. It suggests that twinning was produced by local seismic stresses that caused fracture and frictionalmelting 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 creepingsegment of the San Andreas Fault at Parkfield. From these observations it appears that Dauphiné twinmicrostructures can be diagnostic of high local and transient stresses.

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1. Introduction

It has long been known that quartz undergoes mechanical twinningwhen exposed to high stresses (Schubnikov, 1930; Schubnikov andZinserling, 1932). The significance of these twins in deformed quartzaggregates was first investigated by Tullis (1970) and Tullis and Tullis(1972). Mechanical twins occur in many materials (e.g., Klassen-Neklyudova, 1964) but Dauphiné twins in quartz are rather specialcompared, for example, with classical twins in carbonates (e.g. BarberandWenk, 1979; Pfaff, 1859) or hexagonalmetals (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 isachieved by a slight distortion of the structure (Fig. 1), withoutsignificant change in macroscopic shape of the quartz crystal. Twinningdoes not change the orientation of the c-axis or a-axes but reversespositive rhombs such as {1011} and negative rhombs {0111}. This is ofprofound mechanical importance, as directions normal to positiverhombs are half as stiff as those normal to negative rhombs (e.g.,McSkimin et al., 1965). In a compression experiment with a quartz

aggregate, crystals with normal to negative rhombs parallel to thecompression direction will become twinned, resulting in a neworientation with poles of positive rhombs parallel to the compressiondirection. In situ neutron diffraction experiments indicate that twinninginitiates at 50–100 MPa and that activation of twinning is temperature-dependent (Wenk et al., 2006, 2007).

Dauphiné twinning is expressed in the bulk preferred orientationof quartz crystals in a rock. If the orientation of c-axes and a-axes israndom, but positive and negative rhombs show an inverse patternwith corresponding minima and maxima, then it is likely that thispattern was produced by twinning; but the volume fractions of twinsand hosts must be different (e.g. Tullis, 1970). If positive and negativerhombs show the same orientation distribution (i.e., identical polefigures), this could be interpreted as grains that are divided into equalfractions of host and twin domains. It could also be due to a statisticaldistribution of untwinned grains in one orientation and anotherorientation related to the first orientation by a 180° rotation about thec-axis. For most metamorphic quartzites pole figures of positive andnegative rhombs are distinctly different (e.g., Baker and Wenk, 1972;Pehl andWenk, 2005; Wenk et al., 2009, 2010). This precludes a largefraction of twins in individual grains.

The actual presence of twins needs to be investigated at themicrostructural scale. Contrary to calcite twins, Dauphiné twins cannot

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 areshown with gray shades for different z-coordinates. Trigonal distortion is exaggerated(from Schubnikov and Zinserling, 1932).

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be detected with a petrographic microscope, due to the coincidenceof c-axes between twin and host. Thus, either transmission (e.g.,Barber andWenk, 1991) or scanning electronmicroscopy (e.g., Lloyd,2000, 2004; Trimby et al., 1998) is required to image them. Theelectron backscatter diffraction (EBSD) technique is most suitableand has been first applied by Heidelbach et al. (2000) to map twinboundaries in metamorphic quartz. This is themethod which wewillapply in this study.

There is no doubt that Dauphiné twins in quartz can be producedunder tectonic conditions, just as calcite twins form in metamorphicmarbles. It has also been established that twins develop during theβ–α phase transformation (Van Tendeloo et al., 1976). Here, we arefocusing on quartz in rocks that were subjected to high local dynamicstresses, such as in shock experiments, meteorite impacts and seismicevents.

2. Methods

From rock slabs 30 μm thick petrographic thin sections wereprepared and subsequently polished. First a 3 μm diamond polish wasapplied for roughly 2 h, then a 1/4 μmdiamond polish for half an hour,and finally the sample was polished for 5 min by hand with colloidalsilica. No coating was applied to the sample.

Fig. 2. EBSD diffraction patterns of quartz from two domains related by Dauphiné twinningtrigonal reflections with different intensity.

The thin section was first investigated with a petrographicmicroscope to identify regions of interest. Then selected regions werestudied in a Zeiss EVO MA10 scanning electron microscope (SEM) at25 kV, 100 μA beam current, 5 nA I Probe current, 10 Pa variablepressure vacuum to avoid charging, and a working distance of 18–25mm. The sample surface was tilted 70˚ relative to the horizontal.Diffraction patterns were recorded with a Digiview IV high resolutiondigital camera. Data collection and pattern indexing was performedwith the TSL-OIM software. Images with 1024×1024 resolution werebinned 2×2 or 4×4. Scans were performed over rectangular regions of200–300 μm in 1 μm steps. Such a fine step size is necessary to resolvethe twin boundaries satisfactorily. A scan usually took about 24 h.

Indexing of trigonal quartz is not trivial. Identification of the trigonalorientation relies on intensity differences between diffractions ofpositive and negative rhombs. Contrary to calcite, where rhombohedralspace group symmetry causes systematic extinctions for unambiguousindexing, in quartz both positive and negative rhombs diffract at thesame Bragg angle, though with different intensity. The intensitydifferences rely on the crystal structure, and hereby further confusionmay occur for quartz. Traditionally {1011} is the morphologicallydominant rhomb (e.g., Frondel, 1962; Goldschmidt, 1897; Hauy, 1801).This setting for theunit cellwasusedbyGibbs (1926) for thedescriptionof the crystal structure in space groupP3121, and it follows that for X-rayand electron diffraction the following intensity relationships existsbetween positive and negative rhombs: 1011N0111, 1012b0112,2011b0211, 2022N0222, 1013N0113. Unfortunately, some later de-scriptions of the quartz crystal structure have not followed thisconvention (e.g., discussion by Heaney et al., 1994, p 8) which is criticalfor an unequivocal definition of crystal orientation, as well as regardingphysical properties such as elasticity.

Before entering a quartz structure into EBSD indexing software it isnecessary to carefully check reflectors. Mostly EBSD systems do notdiscriminate intensity and, thus, only the more intense rhombohedralreflections should be used for indexing. Fig. 2 shows two diffractionpatterns which are related by Dauphiné twinning. Note that mostlines are identical. They define the geometry of the hexagonal unitcell. A few lines are different in intensity and one is indicated byarrows. If image quality is low, there is a fair probability that auto-matic indexing chooses the wrong orientation, resulting in individualspots which are related by the twin orientation.

We describe the procedure in some detail for a quartz crystal fromthe Vredefort impact site which will be described later in more detail.Fig. 3a shows an opticalmicrograph of a grainwith parallel deformationlamellae that was selected in a thin section. The SEM image withbackscattered electron (BE) contrast (collected with the forwardscattering detector on the tilted sample) (Fig. 3b) displays surface

. Note that only some bands are different. Arrows point towards corresponding band of

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 someDauphiné 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 mapsas 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 twinand host with a single c-axis maximum (determined by Euler angles ϕ1Φ) and 103 as well as 013 pole figures. The diffraction peak intensities (arrows, pole densities inmultiples of arandom distribution—m.r.d.) correspond to the volume fractions of host and twin in the image (c). Equal area projection.

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morphology (with some holes) and contrast variations that are partiallydue to the presence of Dauphiné twins. BE contrast depends mainly onatomic number and crystal orientation. On this image a region wasselected for a detailed scan (green square). Fig. 3c shows a correspond-ing orientation map of angle ϕ2. Based on indexing of diffractionpatterns, orientations of crystals relative to sample coordinates aredefined with three Euler angles ϕ1,Φ, ϕ2 (in Bunge notation, Bunge,1965). Anglesϕ1 andΦ define the orientation of the c-axis and angle ϕ2

the rotation of a-axes around the crystal c-axis and is thus sensitive toDauphiné twinning.

Scan data with orientations, confidence index, which is a measureof pattern identification, and image quality were then exported fromthe TSL-OIM software to BEARTEX (Wenk et al., 1998) for mappingand identification of Dauphiné twin boundaries with the routineMAPTEX. Dauphiné boundaries must satisfy two conditions: c-axesacross the boundary are the same, thus Euler angles ϕ1 and Φ areidentical (In our processing we allow a ±2° variation) and the

rotation around c defined by Euler angleϕ2 is 60° (180°) (also here weallow for a 60°±2° variation). If both conditions are satisfied, twinboundaries are plotted as red lines on the maps.

We can compile misorientation statistics between each cell on themap and surrounding cells and represent them on a histogram (Fig. 3d).For a single crystal all misorientations are close to zero with smalldeviations due to subgrain misorientations. For quartz, there is almostalways a peak at 60°, which is partly due to the presence of Dauphinétwins but can also be produced by misindexing. To some extentmisindexing can be minimized by rejecting orientations with lowconfidence index or low imagequality, butweneed to keep inmind thatthe 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 grayshades). These orientations are represented in pole figures, whichdisplay two orientations related to Dauphiné twinning (Fig. 3e). Theangle ϕ2 is most critical for identifying Dauphiné twins, as the two

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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. Thepole density for the host is higher (43 multiples of a randomdistribution, m.r.d.) than that of the twin (25 m.r.d.), indicating thatabout 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 astatistical result and we have not even attempted to quantify twinfractions in grains or relationships between twins and grainorientation, but the results obtained so far appeared to us convincingenough to support our conclusions.

For two samples, a granitic breccia from the Nördlinger Ries and ametamorphic quartzite from the Bergell Alps, we also display polefigures to illustrate bulk preferred orientation, especially the differencebetween positive and negative rhombs. Textures have been measuredon 1 cm cubes by time-of-flight neutron diffraction with the HIPPOdiffractometer 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 under-went shock-loading. Cratering experimentswere performed at the two-stage light-gas acceleration facility at Fraunhofer Ernst-Mach-Institute

Fig. 4. Seeberger sandstone, experimentally shock-deformed within the MEMIN project. (a)maps of Euler angle ϕ2, 1 μm steps. Dauphiné twin boundaries are red. In material distanboundaries coinciding with grain boundaries may be artifacts of misindexing. (c, d) Near thand at edges of grains. White regions could not be indexed or were rejected because of poo

in Efringen-Kirchen, Germany, as part of the MEMIN program (Multi-disciplinary Experimental and Modeling Impact Research Network,Kenkmann et al., 2011). MEMIN focuses on impact cratering exper-iments in geologicalmaterials to comprehensively understand details ofthe cratering process through in situ measurements, extensive post-impact analysis, and numerical modeling. The investigated samplestems from a calibration test shot in which a 1 cm steel projectileweighing 4.1 g was accelerated horizontally to ~4500 m s−1 andimpacted onto the flat, vertical surface of a 40 cm cube of Seebergersandstone. The kinetic energy of the experiment was 41.5 kJ and theexpected peak shock pressures were 50–55 GPa at the contact of theprojectile with the target. The crater volume was ~620 cm3. However,impact-induced fractures reached the edges and completely disjointedthe target cube.

The 20×15×2 cm sample investigated here contains the craterfloor. Some material around the projectile impact was partlypulverized and ejected. As the shock pressure rapidly decays inporous targets with increasing distance from the point of impact, theassumed shock pressure in the sample is estimated to be of the orderof a few GPa.

An optical examination of the thin section reveals rounded andslightly flattened grains of quartz with interstitial phyllosilicates(Fig. 4a). There are no obvious deformation features such as planardeformation lamellae in quartz. EBSD scans were performed onseveral grains from about 4 cm beneath the central crater floor and ongrains immediately on the crater floor. Grains from the subsurface are

Microstructure viewed with a petrographic microscope, crossed polarizers. (b-d) EBSDt (20 mm) from the point of projectile impact, grains are largely uniform (b). Somee point of impact grains display Dauphiné twinning, particularly near grain boundariesr confidence index, or because they represent other phases between quartz grains.

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uniform with only occasional twin boundaries (Fig. 4b). Grains in theimpact region clearly show an abundance of twins, particularly at themargins of grains and on surfaces where two quartz grains are indirect contact (Fig. 4c,d). Some grains are more intensively twinnedthan others, which could be related to the relation between shockwave direction and crystal orientation.

3.2. Quartz Deformed During Meteorite Impact

Based on preferred orientation patterns of quartz it was suggestedthat shock stresses may have caused Dauphiné twinning in stishovite-bearing quartzites from the Vredefort impact site in South Africa (Wenket al., 2005). Thiswas recently confirmed by EBSDmeasurements (Chenet al., 2011). Here, we reinvestigatemicrostructures in the same samplefromWeltevrede farm northeast of Parys, about 30 km northeast of theimpact center (Gibson et al., 1997). Dauphiné twins occur in grains thatare oriented with the c-axis in the plane of the section (Figs. 3a, 5a) andthe c-axis perpendicular to the thin section (Fig. 5b). Figs. 3a and 5adisplay deformation lamellae oriented at high angles to the c-axis.Orientationmapping clearly shows abundant twinswithdominant twinboundaries that are parallel to the deformation lamellae (Figs. 3c, 5c).This suggests a close relationship between the twomicrostructures. Thelamellae are subparallel to {1013} or {0113} (Fig. 3e, Trepmann andSpray, 2005; Vernooij and Langenhorst, 2005). Dauphiné twinningresolves this puzzling relationshipwith equivalentpositiveandnegativerhombs: a boundary that is parallel to a positive rhomb in the host is alsoparallel to the adjacent negative rhomb in the twin. In grains viewedalong the c-axis (Fig. 5b) no such relationship is evident. Also, there areabundant twins here (Fig. 5d). The domains are larger and less regular,though someboundaries display approximate 120° / 60° angles (arrow).

Fig. 5. Vredefort quartzite from Weltevrede farm. (a,c): Grain viewed perpendicular to thephotomicrographs with crossed polarizers. (c,d) EBSD maps of Euler angle ϕ2, 1 μm steps. Dadeformation lamellae (a) and twin boundaries (c). Arrow in (d) points to an approximate 1

A second natural impact sample investigated originates from theRies impact structure (sample 2008-02385 from the Museum fürNaturkunde, Berlin). The “granite breccia” from W of the town ofSchmähingen consists of coherent granitic fragments in a more fine-grained matrix of the same material (Von Engelhardt, 1974). Thefragments are rotated; we analyzed the microstructure of one of thefragments (our label R8). In the thin section we can see elongatedquartz grains (Fig. 6a). Texture analysis with the HIPPO neutron time-of-flight diffractometer establishes a strong preferred orientation ofc-axes (Fig. 6c), with a c-axis maximum exceeding 6 multiples of arandom distribution and distinct differences between positiverhombs {1011} and negative rhombs {0111}. The c-axis pattern isclearly not caused by impact and the rock represents a foliated gneissrather than a granite and has undergone extensive ductile deforma-tion. However, the orientation of the rhombs may be related to theimpact. Note that in pole figures of the positive rhomb {1011}subsidiary concentrations corresponding to maxima of the negativerhomb {0111}, are indicated by arrows (Fig. 6c). This is likelyevidence for reorientation by partial twinning. Quartz grains in thisRies sample do not display visible deformation lamellae (Fig. 6a),suggesting lower stresses than in the case of Vredefort. EBSD mapsnevertheless display abundant twinning (Fig. 6b).

3.3. Quartz Associated with Pseudotachylites

Having established that Dauphiné twins are common in shock-deformed materials, we wanted to explore whether stresses generatedduring seismic events could induce similar features. Obvious rocks toinvestigate are pseudotachylites that are thought to have originated byfrictional melting during seismic rupture (Sibson, 1975). We exploredtwinning in quartz-bearing rocks from the Santa Rosa mylonite zone in

c-axis shows deformation lamellae. (b,d): Grain viewed parallel to c-axis. (a,b) Opticaluphiné twin boundaries shown in red. Note the clear morphologic relationship between20˚ angle at a twin boundary.

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 agrain, 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 asingle crystal. Arrows in (1011) pole figure point at subsidiary maxima, corresponding to principal maxima of the negative rhomb (0111). Equal area projection. Linear contourlevels in multiples of a random distribution.

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Southern California. Here, there is a gradient from ductile mylonites atdepth to brittle deformation above (Wenk, 1998). Pseudotachylites areobserved in the vicinity of the brittle–ductile transition zone (Wenket al., 2000). In largely ductile mylonites, texture analysis reveals astrong difference between positive and negative rhombs (Pehl andWenk, 2005). Thus, twins cannot be prevalent.

Two examples of moderately deformed granite from the ductilezone obtained in upper Palm Canyon are shown in Fig. 7. Typicallythere are large grains of quartz with undulatory extinction, as in PC 89that was investigated previously for residual strain (Kunz et al., 2009)(Fig. 7a). With increasing deformation quartz recrystallizes, first alonggrain boundaries as in PC 88 (Fig. 7b), and ultimately throughout asample. EBSD scans of ductilely deformed quartz in both rocksindicate that grains are largely uniform and devoid of twins (Fig. 7c,d),although there are slight variations in orientation, visible as variationsin gray shades that are indicative of subgrain formation. This is verydifferent from observations made on quartz from the vicinity ofpseudotachylite veins, as shown by two examples: one is afragmented quartz grain directly adjacent to a pseudotachylite veinin PC 825 from the contiguous pseudotachylite zone exposed atelevation 1900 ft E of Martinez Mountain (Fig. 8a), and the secondone is a large quartz grain from about 2 cm from the pseudotachylitein PC 738c from Deep Canyon SE of Black Hill (at 3000 ft) (Fig. 8b).Both grains are fractured, with misplaced and slightly rotatedfragments. 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 samplerecovered from 3141 m depth (measured along the borehole) of the

phase 3 borehole of the San Andreas Fault Observatory at Depth ICDPproject (SAFOD; Hole E, Run 1, Section 6). This brecciated sample wastaken from a sequence of arkosic sandstone, which belongs to theSalinian Block (Springer et al., 2009). In thin section, the matrix ispredominantly composed of coarse- to very coarse, subrounded tosubangular quartz (36 wt.%), plagioclase (22%) and microcline (17%)(Janssen et al., 2011). The sample position was 16 m from thepresently inactive (non-creeping) ‘geological’ San Andreas Fault thatforms the eastern limit of the Salinian Block and 50 m from the activefault trace (southwest deforming zone/SDZ).

Large quartz grains are often fracturedwith displaced fragments in afine-grained cataclastic matrix (Fig. 9a,b). Some of these quartz clustershave been analyzed in detail. A very interesting pattern emerges: largecrystals are generally uniform, withoutmuch twinning, but corners andedges, as well as small fragments are pervasively twinned (Fig. 9b,c).Stresses which caused fragmentation were high enough to induce localtwinning.

4. Discussion

Mechanical twinning in quartz was discovered in experimentallystressed single crystals (Schubnikov, 1930; Schubnikov and Zinserling,1932). Localized twins were produced by mechanical action with steelpins or spheres. Later, stresses during growth were used to preventgrowth twins in piezoelectric material or to remove existing twins byapplying a torque at elevated temperature (Thomas andWooster, 1951;Wooster et al., 1947). At ambient temperature twinning occurs ataverage stresses around 500 MPa (Bertagnolli et al., 1979) and ispervasive at 1 GPa (Tullis and Tullis, 1972). At elevated temperaturesand pressures twinning is already initiated at stresses of 50–200 MPa(Wenk et al., 2006, 2007). But twins are induced at local stressconcentrations (Schubnikov, 1930) that are likely much higher than

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) Opticalphotomicrographs 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 undulatoryextinction due to moderate plastic deformation. (b) In this sample, quartz has become recrystallized along grain boundaries. (c,d) In both samples division into subgrains (variationin gray shades) is noted, but Dauphiné twins are largely absent.

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average stresses. The experimentally shocked sandstone was subjectedto stresses up to 50–55 GPa. (Kenkmann et al., 2011). However, thecoherent material studied here, contrary to ejected fragments, repre-sents 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 notrestricted to shock deformation but are also produced in conventionaldeformation experiments at elevated pressures and temperatures(e.g. Vernooij and Langenhorst, 2005) and observed in metamorphicrocks. As we have shown in Fig. 5c, these lamellae are closely linked toDauphiné twins, at least in Vredefort quartzite. The morphology, withtwin boundaries parallel to the rhombohedral lamellae, explains whythe lamellae are either of {1013} or {0113} orientations. In this samplethere is no evidence for amorphous zones or high dislocation densitiesalong lamellar boundaries, but obviously the EBSD resolution is not atthe nanometer scale. The close geometric relationship betweenlamellae and twin boundaries suggests that the two features wereproduced simultaneously.

Dauphiné twinning is pervasive in quartz rocks subjected tometeorite impact. Apart from Vredefort and Ries, it has beendocumented for the Charlevoix impact structure in Canada (Trepmannand Spray, 2005) and the Rochechouart impact in France (Trepmann,

2008). Shock pressures for their production are above the Hugoniotelastic limit and have been estimated at N3–8 GPa (Stöffler andLangenhorst, 1994). This is consistent with the partial conversion ofquartz to stishovite in theVredefort quartzite (Martini, 1978)whichalsosuggests pressures N7 GPa.

While twinning in quartz subjected to meteorite impact is wellestablished, it has been surprising to find similar microstructures inrocks exposed to seismic stresses. For this a survey through the brittle–ductile transition in the Santa Rosamylonite zone in Southern Californiahas been most revealing. Quartz in granitic rocks from the ductiledeformation zone, also associated with sillimanite and cordierite-bearing paragneisses and tremolite-bearing marbles, shows signs ofplastic deformation such as undulatory extinction, grain flattening andrecrystallization. This implies high temperatures and low stresses. Insuch rocks Dauphiné twins are largely absent. This situation changes asone passes the brittle–ductile transition zone, with a distinct zone ofpseudotachylites that can be attributed to ancient seismic activity. If oneaccepts the hypothesis that these enigmatic rocks are produced byfrictional melting on ruptured fault surfaces (Sibson, 1975), thenstresses had to exceed the shear strength of the rocks. Local stressconcentrations had to be higher than the shear strength of quartz toproduce extensive failure. Quartz is one of the strongest componentsof gneisses. There is abundant twinning in quartz associated with

Fig. 8. Quartz associatedwith pseudotachylite from the Santa Rosamylonite zone. (a,c): Sample PC 825,MartinezMountain. (b,d): Sample PC 738c, Deep Canyon. (a,b) Photomicrographstaken 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.

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pseudotachylites. If frictional melting occurred during rupture, then atleast locally temperatures were elevated, facilitating twinning. There isno evidence that local conversion to hexagonal β-quartz was achieved(650 °C at 1 GPa). The pseudotachylites studied here are of tectonic/seismic originbut it should bementioned that they also occur associatedwith impacts (Reimold, 1995).

Quartz in brecciated fault rocks from the San Andreas Fault atdepth also shows twinning, and it is likely that also in this case it wascaused by seismic stresses. These are likely local stresses during anearthquake, which is not inconsistent with the current creep mode ofthe fault at Parkfield, with very low average stresses (Hickman andZoback, 2004; Townend and Zoback, 2004). SAFOD stresses based ondislocation densities, twinning microstructures and preserved resid-ual 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 isevidence of local stress exceeding the strength of quartz. What is thestrength of quartz?

The compressive strength of quartz is very high (1–5 GPa, Griggset al., 1960; Kimberley et al., 2010), but the tensile and shear strengthsare two orders of magnitude lower (30–50 MPa, Ball and Payne,1976). Thus, in brecciated and fragmented quartz such stresses musthave been exceeded. This corresponds to stresses that can induceDauphiné twinning at low temperature and we should again keep in

mind that twins nucleate at local stress concentrations which arelikely much higher than average applied stresses. The experiment ofSchubnikov (1930) demonstrated that twinning occurs well belowfracture. The relative ease by which twinning occurs, raises thepossibility that some twins could be artifacts and have been producedduring sampling, e.g. extracting specimens with a geological hammer.We do not think that this was the case for the specimens describedhere. MEMIN and SAFOD samples were carefully cut with a microsaw.And while pseudotachylite and mylonites were collected with ahammer in the field, they were later also cut and internal portionswere used. So far bulk texture analyses of quartzites always show verysystematic orientation distributions that can be followed over largedistances (e.g. Pehl andWenk, 2005). If sample extraction would altercrystal orientations, this would not be the case. Nevertheless wewanted tomention this possibility and should be aware of conceivableartifacts.

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 figuresalways display a strong difference between positive and negativerhombs, which precludes pervasive twinning. This is also illustratedfor a greenschist-grade muscovite quartzite from the Bergell Alps inFig. 10. The microstructure consists of slightly flattened grains,separated by muscovite flakes (Fig. 10a). Neutron diffraction pole

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) EBSDmaps 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 matrixthat is largely feldspar.

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figures measured on a 1 cm cube show a bimodal c-axis distributionand a very distinct difference between positive and negative rhombs(Fig. 10c,Wenk et al., 2010). Positive rhombs {1011} have amaximumperpendicular to the schistosity plane s (horizontal). Below (Fig. 10d)are EBSD pole figures measured on a small surface segment. Positiverhombs show a clearmaximumperpendicular to the schistosity plane.This requires that there could not be equal fractions of twins and hostin each grain. The EBSD map (Fig. 10b) shows some twinning buttwins comprise only minor fractions, particularly at grain boundaries,and at boundaries between quartz and muscovite. Some grains aremore profusely twinned than others, suggesting orientation control.Indeed, in the EBSD pole figures (Fig. 10d) twin orientations can beclearly identified (arrows). Statistically, over a large volume, this isless 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 ortwinning? If twinning has occurred at metamorphic conditions, whyhas it not gone to completion? The morphology of twins in thismetamorphic quartzite is different from that in the stressed samples,where a clear tendency exists for 120° angles of boundaries and twinconcentrations at grain corners. The role of twinning in metamorphicrocks will be the subject of a future study.

Dauphiné twinning is not really a paleopiezometer because it doesnot provide information about exact stress magnitudes (Tullis, 1980).Initiation of twinning is highly orientation dependent (Schubnikovand Zinserling, 1932). It also depends on temperature and, atintermediate temperatures, nucleation of twins has been observedat 50–100 MPa (e.g. Wenk et al., 2006, 2007). Nucleation of twins is

followed by propagation of twin boundaries and, in an equilibriumsituation, twinning should go to completion, whichmay be the reasonwhy quartz twin boundaries are rare in higher grade metamorphicrocks. But high localized and transient stresses appear to imposecharacteristic twin domain structures in quartz of impactites andpseudotachylites, linking seismic events to meteorite impacts. Thisinvestigation, mainly on natural rocks, cannot determine magnitudesof local stresses to induce twinning and this should be approachedwith modernmethods such as nanoindentation (Wang et al., 2005) orin situ electron microscopy (e.g. Ye et al., 2010). Similarly, deforma-tion experiments, combined with EBSD analyses should be conductedto determine the influence of crystal orientation relative to an appliedstress. With such additional data one may be able to better constrainstress magnitudes.

5. Conclusions

The investigation documents abundant Dauphiné twinning inquartz subjected to intense dynamic stresses (N100–200 MPa). Innatural rocks such conditions occur during meteorite impacts andseismic rupture, with high local stress concentrations. This investiga-tion has focused on a few grains in a few samples. Thus, our generalconclusions need to be corroborated in the future. For example, istwinning common in brittle rocks, such as fault gouges? Is twinningobserved during spallation such as in deepmines?Wewould expect ahigh twin density in quartz fragments ejected during impact. And asystematic investigation of twinning should be conducted in meta-morphic rocks. Interestingly, most texture studies of quartz stillemphasize pole figures of c-axes and a-axes, even though modern

Fig. 10. Metamorphic muscovite quartzite Brg 980 from the Bergell Alps. (a) Optical photomicrograph taken with crossed polarizers; schistosity plane is near horizontal. (b) EBSDmap 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 differencebetween 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 areequal area projection, linear contours in multiples of a random distribution, s is the schistosity plane (horizontal).

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diffraction techniques allow to distinguish between positive andnegative rhombs, and this trigonal orientation of quartz crystalsappears to be very relevant. Finally, twinning should depend oncrystal orientation. Are some orientations more twinned than others?If this were established, it could become a method to determine thedirection of the highest stress.

Acknowledgements

This research was supported by grants from NSF (EAR-0836402),DOE-BES (DE-FG02-05ER15637), DFG-MEMIN and DFG-SAFOD(JA573/4-1). We are appreciative for samples obtained from theFraunhofer Institute in Efringen/Kirchen, the SAFOD team, from Prof.U. Reimold (Berlin) and the Berlin Museum of Natural History. Weappreciate access to the HIPPO neutron diffraction facilities at LANSCEand Sven Vogel for help with experiments. We are especially grateful

for reviews by Jan Tullis and an anonymous reviewer which helped usto improve the presentation.

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