Tectonophysics 603 (2013) 162–178

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Tectonophysics

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Seismic anisotropy in the Morcles nappe shear zone: Implications for seismicimaging of crustal scale shear zones

Bjarne S.G. Almqvist a,b,⁎, Ann M. Hirt a, Marco Herwegh c, Andreas Ebert d, Jens M. Walter e,Bernd Leiss e, Luigi Burlini b,†

a Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerlandb Institute of Geology, ETH Zürich, Sonneggstrasse 5, CH-8092, Zürich, Switzerlandc Institute of Geological Sciences, Bern University, Baltzerstrasse 1 + 3, CH-3012 Bern, Switzerlandd Geo Explorers International Ltd, Schlossstrasse 3, CH-4133 Pratteln, Switzerlande Geoscience Centre of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

⁎ Corresponding author at: Department of Earth Science16, 752 36, Uppsala, Sweden. Tel.: +46 703252066.

E-mail address: bjarne.almqvist@geo.uu.se (B.S.G. A† (Deceased Dec. 22, 2009).

0040-1951/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2013.05.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 January 2013Received in revised form 28 April 2013Accepted 21 May 2013Available online 3 June 2013

Keywords:TextureMicrostructureSeismic anisotropySecond phasesCarbonate mylonitesMetamorphic gradient

Microstructures and textures of calcite mylonites from the Morcles nappe large-scale shear zone in south-western Switzerland develop principally as a function of 1) extrinsic physical parameters including temper-ature, stress, strain, strain rate and 2) intrinsic parameters, such as mineral composition. We collected rocksamples at a single location from this shear zone, on which laboratory ultrasonic velocities, texture and mi-crostructures were investigated and quantified. The samples had different concentration of secondary miner-al phases (b5 up to 40 vol.%). Measured seismic P wave anisotropy ranges from 6.5% for polyphase mylonites(~40 vol.%) to 18.4% in mylonites with b5 vol.% secondary phases. Texture strength of calcite is the main fac-tor governing the seismic P wave anisotropy. Measured S wave splitting is generally highest in the foliationplane, but its origin is more difficult to explain solely by calcite texture. Additional texture measurementswere made on calcite mylonites with low concentration of secondary phases (≤10 vol.%) along the metamor-phic gradient of the shear zone (15 km distance). A systematic increase in texture strength is observed mov-ing from the frontal part of the shear zone (anchimetamorphism; 280 °C) to the higher temperature, basalpart (greenschist facies; 350–400 °C). Calculated P wave velocities become increasingly anisotropic towardsthe high-strain part of the nappe, from an average of 5.8% in the frontal part to 13.2% in the root of the basalpart. Secondary phases raise an additional complexity, and may act either to increase or decrease seismic an-isotropy of shear zone mylonites. In light of our findings we reinterpret the origin of some seismically reflec-tive layers in the Grône–Zweisimmen line in southwestern Switzerland (PNR20 Swiss National ResearchProgram). We hypothesize that reflections originate in part from the lateral variation in textural and micro-structural arrangement of calcite mylonites in shear zones.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Active source seismic reflection, which employs networks ofseismometers, is used to image the structure of the Earth's crust andupper mantle. Interpretation of these images is based in large onwhat is observed on the surface of the Earth, in terms of geologicalstructures, coupled with the rock elastic properties. Seismic reflectorsin orogens are often interpreted as lithological contacts or as defor-mation structures related to high strain zones (Fountain et al.,1984). The zones of high strain, or shear zones, appear as shallowly dip-ping reflective layers, sometimes extending over several kilometers in

s, Uppsala University, Villavägen

lmqvist).

rights reserved.

length (du Bois et al., 1990; Hedin et al., 2012; Levato et al., 1993).Thorough investigations of the reflective properties across shearzones have been dealt with in detail (e.g., Rey et al., 1994), whereaslateral change in reflective properties (i.e., along the shear zone)have been less recognized. Nonetheless lateral changes in reflectivitymay contain information on variation of the thickness, strain, andother physical and chemical characteristics of the shear zone. Inthis contribution we address the physical properties both across ashear zone, as well as along the transport direction of the shearzone. In particular the seismic anisotropy is addressed, because thisis a possible source for reflection in highly strained, ductile deformedmylonites that exist in the middle and lower crust, where brittledeformation is less important.

Sources for seismic anisotropy include, 1) crystallographic andshape preferred orientation of minerals (CPO and SPO, respectively;e.g., Baker and Carter, 1972; Crosson and Lin, 1971; Mainprice, 2007;Mainprice and Humbert, 1994; Owens, 1974; Owens and Rutter,

163B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

1978; Wenk, 2002). 2) Chemical and mineral composition in rocksfrom shear zones are also important. Volumetrically small amountsof highly anisotropic minerals, such as sheet silicates or amphibole,may contribute significantly to the seismic anisotropy, given thatthey develop a CPO (Burlini, 1994; Burlini and Fountain, 1993;Dempsey et al., 2011; Lloyd et al., 2009; Lloyd et al., 2011a,b; Tathamet al., 2008; Weiss et al., 1999). Several studies, both theoretical(e.g., Naus-Thijssen et al., 2011; Ward et al., 2012) and experimental(e.g., Almqvist et al., 2010a), have shown that interactions between amatrix phase and secondary phases can yield complex seismic prop-erties and anisotropy, N.B., in this case the matrix phase refers to themodally dominant phase, whereas secondary phases are modally sub-ordinate. Depending on the interaction and orientation of the matrixand secondphases, the anisotropy can be either constructive or destruc-tive. The first case is exhibited by deformed calcite mixed with musco-vite, where the crystallographic c-axis have a preferred alignmentnormal to the foliation plane, which is also the slow axis of the rock(Almqvist et al., 2010a). In contrast the alignment of quartz andmusco-vite c-axes leads to a decrease of the overall anisotropy, becausethe c-axis represents the fast velocity axis in quartz (Ward et al.,2012). 3) Finally, the crack and grain-boundary networks that developin strained, as well as in relatively undeformed rocks, may play a keyrole for the development of seismic anisotropy (Babuska and Cara,1991; Burlini and Kunze, 2000; Crampin and Peacock, 2005; Kernet al., 2008).

The present study investigates seismic anisotropy as a function oftexture andmicrostructures for plastically deformed calcite mylonitesfrom the Morcles Nappe shear zone. In the Morcles nappe it is possi-ble to investigate this relationship in terms of 1) local variation inphysical properties as a function of texture development, modal min-eral composition and crack network distribution, and 2) changes instrain and physical conditions (i.e., temperature, stress and strainrate). The first point is addressed by focusing on samples collectedfrom a single location across the shear zone, at constant physical con-ditions but containing varying amounts of second phases. The secondfactor is approached by calculating the physical properties as a func-tion of changing strain and texture strength along the transport direc-tion of the shear zone.

Fig. 1. (a) Cross-section of the Morcles Nappe shear zone, located in southwest Switzerlanindicated by a star (redrawn from Ebert et al., 2007; Escher et al., 1993). Samples collecwhite diamonds; (b) locality indicated by the star in (a), which shows the inverted stratigr

2. Locality and geological description

TheMorclesNappe shear zone is located in southwestern Switzerland,and constitutes the contact between the Helvetic nappe stack and under-lying crystalline basement, i.e., Aiguilles Rougesmassif and its autochtho-nous sediment cover (Fig. 1a). The intensity of deformation is highest inthe root of the shear zone in the Rhône valley and decreases towardsthe northwest, the frontal part of the nappe. The rock units in the rootof the shear zonehavebeen intensely stretched,with a thickness to lengthratio of over 1:400 (Ramsay, 1981; Siddans, 1983). Austin et al. (2008) es-timated strain rates between 10−10 and 10−12 s−1 using a combinationof paleowattmeter and paleopiezometer methods, which correlates wellwith estimates of Ebert et al. (2007) of 10−11–10−13 s−1. Metamorphicconditions reached greenschist facies in the root zone (350–400 °C),and gradually decrease toward the northwest, where the lowest gradeof anchimetamorphic conditions is reached (280 °C) (Ebert et al., 2007,2008; Frey et al., 1980). Part of the highly strained root has been displacedalong the Simplon-Rhône fault line and is now located near the townof Martigny. A number of researchers have addressed the conditionsof deformation and microstructures of the Morcles nappe shear zone(e.g., Austin et al., 2008; Dietrich, 1986; Dietrich and Song, 1984; Ebertet al., 2007, 2008; Ramsay, 1981; Ratschbacher et al., 1991; Schmidet al., 1981), and the origin of magnetic anisotropy in rocks from theMorcles Nappe recumbent fold and shear zone (Almqvist et al., 2009,2010b, 2011; Ihmlé et al., 1989). Preserved textures and microstructuresmainly represent steady-state dynamic conditions of deformation in theshear zone, with minimal influence of static recrystallization (annealing)subsequent to deformation or during uplift and erosion. Calcite twinningis observed in rocks from the shear zone, and its presence is related toretrograde strain localization and late stage deformation in the shearzone (Dietrich and Song, 1984; Ebert et al., 2007).

3. Methods

3.1. Texture measurements and imaging

The CPO for Malm, Barremian, White and Gray Urgonian sampleswas determined using electron backscatter diffraction (EBSD). EBSD

d. The sample site for the sub-sampled Malm, Barremian, Gray and White Urgonian isted for the magnetic anisotropy measurements presented in Fig. 8c are shown withaphic sequence of the four sampled units treated in this study.

164 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

measurements were performed at the Institute of Geology, BerneUniversity, with a Zeiss Evo 50XVP scanning electron microscope(SEM) equipped with a Digi View II EBSD camera and the OIM 5.31data acquisition software (Ametek, TSL). Details on the settings forthe scanning electron microscope used during measurements are de-scribed by Ebert et al. (2007), Herwegh et al. (2008) and Almqvistet al. (2010b). In addition, EBSD texture measurements and micro-structures, which were measured in Urgonian samples along theshear zone, have been used as a basis for calculation of physical prop-erties. The CPO is presented with respect to a macro-structural refer-ence frame where the X-axis is parallel to the lineation and foliation,the Y-axis is parallel to the foliation and perpendicular to the linea-tion and the Z-axis is normal to the foliation. Texture strengths are in-dicated by the texture index J (Bunge, 1982) and by the maximumintensities in the pole figures; i.e., multiples of random distribution(m.r.d.), or multiples of uniform distribution (MUD).

Neutron texture diffraction measurements were performed at theFRM II research reactor in Garching near Munich, Germany. As theCPO of mica is particularly challenging to measure with EBSD, neu-tron diffraction was chosen to measure the CPO of this mineralphase. These neutron diffraction measurements were performed onthe same sample cores that were used for laboratory seismic mea-surements, and provide a bulk texture measurement of the completecore volume due to the high penetration capabilities of neutrons.Neutron diffraction measurements additionally serve as a constrainton the suitability of using EBSD data to calculate the seismic velocitiesand anisotropy, by comparison of the CPO's obtained from eithermethod. At FRM II, the monochromatic stress and texture diffractom-eter STRESS-SPEC (Hofmann et al., 2006) was used in continuousmode as described in Brokmeier et al. (2011) with a wavelength of2.51 Å and a sample detector distance of 843 mm. Sample orientationduring measurement was selected to obtain lower hemisphere pro-jection with a foliation oriented horizontally in the view of the polefigures. Data processing was performed for the pole figure calcula-tion with the STeCa software (Randau et al., 2011). The experimentalpole figures are normalized to multiples of random distribution(m.r.d). Also Rietveld refinement was performed on the sum diffrac-tion pattern with the Fullprof package to verify the phase and struc-ture measured for texture analysis and thereby for indexing the polefigures (Rodriguez-Carvajal, 1993; Roisnel and Rodriguez-Carvajal,2000).

Microstructural data are compiled from Ebert (2007), and only abrief summary is provided here on the preparation of samples andthe measurements and analyses that were performed. Thin sectionsand thin slab samples were cut from hand samples, which werepolished and etched using a dilute hydrochloric and acetic acid, fol-lowing the two-step etching technique of Herwegh (2000). Imagingwas done with an SEM at the Geological Institute at Berne Universityin secondary electron (SE) mode and backscatter diffraction mode(BSE). A quantitative description of the microstructures was obtainedby tracing grain boundaries of calcite and secondary phases on thedigital SEM images using Adobe Photoshop. Analysis of the traced im-ages was made with the NIH-image software, which is available fromthe National Institute of Health (USA). Microstructures are describedin terms of grain-size and aspect ratio of calcite and secondary phases(b/a; grain short axis/grain long axis). The ratio between mean grainsize (dp) and the volume fraction (fp) of second phases is further usedto describe the microstructures. This ratio is referred to as the Zenerparameter, Z = (dp/fp)m⁎, where the superscript m⁎ is an exponentialfactor acting on the ratio dp/fp. Low values of Z (b100) imply that sec-ondary phases exert a large control on calcite recrystallization by ac-tion of pinning the migration of grain boundaries, whereas highvalues of Z indicate that recrystallization of calcite is not hinderedwith respect to second phases. We utilize this parameter to investi-gate its potential role in texture development and seismic anisotropy.Modal composition of the secondary phases quartz and dolomite

were determined using the element-based contrast of these twophases with respect to calcite (SEM BSE mode), in images orientedparallel to the XZ plane and the YZ plane. This allowed evaluation ofthe modal composition taking into consideration the shape preferredorientation of grains. However, a bigger challenge in accurately esti-mating the modal composition is related to heterogeneity in the dis-tribution of secondary phases in the sample, in particular whensecond-phase content is high. In this study we have simplified theevaluation of secondary phases to type-sections for sample, and seis-mic anisotropy calculation are based on this type case.

SEM analysis was complemented with Synchrotron-based X-rayTomographic Microscopy (SRXTM), performed at the Swiss LightSource (PSI Villigen). This analysis was made chiefly to investigatethe three-dimensional distribution of grain boundaries andmicrocracks present in samples with a small contribution of second-ary phases. The 2.4 GeV electron storage ring produces photons(X-ray light) with high brightness. The bright beam is diverted, inpart, to the TOMCAT (TOmographic Microscopy and Coherent rAdiol-ogy experimenTs) beamline, with an X-ray energy that covers therange between 8 and 45 keV. In our experimental setup the beam en-ergy was kept at 22.6 keV, with an exposure time of 300 ms, and 20×magnification. Specimens of 2 mm diameter were drilled from thesame cores used for elastic-wave measurements. High-resolution16-bit images were obtained using these operating conditions, witha pixel resolution of 0.38 μm. Each tomographic image covered anarea of 2048 × 2048 pixels (778 × 778 μm). Raw projections werepost-processed online and tomographic volumes are reconstructedusing a highly optimized Fourier-based algorithm (Marone et al.,2009). Image segmentation for 3D (volumetric) reconstructed imageswas performed using Avizo Fire.

3.2. Calculating seismic velocities and anisotropy

If the modal composition of each type of mineral present inthe sample is known, together with their CPO, it is possible topredict theoretically the physical anisotropies (e.g., Mainprice, 1990;Mainprice and Humbert, 1994). For this purpose it is also necessaryto specify the single crystal tensor properties of each type of mineralthat constitute the sample. Predicted seismic velocities are deter-mined, using the same reference frame as for the CPO. For calcitewe used the elastic stiffness matrix of single crystal calcite, given byChen et al. (2001), whereas dolomite and quartz are from Humbertand Plique (1972) and McSkimin et al. (1965), respectively. For thecalculations, the density of calcite is 2.71 g/cm3, dolomite is 2.83 g/cm3

and quartz is 2.65 g/cm3. The software used to calculate the elastic prop-erties is provided by D. Mainprice, and is available for download ascareware, on the Internet (ftp://www.gm.univ-montp2.fr/mainprice//CareWare_Unicef_ Programs). The calculated seismic velocities andanisotropy presented here are Hill averages (Hill, 1952), which isthe arithmetic mean of the Voigt (1928) and Reuss (1929) bounds.

As indicated by Mainprice and Silver (1993) and Mainprice andHumbert (1994) the anisotropy for P and S waves is calculated asAV = (Vmax − Vmin)/Vmean, where Vmean is (Vmax + Vmin)/2. S wavesplitting (dVs) is calculated as Vmax − Vmin.

3.3. Elastic-wave measurements and velocity anisotropy

Velocity measurements of ultrasonic waves were performed witha Paterson gas medium apparatus using the pulse transmission tech-nique (Birch, 1960, 1961). A cylindrical specimen of 2.54 cm diameterand 3–4 cm length is fitted between two ceramic transducers, whichare able to transmit and receive acoustic compressional and shearwaves at 1 MHz ultrasonic frequency. Two shear waves are generat-ed, which are polarized at right angles to each other. The sample col-umn assembly, which consists of the rock specimen and thetransducers, is covered with a copper jacket to prevent the confining

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gas from entering the sample assembly. All measurements reportedhere were performed at room temperature (~20 °C), but due to thegas pressurization of the sample, temperatures may fluctuate byabout ±10 °C throughout the pressurization and de-pressurizationcycle. The error estimate for determining first arrival of the P- andS-waves is partly dependent on the frequency of the transmittedwaves, and for 1 MHz frequency the error is about ±0.3%. The hydro-static pressure is measured with an error of ±1 MPa.

The sample assembly is initially pressurized in increments of~50 MPa, up to 450 MPa; P- and S-wave arrival times are recordedat each pressure increment. Subsequently the sample assembly isdepressurized in increments of ~25 MPa. After letting the sampleequilibrate for several minutes at each pressure step, the arrivaltimes for the waves are recorded. A hysteresis effect of the seismic ve-locities becomes apparent after the pressurization cycle, becausecracks seal and pores collapse with increasing pressure and do notre-open as the specimen is depressurized. Velocities are thereforehigher during depressurization compared to when the sample is pres-surized. The goal of the pressurization is to minimize the contributionof the influence of cracks on wave propagation, in order to obtain alinear relationship between pressure and crystal lattice compression.However, the crack closure effect as a function of confining pressureis mainly dependent on the shape of the pore and not all pores maybe closed, even at confining pressures in excess of 450 MPa. It is com-mon to extrapolate the high-pressure portion of the acoustic mea-surements (≥200 MPa) back to intersect with zero pressure, usinglinear regression, to obtain a value for the room pressure acoustic

Table 1Summary of elastic wave velocity (in km/s), Vp anisotropy and shear-wave splitting.a

Measured seismic properties Predicted seismic properties

Z -core X -core Y -core Z-axis X

M – MalmVp0 6.41 6.76 6.84 Vp 6.70 7Max Vs0 3.43 3.50 3.33 Max Vs 3.62 3Min Vs0 3.40 3.27 3.33 Min Vs 3.60 3dVs 0.03 0.23 0.00 dVs 0.02 0Vpmean (km/s) 6.67 Vpmean (km/s) 6AVp (%) 6.54 AVp (%) 5Max dVs (km/s) 0.23 Max dVs (km/s) 0AVs (%) 6.77 AVs (%) 3

B – BarremianVp0 6.14 6.83 6.75 Vp 6.22 6Max Vs0 3.35 3.31 3.30 Max Vs 3.58 3Min Vs0 3.31 3.30 2.90 Min Vs 3.34 3dVs 0.04 0.01 0.41 dVs 0.24 0Vpmean (km/s) 6.57 Vpmean (km/s) 6AVp (%) 10.44 AVp (%) 10Max dVs (km/s) 0.40 Max dVs (km/s) 0AVs (%) 13.13 AVs (%) 8

Ug – Grey UrgonianVp0 5.67 6.46 6.83 Vp 6.07 6Max Vs0 2.99 3.60 3.50 Max Vs 3.37 3Min Vs0 2.86 3.23 3.32 Min Vs 3.35 3dVs 0.13 0.37 0.18 dVs 0.02 0Vpmean (km/s) 6.32 Vpmean (km/s) 6AVp (%) 18.44 AVp (%) 12Max dVs (km/s) 0.37 Max dVs (km/s) 0AVs (%) 10.82 AVs (%) 7

U2 – White UrgonianVp0 6.28 6.93 6.83 Vp 6.28 6Max Vs0 3.32 3.34 3.49 Max Vs 3.41 3Min Vs0 3.16 3.26 3.11 Min Vs 3.35 3dVs 0.17 0.07 0.37 dVs 0.06 0Vpmean (km/s) 6.68 Vpmean (km/s) 6AVp (%) 9.81 AVp (%) 8Max dVs (km/s) 0.37 Max dVs (km/s) 0AVs (%) 11.24 AVs (%) 3

a Anisotropy (AVp, Avs) and max dVs are calculated based on velocities in X-, Y-, Z-core

velocity of the specimen with minimized influence of porosity(Birch, 1960; Burlini and Kunze, 2000; Wepfer and Christensen,1991). For this reason, the depressurization cycle is used for fittingthe linear regression because the influence of cracks is reduced com-pared to measurements during pressurization of the specimen. Thevalue that intersects with atmospheric pressure (strictly the 0 pres-sure) is known as Vp0 (e.g., Table 1). Velocity anisotropy is calculatedfrom the high-pressure portion of the data collected from the individ-ual cores, where AVp,s = 200 * (Vp,smax − Vp,smin)/(Vp,smax + Vp,smin). The maximum S wave splitting was calculated as Vsmax −Vsmin, where max and min are the maximum and minimum S wavevelocities, respectively.

4. Results from texture and laboratory measurements ofseismic properties

4.1. Sample description and imaging

Four type lithologies, which are present throughout the shear zone,were sampled for laboratory seismic and texture measurements. Thesamples were taken from the Malm, Barremian, and Urgonian Forma-tions, and comprise mainly mylonitized carbonates with varying vol-ume concentrations of accessory minerals or secondary phases (seeFig. 1a for sample location; Swiss coordinates 578.950/113.500). Rocksfrom the Urgonian Formation occur in gray and white varieties,depending on their organic content. Stratigraphically, the WhiteUrgonian is on top of the Gray Urgonian, and the two are typically

CPO properties (calcite) Second phases

-axis Y-axis J-index max MUD (c-axis) Vol.% Minerals

.06 7.01 1.59 2.57 15-40 Dolomite, quartz

.69 3.72

.60 3.61

.09 0.11

.92

.37

.11

.00

.88 6.79 2.79 4.72 b10 Quartz, pyrite

.65 3.67

.35 3.43

.30 0.24

.63

.61

.42

.57

.84 6.84 3.76 6.24 b5 Mica

.58 3.59

.36 3.35

.22 0.24

.58

.69

.24

.16

.71 6.80 1.88 3.37 b5 Mica

.48 3.48

.35 3.41

.13 0.07

.60

.28

.13

.81

s for measurements and along X, Y, Z-axes for modelled velocities.

Fig. 2. (a, b) SEM images in backscatter electron mode for the Malm and Barremian samples, respectively. Mineral abbreviations are dolomite (Do), calcite (Ca) and quartz (Qtz);(c–e) Micro-CT images of Malm, White and Gray Urgonian, respectively. Note that the Malm has a considerable volume of dolomite, in agreement with the SEM backscatterimage in (a), whereas both Urgonian samples are nearly pure calcite; (f–i) 3D reconstructions of solid and pore space in the Gray (f, g) and White (h, i) Urgonian samples.The yellow shading represent pores and cracks in the two samples, a distinct crack-network has developed parallel to the foliation in the Gray Urgonian, which is related tothe white banding observed in the inset image of the sample core. This crack-network is notably absent in the White Urgonian. Inset images in (a), (b), (f) and (h) are actualcores used for ultrasonic measurements (2.54 cm diameter and ~3.5 cm length).

166 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

167B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

referred to as the upper and lower Urgonian, respectively. The domi-nant secondary phases are quartz, dolomite and mica. Malm has thehighest concentration of second phases, and represents the case of apolyphase sample, with up to 40 vol.% secondary phases, mostly

2.08

.18

X

Z(110)

N = 170442

.40

.80

1.20

1.60

(001)

(a) Barremian - calcite

1.37

.79

N = 28449

.88

1.04

1.20 Barremian quartz

N = 155556

(b) Malm calcite

Malm dolomite

Sense of shear

1.79

.41

N = 1858

.80 1.00 1.20 1.40

(c) W. Urgonian calcite

(d) G. Urgonian calcite

1.80

.37

N = 5208

.60

1.00

1.40

2.28

.13

N = 3843

.40

.80 1.20 1.60 2.00

X

Z

X

Z

X

Z

X

Z

X

Z

(110) (001)

(110) (001)

(110) (001)

(110) (001)

(110) (001)

1.37

.61

.72

.88

1.04

1.20

Fig. 3. Texture measurements, shown by pole figures of a (2–10), c (001), r (104) and f (012and dolomite are shown for the Barremian and Malm units, respectively. In the pole figurecontour lines represent multiples of uniform distribution (MUD).

dolomite and quartz (e.g., Fig. 2a, c). The Barremian consists mainly ofcalcite (90 vol.%), with second phases being a mixture of quartz andmica (Fig. 2b). Urgonian samples generally have lower concentrationof second phases, compared with Malm and Barremian. The content of

4.72

.13

1.0

2.0

3.0

4.0

(104) (012)

2.34

.67

1.00

1.40

1.80

3.52

.23

1.0

2.0

3.0

3.37

.08

1.0 1.5 2.0 2.5

6.24

.06

2.0 3.0 4.0 5.0

(104)

(012)

(104) (012)

(104) (012)

(104)

(104) (012)

2.03

.33

.60

1.00

1.40

1.80

1.95

.36

.60

.801.001.201.401.60

1.41

.61

.72

.88 1.04 1.20

1.56

.60

.80

1.00

1.20

1.40

2.40

.50

.80

1.20

1.60

2.00

1.53

.52

.70

.90 1.10 1.30

1.43

.59

.72

.88 1.041.20

(012)

1.97

.37

.60

.80 1.00 1.20 1.40 1.60

1.54

.32

.60

.801.001.20

1.38

.55

.64

.80

.96 1.12 1.28

1.35

.64

.72

.88

1.04

1.20

1.48

.73

.88

1.04

1.20

1.36

1.51

.78

.88

1.04

1.20

1.36

) with gray shading indicating the intensity of the CPO. Additional pole figures of quartzs the maximum (minimum) pole density is indicated by a white square (black circle);

168 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

second phases is low (b5 vol.%) in the two Urgonian samples that areinvestigated in this study, although the microstructures are consider-ably different in the White Urgonian versus the Gray Urgonian(Fig. 2d–i). For seismic measurements, three mutually perpendicularcores of 2.54 cm diameter and approximately 3.50 cm length, weredrilled from block samples, parallel to the lineation and foliation inthe foliation plane (x-core), perpendicular to lineation (y-core), andnormal to the foliation plane (z-core). Although the foliation is easy toidentify, it is more difficult to identify the lineation of the samples.

Ebert et al. (2007) observed a gradual change in deformationmechanisms that coincides with the metamorphic gradient, texturestrengthening and an increase in recrystallized calcite grain size fromthe frontal part towards the root zone of the shear zone. In terms of spe-cific mechanisms this corresponds to a transition from deformationdominated by viscous granular flow (i.e., sliding of grains past eachother without crystal-plastic deformation) to dislocation creep. Aconsistent variation in the grain-size is observed for the carbonatemylonites along the thrust, with grains up to 60 μm diameter in theproximal portion of the Morcles thrust (~380 °C), with a reduction to5–10 μm in diameter in the distal part where temperatures reach~280 °C.

4.2. Texture measurements

A summary of the results from texture measurements is providedin Table 1. The CPO of calcite is shown in Fig. 3, which shows for thecalcite a (110) c (001), r (104) and f (012) pole figures, for the foursamples. Dolomite and quartz pole figures are additionally shownfor the Malm and Barremian units, respectively. The Malm has theweakest CPO in the sample group, with a J-index = 1.59, followedby the White Urgonian, which has only a slightly stronger CPO (J =1.88). Barremian (J = 2.79) and Gray Urgonian (J = 3.76) have con-siderably stronger textures (Table 1). The texture of the Barremianand Malm display oblique concentration of c-axes with respect tothe pole to the foliation plane and a-axes are oriented in a girdle in-clined about 30° to the foliation plane. This offset of c-axes with respectto the normal to the foliation plane may be partly due to grain-boundary sliding (slipmechanism),which also yields the comparativelyweak texture (see also argumentation in Ebert et al., 2007, 2008). Thetwo Urgonian samples display c-axes concentrations near the pole tofoliation, with their a-axes oriented in a girdle sub-parallel to the folia-tion plane.

30 40 50 60 70

Inte

nsity

(co

unts

)

1.8×104

1.4×104

1.0×104

0.6×104

0.2×104

-0.2×104

2θ (°)

a

Fig. 4. Neutron texture diffraction as refined sum diffraction pattern (left) and experim

Neutron diffraction texture measurements were performed on theGray Urgonian sample, in order to investigate the possible influenceand coupling of calcite and oriented mica in this sample. As shownby the Rietveld refinement in Fig. 4 no mica reflexes were measured.This results from the comparative low phase proportion of mica in thesample volume. Thereby calcite reflexes dominate the diffraction pat-tern and the mica is not detectable (mica makes up less than 5 vol.%of the total rock specimen). The Goodness of Fit (GoF) obtained forcalcite from the Rietveld refinement is given by the Bragg R-factor9.60 and by the RF-factor 7.28. These values are slightly high due tothe strong texture in the material as shown in the (006) and (110)pole figures with maximum texture intensities of 5.67 m.r.d. and2.38 m.r.d. respectively. This effect is also shown by the differenceline between the measured and recalculated patterns in Fig. 4 (blueline). The refinement shows that all peaks of the diffraction patterncan be indexed to calcite. The Gray Urgonian texture strength for cal-cite determined by neutron texture diffraction is similar to the EBSD(Fig. 4).

4.3. Calculated seismic anisotropy

Predicted seismic velocities and anisotropies are computed fromthe calcite texture, as well as volumetrically significant secondaryphases in the case of Malm and Barremian samples. The results areshown in Fig. 5. Fast P wave propagation is sub-parallel to the folia-tion plane, with a girdle distribution, for the Gray andWhite Urgoniansamples, whereas the Barremian and Malm has its fast propagation ina girdle that is oblique to the foliation plane. Slow P wave velocitiesare oriented normal to the foliation plane, with the exception ofthe Malm. Predicted maximum anisotropies are significantly differentbetween samples, with 6.6% for theMalm, 8.8% for theWhite Urgonian,12.2% for the Barremian and 12.8% for the Gray Urgonian. Shear-wavesplitting is greatest sub-parallel to the foliation plane in the case ofWhite and Gray Urgonian (Fig. 5). For these samples the maximumAVs and dVs are similar, ranging from 5.62–7.28%, and 0.19–0.26 km/s,respectively (Fig. 5). Malm displays maximum AVs of 4.07% and dVsof 0.15 km/s at an angle of about 30° offset to the pole to the foliationplane, which is slightly lower than the other samples. The Barremiansample has a maximum AVs of 11.75% and dVs of 0.42 km/s, alsooblique to the foliation plane but considerably higher than the Malm.Note that the seismic velocities and anisotropy listed in Table 1 aregiven with respect to the X- (lineation), Y- and Z-axis (normal to

1.02.03.04.05.0

0.51.01.52.0

80 90

measured

calculated

difference

(006)

(110)

m.r.d.

m.r.d.

b

ental pole figures (right) of c (006) and a (110) for the gray Urgonian specimen.

6.90

6.10

Vp Contours (km/s)

Anisotropy = 12.2%

6.24

6.40

6.56

6.72

11.75

.26

7.09

6.63

Vp Contours (km/s)

Anisotropy = 6.6%

6.70

6.80

6.90

7.00

4.07

.03

Max AVs (%)b) Barremian (94 vol% calcite, 6 vol% quartz)a) Malm (64 vol% calcite, 36 vol% dolomite)

c) Gray Urgonian (100 vol% calcite) d) White Urgonian (100 vol% calcite)6.84

6.26

Vp Contours (km/s)

Anisotropy = 8.8%

6.406.486.566.64

5.62

.12

6.88

6.06

Vp Contours (km/s)

Anisotropy = 12.8%

6.24

6.40

6.56

6.72

7.28

.15

Min AVs (%)

Max AVs (%)

Min AVs (%)

Max AVs (%)

Min AVs (%)

Max AVs (%)

Min AVs (%)

Fig. 5. Predicated P wave, S wave anisotropy (AVs). The square (circle) for predicted velocities indicate the axis of fast (slow) wave-propagation in the plot of Vp contours. Note thatVs1 shows the plane of polarization for fast shear wave, with respect to the sample coordinate system (X, Y, Z), as shown in Fig. 3. Foliation plane is indicated by a horizontal blackline in each stereographic projection; projections are lower hemisphere equal area projections.

169B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

foliation) structural coordinate system, in order to provide a compari-son with measured seismic properties.

4.4. Ultrasonic velocity and anisotropy measurements

Seismic P and S wave velocities for the selected samples have beenmeasured as a function of confining pressure (Figs. 6 and 7, Table 1).The fast wave-propagation axis in all samples is either along the X-axis(stretching lineation) or in the X–Y plane. Only for the Gray Urgonianis there a marked difference in velocities between the X- and Y-cores.The slow axis ofwave-propagation is consistently normal to the foliationplane. Seismic P wave anisotropy is 18.4% in the Gray Urgonian, 10.4% inthe Barremian, 9.8% in the White Urgonian and 6.5% in the Malm(Table 1). The shape of the velocity curve in Fig. 6c for the GrayUrgonianindicates a different behavior during depressurization of the sample incomparison with the other samples, for which the velocity curves areflatter. The velocities of two perpendicularly polarized shear-waves fordifferent sample orientations are shown in Fig. 7, providing a measureof shear-wave splitting (dVs) and anisotropy (AVs; Table 1). Thesmallest difference between the polarized shear-waves is observed forthe Z-core, and only in the White and Gray Urgonian mylonites is thedifference slightly larger than 0.1 km/s. Shear-wave splitting is largestin the X–Y plane (Y-core, X-core), with the highest observed value forthe Barremian of 0.41 km/s (AVs = 13.1%), followed by White andGray Urgonian, which have similar dVs and the lowest for Malm, withdVs of 0.23 km/s (AVs = 6.8%).

5. Discussion

5.1. Calculated and measured seismic properties

Measured and predicted seismic velocities and anisotropy agreewell in general for the P wave, when comparing seismic velocitiesin the principal axes, X, Y and Z (Table 1). The difference in seismicP wave anisotropy agrees within 2% of the measured anisotropy forthe Malm, Barremian and White Urgonian samples, indicating thatcalcite texture is mainly responsible for the physical anisotropies ofthese samples. The minor difference observed between measured and

calculated velocities may be related in part to the relatively small vol-ume considered by EBSD texture measurements, or alternatively totheminor influence from alignedmicrocracks/grain boundaries. Calcitetwins formed during experimental deformation cannot be ruled outbecause the CPO of the mylonites may be responsible for a small differ-ential stress. This is, however, unlikely due to the use of hydrostaticpressure at room temperature. Even though a considerable volume ofsecondary phases occur in theMalm, the anisotropy is governed largelyby calcite. However, indirectly the strength of the CPO and recrystalliza-tion of calcite is governed by the large volume of second phases presentparticularly in Malm, but also the Barremian; the influence of secondphases on the symmetry and strength of the CPO is discussed inSection 5.3. In contrast, the predicted seismic P wave anisotropy forthe Gray Urgonian, based on the calcite texture, is ~6% lower than themeasured P wave anisotropy. The disagreement between calculatedand measured seismic properties is also discussed below.

Shear wave velocity and splitting, obtained frommeasurements andcalculation, do not compare as well as the comparisons with the P wavevelocities. In some cases the predicted S wave velocity agree with themeasurements, but not in other cases; there is no obvious systematictrend in the comparison of measured and calculated S wave velocities.Shearwave splitting is, as expected, lowest for Swaves propagating nor-mal to the foliation plane both from measurements and calculated dVs.However, the magnitudes of the measured and calculated maximumdVs are significantly different, and are consistently higher for the mea-surements. One reason for the disagreement could be the uncertaintyin determining the structural lineation of the samples. Ivankina et al.(2005) noted that the trend of calculated shear wave splitting, basedon modal mineral composition and texture, was significantly differentfrom measurements in cases where the shear waves were not ideallypropagating perpendicular to foliation, or parallel to lineation; betteragreementwas foundwhen thiswas the case. Pwaveswere not affectedto the same extent to propagation parallel or normal to macroscopicstructural elements. The distribution of microcracks and grain bound-aries, which influences S wave propagation, represents another possiblereason for disagreement.

Microstructural differences for the Gray and White Urgonian sam-ples are shown in Fig. 2, which highlights in particular the different

4

4.5

5

5.5

6

6.5

7

7.5

8

0 100 200 300 400 500

Z - core

X - core

Y - core

4

4.5

5

5.5

6

6.5

7

7.5

8

0 100 200 300 400 500

Z - core

Y - core

X - core

4

4.5

5

5.5

6

6.5

7

7.5

8

0 100 200 300 400 500

Z - core

Y - core

X - core

4

4.5

5

5.5

6

6.5

7

7.5

8

0 100 500200 300 400

Z - core

Y - core

X - core

a) M - Malm b) B - Barremian

c) Ub - Gray Urgonian d) U2 - White Urgonian

Confining Pressure (MPa) Confining Pressure (MPa)

Confining Pressure (MPa) Confining Pressure (MPa)

km/s km/s

km/s km/s

XY

Z

Shear-direction

foliation

Stretchinglineation

Fig. 6. P wave velocities as a function of confining pressure and three sample-axes (X, Y, Z), for (a) Malm, (b) Barremian, (c) Gray Urgonian and (d) White Urgonian; X is parallel tothe lineation and in the plane of foliation; Y is in the foliation, but perpendicular to the lineation; Z is normal to the foliation.

170 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

porosity character of the two samples.Whereas theGrayUrgonianhas aheterogeneous, layered structure, theWhite Urgonian is nearly homog-enous. This is also evident from the micro-CT images of these two sam-ples. The Gray Urgonian has a clear network of grain-boundariesparallel to the foliation (Fig. 2d). These grain-boundaries are similar tomicrocracks, with an aperture of ~1–2 μm. In contrast the WhiteUrgonian does not show the same type of grain-boundary network,but rather only the junction that is characteristic of adjacent grains(Fig. 2e, h, i). Three-dimensional reconstructions show the samecharacteristics, and the volume of void space (porosity) is signifi-cantly higher in the Gray Urgonian (Fig. 2f). The grain-boundaries ofthe Gray Urgonian (shown by yellow of Fig. 2g) outline the individualgrains (N.B., the solid component is not visible). A more detailed inves-tigation of microstructures in the Urgonian is given by Ebert et al.(2007).

As suggested by micro-CT imaging of the Gray Urgonian, the highseismic anisotropy arises because of aligned micro-cracks, whichwould effectively increase the difference in velocities measured parallel

Fig. 7.Measured velocities of two perpendicularly polarized shearwaves, as a function of confinUrgonian. X is parallel to the lineation and in the plane of foliation; Y is in the foliation, but perprefer to the two polarizing transducers, without reference to a “fast” or “slow” shear wave, and

and perpendicular to the foliation plane (e.g., Figs. 6 and 7; Crampin,1978, 1984). As was previously mentioned in the sample description,mm to sub-mmwhite bands are apparent in theGrayUrgonian samples.The concentration of micro-cracks is highest along these white bands,which effectively lowers the Pwave velocitymeasured normal to the fo-liation (Z-core of Fig. 6c). The non-linear increase in Vp at low confiningpressures, which is unique for the Gray Urgonian samples, suggest asignificant contribution from crack closure. The influence of crack clo-sure is also seen in the S wave velocities. A detailed quantification ofthe microcrack contribution is beyond the scope of this study, butwould represent a useful follow-up study. At confining pressures>200 MPa the increase in Vp and Vs become nearly linear as a functionof pressure, and the difference betweenmeasurements normal and par-allel to foliation likely arises from the CPO and possibly a minor contri-bution from secondary phases, such as micas. However, thecontribution from micas is probably not substantial, because neutrondiffraction measurements did not identify its presence by volume (i.e.,b5 vol.% mica). The combined effect of the high compressibility of

ing pressure and orientation, for (a)Malm, (b) Barremian, (c) Gray Urgonian and (d)Whiteendicular to the lineation; Z is normal to the foliation. Note that the s7 and s8 in the legendare used only to indicate the maximum shear wave splitting along each core-axis.

d) U2 - White Urgonian

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 500

c) Ug - Gray Urgonian

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 500

b) B - Barremian

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 500

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 500

s8s7

2.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 5002.60

2.80

3.00

3.20

3.40

3.60

3.80

4.00

0 100 200 300 400 500

a) M - MalmZ-core Y-core X-coreVS (km/s) VS (km/s) VS (km/s)

VS (km/s) VS (km/s) VS (km/s)

VS (km/s) VS (km/s) VS (km/s)

VS (km/s) VS (km/s) VS (km/s)

Confining Pressure (MPa) Confining Pressure (MPa) Confining Pressure (MPa)

Confining Pressure (MPa) Confining Pressure (MPa) Confining Pressure (MPa)

Confining Pressure (MPa) Confining Pressure (MPa) Confining Pressure (MPa)

Confining Pressure (MPa) Confining Pressure (MPa) Confining Pressure (MPa)

171B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

6.40 - 6.78 km/sVpANIS = 5.8 %R = 0.014

6.09 - 6.95 km/sVpANIS = 13.2 %R = 0.036

6.33 - 6.81 km/sVpANIS = 7.4 %R = 0.017

6.15 - 6.88 km/sVpANIS = 11.2 %R = 0.032

Dislocation creepdominated deformation

Diffusion creepdominated deformation

Sub-greenschist (280 °C)

“Root zone”Greenschist (395 °C)

NW

SE

0 5 km

280

299309

337

358364395

0 5 10 15 200

10

20

30

40

50

60

70

Cal

cite

Gra

in S

ize

(µm

)

Distance along the shear zone (km)

Temperature ( °C)d

Δk (SI)

0.00E+00

1.00E-07

2.00E-07

3.00E-07

4.00E-07

5.00E-07

6.00E-07

7.00E-07

8.00E-07

0 5 10 15 20

Distance along shear zone (km)

Vp anisAMS

0

2

4

6

8

10

12

14

16

18

PredictedVp anis (%)

c

T (°C)

Predicted Vp anis (%)

200 300 400 500

18

16

14

12

10

8

6

4

2

0

b

a

Fig. 8. (a) Cross-section of theMorcles nappe, showing the Pwave seismic anisotropy and reflection coefficients, calculated at different positions along the shear zone.Metamorphic tem-peratures, as well as inferred deformation mechanisms are shown in italic script. (b) Predicted P wave anisotropy as a function of peak metamorphic temperature (°C) of Urgonianmylonites (mixed White and Gray Urgonian specimens), based on calcite texture EBSD measurements made by Ebert (2007) from samples at different locations along the shear zone.Error bars indicate 1σ confidence interval, and are estimated based on texture measurements from several separate samples at each location. (c) The average calculated P wave seismicanisotropy is shown as a function of location along the shear zone (left Y-axis with gray diamonds). On the right Y-axis the difference in principal axes of diamagnetic suscep-tibility, Δk = k1 − k3 (for details, see Almqvist et al., 2011). Error bars indicate the 1σ confidence interval. (d) Calcite grain-size as a function of location along the shear zone(i.e., a function of temperature; data compiled from the measurements of Ebert et al., 2007, and redrawn from Austin et al., 2008).

172 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

sheet silicates and basal-plane parallel cracks has also been observed byCirrincione et al. (2010) in leucogneisses from Calabria.

5.2. Seismic anisotropy inferred along the shear zone, in monophasemylonites

Fig. 8a shows the predicted P wave anisotropy based on EBSD tex-turemeasurements of Gray andWhite Urgonianmylonites for differentpeak metamorphic temperatures along the Morcles nappe shear zone.

The anisotropy is high for peak conditions with a temperature near400 °C, and gradually decreases towards the frontal part of the nappewhere the peak temperature decreases below 300 °C (Fig. 8b, c). Withincreasing metamorphic grade, the strain and correspondingly the tex-ture changes along the thrust. Seismic P wave anisotropy calculatedfrom the calcite textures of Urgonian samples, increases from 6% toalmost 14% over the 15 km distance along the shear zone. The trendof predicted VP anisotropy is supported by the change in magnetic an-isotropy due to diamagnetic minerals (i.e., calcite) along the thrust

173B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

(Fig. 8c; Almqvist et al., 2011). The agreement between the calculatedseismic and diamagnetic anisotropy supports the interpretation thatthe seismic anisotropy arises largely from the calcite CPO. The changesin physical and microstructural properties of the calcite mylonitescan be related to the change in deformation mechanisms alongthe shear zone, from granular flow in the distal and central part todislocation creep dominated deformation in the proximal part, or rootzone (Ebert et al., 2007). As a consequence the texture strength ofcalcite and hence also the anisotropy of physical properties increasestowards the root of the shear zone. There is one deviation from themonotonic increase at 16 km distance along the shear zone. Rocks atthis location display evidence for partial tectonic overprinting, due tothe Rhône-Simplon fault, which has effectively decreased the CPO in-tensity (see Ebert et al., 2007 and Almqvist et al., 2011). In addition,the texture strength and the predicted seismic P wave anisotropyhave a similar trend to the calcite grain coarsening along the thrust(Fig. 8d; Austin et al., 2008).

5.3. Seismic anisotropy in polyphase mylonites

The presence of second phases has been demonstrated to have astrong influence on mechanisms of ductile deformation in rocks(Herwegh et al., 2011). Calcite grain-boundary migration becomesinhibited during dynamic recrystallization by pinning produced bysecondary phases. As a consequence this tends to lead to smallerrecrystallized calcite grain-sizes and weaker textures (Herwegh and

0.5

0.55

0.6

0.65

0.7

0.75Grain aspect ratio (=b

Z (=1 101

b) Calcite

1

10

100

1 101 102 103 104

Grain size (µm)

Z (= dp/f

p)

a) Calcite

Second-phasecontrolled

Recrystallization controlled

Urgonian

Urgonian

Urgonian

Malm

Malm

Barremian

358 °C

337 °C

329 °C

309 °C

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Z (=1 101

d) Second phaseGrain aspect ratio (=b

Legend

Fig. 9. Microstructural data as a function of the Z-value for the different lithologies investid) grain aspect ratio (=b/a) of secondary phases. Grain sizes represent area-weighted estimsurements performed across a thin section or thin slab sample. The angle of grain long-axesary phases; n is the number of data in each bin.

Berger, 2004; Herwegh et al., 2005). However, not only the volumefraction (fp) of second phases is important, but also their grain size,dp. This effectwas highlighted by Zener (in Smith, 1948),who identifieda relationship between the ratio of grain size and volume fraction of sec-ondary phases. Hence the expression of the Zener (Z) parameter, whichis defined as Z = dp/fp. Small Z-values result in a microstructural devel-opment that is controlled by secondary phases, whereas large Z-valuesresult in microstructures whose recrystallization is independent of sec-ondary phases, because of their sparse number. A pertinent questiontherefore arises when comparing monophase with polyphase calcitemylonites. This section examines the possible influence of secondaryphases on the microstructural and textural development, and conse-quently the seismic anisotropy.

To further investigate this influence, we measured calcite grainsize and grain aspect ratio as a function of the Z-parameter in samplesclose to the location at which seismic measurements were performed.The results are summarized in Fig. 9. It is evident that both thegrain-size and aspect ratio vary as a function of Z. Although a limitednumber of data are available for the Malm and Barremian, these sam-ples have lower Z-values in general and correspondingly smaller cal-cite grain-size. The Malm samples deviate slightly from the overalltrend, with higher aspect ratios than Urgonian and Barremian sam-ples, for both calcite and second phases (Fig. 9b, d). This is likelydue to the presence of dolomite second phases in the Malm withouta strong preferred orientation, which do not impose an equal effectduring deformation to that of elongate mica, and therefore result in

/a)

dp/f

p)

102 103 104

dp/f

p)

102 103 104

s/a)

0

1

2

3

4

5

6

-90 -60 -30 0 30 60 90

n

Angle of grain long-axis to foliation

e) Second phases

0

1

2

3

4

5

6

-90 -60 -30 0 30 60 90

Angle of grain long-axis to foliation

nc) Calcite

gated in this study; a) calcite grain size (μm), b) calcite grain aspect ratio (=b/a) andates from the SEM images. Note that each data point represents average values for mea-with respect to the foliation plane, displayed in histograms for c) calcite and e) second-

0.51.01.52.02.53.03.54.04.5

Z <20 µm Z =20-40 µm Z >70 µm

c-ax

es(0

01)

a-ax

es(1

10)

r-po

les

(104

)

m.r.d.

Fig. 10. The development of crystallographic preferred orientation as a function of the Zener value (i.e., amount and size of second phases), for three different calcite pole figures;redrawn from Ebert et al. (2007). Z-values >70 μm imply recrystallization dominated deformation, whereas samples with Z-values b20 μm are controlled by second-phasedominated deformation. The broad lines superposed on the (001) pole figures indicate the mean shape preferred orientation of calcite (black) and secondary phases (gray).

174 B.S.G. Almqvist et al. / Tectonophysics 603 (2013) 162–178

higher aspect ratios for calcite. The preferred grain long-axis of bothcalcite and second phases is within an angle of 30 degrees to the foli-ation plane (Fig. 9c, e).

The boundary between second-phase controlled and recrystalliza-tion controlled microstructures is further temperature dependent(Herwegh et al., 2005). Keeping fp constant as temperature increases,shifts the boundary towards higher values of Z. In other words, graingrowth is more effective at elevated temperature, and shifts the bound-ary of second-phase controlled versus recrystallization-controlleddeformation towards higher values. When considering a constant tem-perature, the texture strength varies according to the Z-value (Fig. 10;cf. Ebert et al., 2007, 2008). For high values of Z, the CPO symmetry is or-thogonal with respect to the sample coordinate system (i.e., the samplesymmetry correlates well with the textural symmetry of calcite). Forthe case in which Z decreases, the symmetry axis of the CPO rotateswith respect to the pole to foliation (i.e.,Malm sample). The obliquenessresults from the stabilizing effect (i.e., pinning) of second phases on cal-cite deformation. This means that grains unfavourably oriented forintracrystalline glide, characterized by an elevated dislocation density,cannot be consumed by grain boundary migration recrystallization,simply because their grain boundaries are immobilized by pinning viasecond phases (see Ebert et al., 2007; Herwegh and Berger, 2004). Inpolyphase mylonites (i.e., second-phase rich), where grains withmetastable oblique c-axis orientations are consumed, oblique crystallo-graphic orientations are preserved due to deactivation of grain bound-ary migration as resetting mechanism. Additionally, the CPO intensitydecreases because pinning of grain-boundaries leads to an enhance-ment in diffusion related deformation processes, in particular diffusionenhanced granularflow (cf. Herwegh et al., 2011: section 6.3). Herweghand Berger (2004) observed that the Z-value influences the calcitegrain aspect ratio, e.g., for a low value of Z (i.e., controlled by second-ary phases) the calcite grains are elongate. SPO and grain shape andtheir respective grain boundaries are typically not accounted for intheoretical averaging models, as the one used in this study, but pre-vious workers have suggested that SPO and grain aspect ratio can

significantly increase the seismic anisotropy (e.g., Burlini andKunze, 2000; Kern et al., 2008). The possibility that SPO and grain as-pect ratio have an impact on seismic anisotropy raise an importantissue in regards to seismic anisotropy, because mineral microstruc-tures in many geological settings develop elongated grains and SPOduring deformation. The geometrical arrangement of microstruc-tures, as well as void space, is predicted to significantly influencethe elastic properties of the deformed rock (Casteñada and Willis,1995; Mainprice, 2000; Matthies, 2010, 2012; Wendt et al., 2003;Wenk et al., 2012; Zheng and Du, 2001).

The influence of second phases on the anisotropy of physical prop-erties can be inferred from the vertical spread in data values observedat individual locations along the shear zone in Fig. 8c and d. Highercontent of secondary phases results in smaller grain size, and weakertexture. Hence the texture strength and correspondingly the seismicanisotropy arises from both the local variation in content of second-ary phases at each location and the regional conditions that changealong the shear zone.

5.4. Seismic properties and reflection of crustal scale shear zones

The possibility that ductile shear zones in the crust and the uppermantle can act as seismic reflectors has received some attention inthe literature (e.g., Barruol et al., 1993; Fountain et al., 1984; Jonesand Nur, 1982; Law and Snyder, 1997; Rey et al., 1994; Siegesmundand Kern, 1991; Vauchez et al., 2012). Several studies have focused onthe possibility of texture-induced seismic reflection (Barruol et al.,1993; Christensen and Szymanski, 1988; Fountain et al., 1984; Jonesand Nur, 1982; Kern and Wenk, 1990; Khazanehdari et al., 1998; Lawand Snyder, 1997; Rey et al., 1994; Sellami et al., 1993; Siegesmundand Kern, 1991). In most of these studies, the seismic reflection isexplained by alignment of highly anisotropic minerals during deforma-tion, such as mica. Khazanehdari et al. (1998) showed for calcitemylonites that the angle of incidence of the P-wave with respect tothe foliation plane of the thrust and the CPO also affected the seismic

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reflection. Other pertinent factors when considering seismic reflectionare the thickness and strain gradient across the shear zone. Rey et al.(1994) demonstrated that seismic waves propagating though a shearzone with a gradational strain gradient does not necessarily show upas a reflector on seismic images, and the reflectivity depends on thewavelength and the thickness of the transition from the protolith tothemylonite. They found that part of the seismic energy could be visiblyreflected, given that 1) the seismic wavelength was short; 2) there isa significant impedance contrast between the adjacent protolithand mylonite; and 3) the mylonitic zone is of reasonable thickness(20–400 m). In general, the shear zone needs to be thicker than onequarter of the seismic wavelength in order to be visible as a seismicreflector (Sheriff and Geldart, 1995). The reflection coefficient is definedas R = (I1 − I2)/(I1 + I2), where I is the product of the P wave velocityand thematerial density (I = Vp * ρ), and the subscripts refer to the im-pedance contrast between two adjacent units (i.e., lithological contrastor zoneof deformation).Warner (1990) has investigated theR that is re-quired to produce visible reflections in the Earth's crust in some detail,particularly targeting the lower crust and Moho. In this case reflectioncoefficients (R) of a minimum 0.1 were needed for visible reflections.

Based on the results presented in the previous sections a hypo-thetical large shear zone can be envisaged, which is situated in theupper crust and spanning 15–20 km in length. The shear zone has ashallow dip with a gradual increase in metamorphic conditions to-wards the deeper parts, similar to the Morcles shear zone. Assuminga scenario that fits with the data in Fig. 8, the shear zone is boundedby a carbonate protolith, which consists of calcite, has a mean Vp of6.55 km/s, and is considered to be isotropic. Note that the Morclesshear zone is bounded by crystalline basement rocks and cover sedi-ments, but the objective here is to investigate the reflection whichcan arise solely due to the CPO in the shear zone. The predicted Vpfast axis is parallel to the plane of the shear zone and ranges from6.74 to 6.95 km/s, whereas the Vp slow axis varies between 6.09and 6.42 km/s normal to the shear plane. The variation in velocity de-pends on the location along the thrust. Note that these predicted Vp'sare due only to calcite; the degree of anisotropy is reported in Fig. 8for selected locations along the thrust. The density of the protolithand mylonite are 2.71 g/cm3. A wave travelling through the crust,with an angle of incidence normal to the plane of the shear zonewill therefore experience a contrast in impedance due to the

NZweisimmen St. Stephan Lenk

TIM

E (

s)

0.00

1.00

2.00

3.00

4.00

Width of stretchedlower Cretaceous

Shear zone width

Distance along shear zone (km)

Wid

th (

m)

00 51 0 15 20

50

100

150

200

250

300

N

W1

Fig. 11. Seismic line reflection profile, W1 (inset map in upper left corner), redrawn from Lsecond inset figure, in lower left corner, shows the shear zone width, and the width of the stEbert et al., 2007). In the latter case, the lower Cretaceous units consist of the Hauterivian, Ugular box shown in the seismic reflection profile.

preferred orientation of calcite, and in this scenario the shear zoneacts as a slow boundary for the propagating wave. However,depending on the CPO symmetry this boundary could be at an obliqueangle to the plane of foliation, particularly in the case of mylonitesinfluenced by secondary phases. Lithologies containing a substantialamount of secondary phases, such as Malm, have lower degree of an-isotropy, which may produce smaller changes in acoustic impedancesin the protolith relative to mylonites. Nevertheless, given a pure cal-cite mylonite (i.e., Urgonian), the cooler, distal part of the shearzone has an impedance of 17.398, which changes gradually to an im-pedance of 16.504 in the warmest part of the thrust; the protolith hasa seismic impedance of 17.751 (km g/cm3 s). This leads to R = 0.014in the cool upper part of the shear zone, where sub-greenschist faciesconditions are prevalent, and R = 0.036 in the hotter bottom part ofthe shear zone.

We haven't considered the effect of increased temperature and pres-sure, aswould be the case in the Earth. In the case of theMorcles nappe,which is a calcite-rich upper to middle crustal structure (anchizone togreenschist metamorphism), the influence of pressure on the elasticproperties can be considered to be small. Dandekar (1968a) showedthat the difference of elastic compressional (C11 and C33) and shear(C44 and C66) related elastic constants of calcite changed much lessthan 1% over the pressure range 0–5 kbar (i.e., room pressure to500 MPa). Elevated temperatures have a stronger influence on the cal-cite single crystal elastic constants (Dandekar, 1968b). The C11 constantdecreases by ~10% going from 25 °C to 275 °C. The C33 constantdecreases only by ~4% in the same temperature interval (measure-ments performed at ambient pressure conditions). In a calcitemylonite with a strong CPO the P wave anisotropy would increaseat temperatures of greenschist conditions (~300–400 °C), whencompared to room temperature conditions. The shear elastic module,C44 and C66, show a similar effect to the compressional moduli, and in-dicate that shear wave anisotropy should increase at temperaturesaround 300 °C. In this context, it can be noted that calcite has a very an-isotropic thermal expansion, which may influence the microstructuralarrangement of crystals in the rock as well as the physical properties(Srinisavian, 1955; Weiss et al., 2002).

The observation of diminishing texture strength along the shearzone, which is coupled to the physical conditions of temperature, stress,and strain-rate, could be used to help explain the nature of some

SRawil

Lake ofTzeusier Icogne PF Rhône

4 km

0.00

1.00

2.00

3.00

4.00

R3R2

R1

evato et al. (1994). R1–R3 are reflective layers in the southern part of the transect. Theretched lower Cretaceous, as a function of distance along the shear zone (redrawn fromrgonian and the Gault. The gray area represents a rough overlap area with the rectan-

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dipping reflectors in the crust of both the Alpine and Caledonideorogens (e.g., du Bois et al., 1990; Juhojuntti et al., 2001; Levato et al.,1993, 1994). These reflectors often appear as semi-continuous layers,at low angle to the horizontal plane, along which thrusting or normalfaulting has taken place. Of particular interest to the present study isthe work of Levato et al. (1994), who recognized three prominentmid-crustal reflective layers in a seismic transect across the Rawil de-pression in southwest Switzerland (Fig. 11). They ascribed the crustalreflections to the contact between nappes in the nappe pile (i.e.,Fig. 1a). A strong reflective layer (R2)was believed to represent the con-tact of the upper limb of the Morcles/Doldenhorn nappe (these twonappes are equivalent structures, but separated by the Rawil axial de-pression). Aweaker reflection horizon (R3)was postulated to representparautochtonous cover sediments that were pinched in a fold of theGastern massif. An equally likely solution for R3 would be that this ho-rizon represents the overturned limb, and hence shear zone of theMorcles/Doldenhorn nappe (Levato et al., 1994). The top-lying R1 re-flector was believed to represent the Malm unit in the Diableret orJägerchrüz nappes. All three reflective layers (R1–R3) are laterally dis-continuous, with R3 giving the weakest true amplitude reflections.The normal incidence reflection coefficient ranged from 0.01 to 0.20,based on laboratory petrophysical measurements of Sellami et al.(1993). The highest value of R = 0.20 represents a special case, for acontact between a shale-marl and limestone unit, whereas theremaining values never exceeded 0.05. The discontinuity of the reflec-tive layers can arise for several reasons, but it is difficult to explainthis phenomenon solely from a lithological contrast. Rather the reflec-tions arise as a combination of thickness of the reflective boundaryand the magnitude of R. Levato et al. (1994) indicated that the mini-mum resolution in the seismic reflection profile is ~50 m, for an ana-lyzed frequency band from 10 to 40 Hz. In this case the shear zones ofthe inverted limbs of theMorcles and Doldenhorn nappes clearly repre-sent an important component in the observed reflectivity record inFig. 11. Ebert et al. (2007) has recently shown that the shear zonewidth increases toward the root of the Morcles shear zone (Fig. 11,inset). In contrast the combined thickness of the lower Cretaceousunits, consisting of Hauterivian, Urgonian andGault units, thins towardsthe root of the shear zone. Note that Hauterivian and Gault samples arenot dealtwith in the present study, although they are considered part ofthe lower Cretaceous units (Ebert et al., 2007). Thicknesses of the shearzone and lower Cretaceous units both play a role in the reflection, andmay help explain the discontinuity of the R3 reflective layer. A com-bined low for these two parameters occur at the center of the shearzone, between 9 and 13 km, where neither exceed 50 m thickness.We hypothesize that this area represents the seismically transpar-ent area in R3 (cf. left-hand side of the rectangular box in Fig. 11). Itshould be noted that the R3 reflective layer continues with prominentreflections farther to the north.

6. Conclusions

This study focuses on the underlying factors that give rise to anisot-ropy of seismic velocities in calcite mylonites from the shear zone of theMorcles nappe, considering 1) the influence of intrinsic factors on alocal scale, including CPO, microstructures and grain-boundary effects,and 2) the influence of physical conditions or extrinsic factors (i.e., tem-perature, stress and strain-rate) on a regional scale. To test the intrinsicinfluence of second phases, samples were collected from one location~15 km along the shear zone that range from almost pure calcite tosamples with up to 40 vol.% secondary phases. Laboratory seismic andtexturemeasurements illustrate that polymineralic samples are less an-isotropic compared to rocks that have a sparse concentration of secondphases. Predicted seismic anisotropy varies between 6 and 14%, basedsolely on the texture strength of calcite for the samples that were stud-ied. During deformation the presence of secondary phases hinders re-crystallization of calcite by action of pinning along the calcite grain

boundaries, which leads overall to smaller grain sizes, weaker texturestrength, and lower anisotropy. Samples that contain b10 vol.% secondphases have higher anisotropy. In general, themeasured seismic anisot-ropies can be explained by the calcite texture.

Strain increases toward the higher grademetamorphic portion of theshear zone (greenschist metamorphism), resulting in calcite grain-sizeincrease and stronger CPO, that arises from the change in deformationmechanism from viscous granular flow to dislocation creep. A gradualstrengthening of the texture along the 15 km long shear zone impliesthat the seismic anisotropy also changes along the shear zone. This isalso shown for another physical property, namely by a systematic in-crease in the magnetic anisotropy along the shear zone. The physicalproperties and anisotropy determined for the deformed carbonatesfrom theMorcles nappe shear zone can yield significant acoustic imped-ance. We predict that textural variations along shear zones are in partresponsible for seismic reflections observed in the upper and middlecrust of orogens, such as the dipping reflectors observed during seismicreflection campaigns in the southwestern Swiss Alps and the centralScandinavian Caledonides. Sufficiently strong texture-induced seismicanisotropy (>10%), can contribute an absolute coefficient of reflection>0.03, at normal incidence, although other factors need to be takeninto consideration, such as seismic wavelength, shear zone thicknessand the Fresnel zone. Our results demonstrate the interplay betweenmicrostructures and texture with physical anisotropies. One wouldexpect similar relationships in other mineral systems, such as quartz,feldspar and olivine dominated rocks located in the lower crust andupper mantle (e.g., Linckens et al., 2011a,b). Future investigations, how-ever, should be applied to polymineralic systems. These systems arestill poorly understood in terms of their microstructural and texturaldevelopment, as well as their importance in rock deformation, strain lo-calization and formation of shear zones in the lower crust andupper mantle; their significance for geophysics remains largely to bediscovered.

Acknowledgments

We thank the Swiss Light source (SLS) and beamline scientistRajmund Mokso for help with the synchrotron X-ray microCT imaging.Robert Hoffmann provided technical assistance in the Rock DeformationLaboratory. This research was supported in part by the Swiss NationalScience Foundation project No. 2-77070-07. We thank Dan Tatham andHartmut Kern for the thoughtful and constructive reviews that improvedthe manuscript.

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