# Transient deformation and long-term tectonic activity in the Eastern Alps recorded by mylonitic pegmatites

Felix Hentschel, Emilie Janots, Valerie Magnin, Lisa Brückner, Claudia Trepmann

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Felix Hentschel, Emilie Janots, Valerie Magnin, Lisa Brückner, Claudia Trepmann. Transient deformation and long-term tectonic activity in the Eastern Alps recorded by mylonitic pegmatites. Journal of Structural Geology, 2022, 155, pp.104507. 10.1016/j.jsg.2021.104507. hal-04983774

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# Transient deformation and long-term tectonic activity in the Eastern Alps recorded by mylonitic pegmatites

Felix Hentschel<sup>a,b,*</sup>, Emilie Janots<sup>c</sup>, Valerie Magnin<sup>c</sup>, Lisa M. Brückner<sup>b</sup>, Claudia A. Trepmann<sup>b</sup>

## ARTICLE INFO

**Keywords:**

Episodic deformation

Quartz microstructures

DAV

Monazite dating

Alpine orogeny

## ABSTRACT

Mylonitic pegmatites from the Austroalpine basement north of the Deferregen-Antholz-Vals shear zone (DAV) in the Eastern Alps record episodic deformation at greenschist facies conditions after the Eoalpine tectonometamorphic event and before uplift in the Oligocene. Fluorapatite-allanite-epidote corona around monazite formed at about 60 Ma. They postdate a main mylonitic foliation and predate a second shear band foliation. Deformation of quartz is controlled by crystallographic orientation: Dislocation glide in favourable orientation forms elongate high-aspect-ratio grains. Localised recrystallisation takes place at sites of increased dislocation densities in less favourable oriented quartz and distributed microcracking in unfavourable orientation. Distributed microcracking of quartz occurred quasi-instantaneously coeval with cataclastic deformation of garnet and tourmaline, indicating transient high stresses. During subsequently decreasing stresses, small isometric quartz grains precipitate within the shear band boundaries from the pore fluid, probably derived from recrystallisation of fluid-rich quartz porphyroclasts. Cataclasites with mylonitic components record a coeval or subsequent stage of transient deformation, reflecting the main activity of the DAV with uplift from greenschist facies conditions. The mylonitic pegmatites record systematically changing deformation mechanisms during episodic deformation at greenschist facies conditions dominantly governed by changing stresses and strain-rates, as opposed to changing temperatures.

## 1. Introduction

Although being of vital importance in Earth Sciences, the relation between long-term lithospheric deformation and episodic deformation related to the seismic cycle remains poorly understood. This is partly because of the complex deformation behaviour of rocks along seismic active faults, where the changing stress and strain-rate conditions during the seismic cycle facilitate deformation by both, brittle and dislocation glide-controlled mechanisms even at low ambient pressure and temperature conditions (e.g. Ohl et al., 2021; Pozzi et al., 2021). Also, the interaction between the seismogenic and the aseismic zone during the seismic cycle, which is crucial for an understanding of the large-scale transport of crustal blocks during orogeny, is largely unknown. Coseismic slip within the seismogenic zone can induce transient high stresses below hypocentral depth (e.g. Nur and Mavko, 1974; Savage and Prescott, 1978), triggering episodic deformation (Ellis and Stöckhert,

2004a, 2004b; Nüchter and Ellis, 2010). In turn, accelerated creep below hypocentral depths can reload the seismogenic zone during interseismic periods, as described by the “deep slip model” sensu Scholz (2002). The base of the seismogenic zone in the continental crust (Marone and Scholz, 1988) coincides with the temperature-depth onset of crystal-plastic deformation of quartz at temperatures around 300 °C, i.e. at greenschist facies conditions (e.g. Van Daalen et al., 1999; Dunlap et al., 1997; Hirth et al., 2001; Hirth and Tullis, 1994; Stipp et al., 2002; Stöckhert et al., 1999; Voll, 1976). It is well known that quartz-rich microstructures formed at these depths can record such episodic ductile deformation during the seismic cycle (e.g. Famin et al., 2004; Küster and Stöckhert, 1999; Nüchter and Stöckhert, 2008, 2007; Trepmann and Stöckhert, 2003). Peak stresses can transiently be several hundred MPa as indicated by jadeite and amphibole twin paleopiezometry (Brückner and Trepmann, 2021; Trepmann and Stöckhert, 2001). The role of stress-loading rates and the characteristic time scales

<footnote><sup>a</sup> Mineralogical State Collection, Bavarian Natural History Collections, 80333, Munich, Germany</footnote>

<footnote><sup>b</sup> Department of Earth and Environmental Sciences, Ludwig-Maximilians-University Munich, Germany</footnote>

<footnote><sup>c</sup> Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTERRE, 38000, Grenoble, France</footnote>

<footnote>* Corresponding author. Mineralogical State Collection, Bavarian Natural History Collections, 80333, Munich, Germany.</footnote>

<footnote>E-mail addresses: hentschel@snsb.de (F. Hentschel), emilie.janots@univ-grenoble-alpes.fr (E. Janots), valerie.magnin@univ-grenoble-alpes.fr (V. Magnin), lisa.brueckner@lmu.de (L.M. Brückner), claudia.trepmann@lmu.de (C.A. Trepmann).</footnote>
for coseismic deformation (minutes) and postseismic creep (hundreds of years) have been constrained by the comparison of natural quartz microstructures and “kick-and-cook” quartz deformation experiments (Trepmann et al., 2007, 2017; Trepmann and Stöckhert, 2013; Trepmann and Seybold, 2019). The deformation behaviour of rocks in this pressure-temperature range of greenschist-facies conditions is consequently extremely relevant for an understanding of the relationship between deformation during seismic cycles and long-term shear zone activity.

Yet, most of the studies on natural microstructures at hypocentral depth document just one episode of deformation at initially high and then relaxing stresses (Nüchter et al., 2013). Other cycles are either masked due to overprinting or are of lower intensity compared to the dominating well-recorded one. For thrust fault settings with the formation of pseudotachylytes, the model of Nüchter et al. (2013) explains

an unique imprint of one single high-stress deformation event at one specific location by coseismic thrust fault propagation. The central idea of the model of Nüchter et al. (2013) is that, depending on the distance to the propagating fault tip, a characteristic sequence of deformation structures develops. With decreasing distances to the fault tip, ductile (e. g. kink bands) to brittle structures (quartz veins) and finally pseudotachylytes, as the last imprint, develop. Strain after passage of the fault front is localised in the next rock increment. Once a crystalline nappe is decoupled, its transport over large horizontal distances requires low shear stresses (Davis et al., 1983). During transport further strain is localised in weak lithologies and areas of high pore fluid pressure (King Hubbert and Rubey, 1959; Rodgers et al., 1962; Cook and Aitken, 1976; Laubscher, 1983; Davis and Engelder, 1985; Koch and Masch, 1992; Bachmann et al., 2009).

For strike-slip tectonic settings, the Austroalpine basement to the

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Simplified geologic map of the DAV shear zone and sample location at Weitental. (b) Stereographic projection (lower hemisphere) of the main mylonitic foliation, shear bands and lineations. (c, d) Field photographs of pegmatitic gneisses with main foliation (solid line) and shear bands (dashed lines) dipping to the NW. e) Field photograph of a cataclasite and its relation to the mylonitic pegmatites.</figcaption></figure>
north of the Deferreger-Antholz-Vals (DAV) shear zone in the Eastern Alps (Fig. 1) represents an ideal field area to investigate episodic deformation at the base of the seismogenic zone. This is because these rocks were resident at the critical P-T range within greenschist facies conditions from the middle Cretaceous to the Oligocene for about 70 Ma in a tectonically active setting (e.g. Mancktelow et al., 2001; Steenken et al., 2002; Stöckhert, 1984). In such a setting, recurrent seismic events are expected, which potentially have imprinted the rock's microfabric. Vein quartz microstructures from this area document transient high-stress deformation at greenschist-facies conditions at relatively slow stress relaxation compared to quartz microfabrics from pseudo-tachylytes related to the Silvretta basal thrust fault (Trepmann et al., 2017).

We use microfabrics from pegmatites of the Austroalpine basement north of the DAV near the village of Weitental (Fig. 1a) to evaluate the long-term deformation and stress history through the prolonged time at greenschist facies conditions and to discuss, whether several stages of transient high-stress deformation are recorded. We focus on this specific site at Weitental because (1) there, the Permian pegmatites show a marked mylonitic foliation and second shear band foliation (Fig. 1b–d; Mancktelow et al., 2001), (2) the pegmatites contain fluorapatite-allanite-epidote coronae around monazite similar to those that were dated to 60 Ma by Hentschel et al. (2020) and therefore serve as important reference microstructure to evaluate the relative sequence of deformation and (3) cataclasites that contain components of mylonitic pegmatites also occur.

## 2. Geological overview

The Austroalpine basement south of the western Tauern Window is divided into a northern and a southern block by the Defereggen-Antholz-Vals (DAV) shear zone (Fig. 1a). The northern block is part of the Kor-alpe-Wölz nappe system (Schmid et al., 2004, 2013) and comprises polymetamorphic rocks (i.e., paragneisses, white mica-garnet schists and gneisses, graphite schists, tourmaline-Mg-chlorite schists, quartz-ites, calc-silicate gneisses, amphibolites, augengneisses, and marbles) as well as Permian pegmatites (e.g. Hofmann et al., 1983; Kleinschrodt, 1987; Stöckhert, 1987; Mancktelow et al., 2001; Hentschel et al., 2019, 2020). The rocks were affected by both (1) the Eoalpine tectonometamorphic event at 450 ± 50 °C and ca. 0.7 GPa and (2) the Tertiary tectonometamorphic event at 350 ± 50 °C and ca. 0.5 GPa (Stöckhert, 1982, 1984, 1987). Coarse muscovite in the pegmatites shows K-Ar ages of 102 ± 2 Ma (Stöckhert, 1984), consistent with muscovite ages from the same area (Borsi et al., 1978; Steenken et al., 2002). In contrast biotite shows Oligocene ages around 30 Ma (e.g. Steenken et al., 2002 and references therein). Zircon fission-track data indicate Oligocene to middle Miocene cooling (Steenken et al., 2002; Klotz et al., 2019). The southern block is part of the Drauzug-Gurktal nappe system (Schmid et al., 2004, 2013). There, no Permian pegmatites occur and the Variscan paragneisses and orthogneisses of Permian Rb-Sr biotite ages of 288–299 Ma show only weak brittle deformation and no Alpine metamorphic overprint (<300 °C; Stöckhert, 1982; Hofmann et al., 1983; Kleinschrodt, 1987; Schulz, 1994). Consistently zircon fission-track data indicate lower Cretaceous to early Oligocene cooling (Steenken et al., 2002; Klotz et al., 2019). The DAV shear zone itself is characterised by mylonites and cataclasites.

As such, the DAV in the study area is part of the “Southern limit of Alpine metamorphism” (Hoinkes et al., 1999). Displacement along the DAV comprises therefore not only some tens of kilometres of horizontal component but also vertical uplift of several kilometres (Kleinschrodt, 1987; Schulz, 1989; Stöckhert et al., 1999; Mancktelow et al., 2001). The significant vertical uplift of the northern block and the position of the DAV in the study area (Fig. 1a) have been confirmed by the recent thermochronological data ((U–Th)/He dating (apatite) and fission-track

analysis (apatite and zircon)) of Klotz et al. (2019).

Rb-Sr ages of newly formed white mica just to the north of the DAV yield ages between 33 and 30 Ma (Müller et al., 2000, 2001). These ages were interpreted to correlate with the sinistral movement and associated deformation along the DAV (Kleinschrodt, 1987; Mancktelow et al., 2001). Throughout the northern block spatially distributed deformation already occurred from the Eoalpine onward (Mancktelow et al., 2001; Hentschel et al., 2020).

All geochronological and microstructural evidence from the study area consistently indicate that the rocks were at lower greenschist facies conditions from the middle Cretaceous to the Oligocene, where the rocks underwent deformation and mineral reactions (Stöckhert, 1982, 1984, 1987; Hentschel et al., 2020).

## 3. Methods

Pegmatites were sampled at Weitental north of the DAV shear zone (46°51'12"N, 11°42'48.6"E, Fig. 1a). Thin sections were prepared from slices perpendicular to the main foliation and parallel to the stretching lineation of the sample. The normal to the foliation is taken to indicate the short (Z) and the stretching lineation the long X-axis of the finite strain ellipsoid, respectively. All images presented are from XZ cuts. To judge the 3D shape of the shear bands also a YZ cut, perpendicular to both, the foliation and lineation, was prepared. The microfabrics were investigated by polarised light and electron microscopy using standard petrographic thin sections. We focus on two specific samples, a mylonitic pegmatite with a well-developed shear band foliation (FH36) and a cataclasite with components of mylonitic pegmatite (FH123).

For electron microscopy by back scattered electron (BSE) imaging, electron dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD), the thin sections were coated with a ~5 nm layer of carbon to prevent charging effects. For EBSD analysis thin sections where chemo-mechanically polished in a colloidal silica solution (Syton). We used a scanning electron microscope (Hitachi SU5000 at LMU Munich) equipped with an HKL NordlysNano high-sensitivity EBSD detector (Oxford Instruments). The EBSD signals were acquired and data processed using the AZtec analysis software (Oxford instruments) and using the MTEX toolbox for Matlab (https://mtex-toolbox.github.io/, v5.7.0, last access: June 2021; e.g. Bachmann et al., 2010). We used a sample holder pre-tilted at 70° with respect to the electron beam, an accelerating voltage of 20 kV and a working distance of 15–19 mm. The step size for automatic mapping was in the range of 1–2 µm, dependent of the required resolution, grain size and size of the area measured. Small non-indexed pixels were filled during data smoothing by a half-quadratic filter (Bergmann et al., 2016). The pole figures show all points from the measured area, unless otherwise stated in the figure caption.

Zeolite minerals in the cataclasite were characterised by Raman spectroscopy with a HORIBA JOBIN YVON XploRa ONE micro Raman machine at the Mineralogical State Collection Munich (SNSB), using a laser with a wavelength of 532 nm.

The chemical composition of monazite was analysed on carbon coated (55–60 nm) thin sections using an electron microprobe (EMP) measurements on a JEOL JXA-8230 EMP at the ISTerre in Grenoble, with operating conditions of 15 kV and 200 nA following the protocol of Hentschel et al. (2020). Concentrations of Pb, Th and U were measured with a counting time of 480 s, 160 and 300 s, respectively. Under these conditions, EMP measurement detection limits are in the range of 135 ppm, 130 ppm, and 105 ppm for U, Th and Pb, respectively. To monitor instrumental drift, Madagascar monazite standard (e.g. Grand’Homme et al., 2018) was run as unknown (3 points) before, in-between and after analyses of the mylonitic pegmatite monazite. Ages were individually calculated by the following formula (Montel et al., 1996):
$$Pb = \frac{Th}{232} [exp(\lambda^{232}\tau) - 1]208 + \frac{U}{238.04} 0.9928 \times [exp(\lambda^{238}\tau) - 1]206 + \frac{U}{238.04} 0.0072 \times [exp(\lambda^{235}\tau) - 1]207 \quad (1)$$

# 4. Results

## 4.1. Sample description

At Weitental, the main mylonitic foliation of the pegmatites dips with 30–50° to the N or NW and the lineation plunges gently E-NE or WNW (Fig. 1b-e). The shear band foliation dips to the NW (Fig. 1b-d). The intersection of the shear band foliation and the first foliation is about normal to the sample’s lineation within the foliation plane (Fig. 1b). Cataclasites with mylonitic components from the same location occur along faults that dip at 30–45° to the ENE (Fig. 1e).

The mineral assemblage of the mylonitic pegmatites from Weitental is quartz (ca. 85%), albitic plagioclase (ca. 5%), K-feldspar (ca. 5%) and white mica (ca. 5%), accompanied by accessory tourmaline, garnet, fluorapatite, monazite, allanite and pyrite (proportions are estimated from thin section observations). In the cataclasites, additionally, zeolites can be present. Monazite is surrounded by allanite-epidote-fluorapatite coronae, as described by Hentschel et al. (2020). The shear bands generally indicate a sinistral sense of shear and cross-cut the mylonitic foliation (S) at an average angle of 40°, ranging within 25–50° (Figs. 1b and 2). The shear bands are short, anastomosing and wavy and are in

synthetic orientation. We classify them as C'-type shear bands (Berthé et al., 1979; Platt and Vissers, 1980; Gapais and White, 1982; Passchier and Trouw, 2005).

The main foliation is defined by elongate quartz grains of tens to hundreds of µm thickness (Fig. 2a-c) sometimes separated by thin mica layers (Fig. 2d-f). Plagioclase is a subordinate phase, occurring as roundish or slightly elongate grains of a few tens of µm in diameter isolated within the foliation. Some sericite occurs in the cores of plagioclase grains. The shear band foliation is evenly spaced by a few hundred µm and is well-developed not only in mica-rich layers but especially within quartz-rich layers (Fig. 2a and b). The shear bands consist of finer-grained zones within quartz grains of specific orientations (Fig. 2b-f). The shear offset along the shear bands is typically low, of the order of a few tens of microns (Fig. 2a-c).

The quartz grains that define the main mylonitic foliation can be categorised into two groups:

(a) Bent elongate quartz grains with long axes of several hundreds of µm and a width of a few tens of µm. They show mostly very low interference colours indicating an isotropic cut with the [c]-axis perpendicular to the thin section plane (Fig. 2c-f), i.e. [c]

<figure><img src="image_5.png" /><figcaption>Fig. 2. Polarised light micrographs of a mylonitic pegmatite from Weitental, sample FH36. (a) Shear planes are typically at an angle of about 40° (red dashed lines) in both the mica-rich (upper) and quartz-rich (lower) part, but as low as 25° (green dashed lines) to the main foliation (S, yellow dashed line). Black box in (a) is indicating a cataclastically deformed garnet, compare Fig. 5. (b) Shear band foliation is horizontal, the first foliation at about 40°, micrograph is taken with crossed polarisers and compensator plate. (c-f) Shear bands in quartz-rich domain at 40° (red dashed lines) to the main foliation, crossed polarisers. Flakes of white mica are indicated by white arrows. A few 10s of µm thick elongate quartz grains in the foliation plane (S) are oriented with [c]-axes perpendicular to the thin section (isotropic cut, no birefringence). 100–200 µm thick elongate grains in the foliation plane either contain deformation lamellae (white dashed lines) in (c, d) or are cut by shear bands in (c-e). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** EBSD-map of quartz shear band foliation, sample FH36. (a) Polarised light micrograph with crossed polarisers and lambda plate inserted, compare Fig. 1b. (b) EBSD-map with Euler angle colour coding. (c, d) Density plots (lower hemisphere, equal angle projections) showing crystallographic orientation of grains with diameters <10 µm within and outside of shear bands. (e) Grain size histogram for grains with diameters smaller than 12 µm. (f–h) Pole figures colour coded after the EBSD-map in (b). (i) Bent elongate quartz grains have preferred orientations with the [c]-axis oriented at high angles to the X-axis and with a maximum at ~30–70° to the Z-axis. (j) Quartz grains that are only weakly affected by the shear bands have a preferred orientation with a [c]-axis about 10° counter-clockwise rotated to the Z-axis. (k) Quartz grains that are truncated by the shear bands show [c]-axes preferentially at 10° clockwise rotated to Z. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

closely subparallel to the Y-axis of the sample (Fig. 3a, b, f). These high aspect ratio grains (in average about 10–12 measured in XZ cuts) are either not affected by the C'-type shear bands (Fig. 2c–f) or are slightly bent (Fig. 3a and b).

(b) Elongate grains of 100–200 µm thickness that either contain sub-basal deformation lamellae (white dashed lines in Fig. 2c and d, 3a) or are truncated by shear bands (Fig. 2c–e, 3a, b). These grains systematically have a different crystallographic orientation with the [c]-axis being oriented at a slight angle (<10°) counterclockwise with respect to the normal of the main foliation, Z (Fig. 2c and d, 3b, g). The sub-basal deformation lamellae are oriented at low angles to the C'-type shear bands (Figs. 2d and 3a).

Quartz grains that are truncated by the shear bands show [c]-axes preferentially at an orientation ca. 10° clockwise with respect to the normal of the foliation, Z (Fig. 3b, h). There, the shear bands (Fig. 4a and b) contain isometric grains with diameters of only few µm that show pores along straight or smoothly curved grain boundaries (Fig. 4c and d). The small grains show preferred orientations of the [c]-axis at high angle to the shear band foliation (Fig. 3b and c). Smaller grains outside the shear bands occur surrounding the elongate grains and show a preferred orientation of the [c]-axis perpendicular to the mylonitic foliation plane (S), i.e. parallel Z (Fig. 3b, d).

Garnet grains are fractured forming bookshelf structures with evenly spaced antithetic subparallel fractures forming aggregates of sheared fragments aligned within the mylonitic foliation (Figs. 2a and 5). The bookshelf structure itself can be affected by synthetic shear planes (Fig. 5a). The sense of shear of the bookshelf structure is sinistral, consistent with that indicated by the shear band foliation. The garnet fragments are rotated around the Y-axis and do not show an internal misorientation (Fig. 5c and d). Garnet composition is almandine rich (Alm60.70Gro20.30Sp3.6Py5.7). Also, tourmaline can show a similar bookshelf structure (Fig. 6a). Single fractures in allanite (Fig. 6b) and zircon (Fig. 6c) are mostly at a higher angle to the foliation. Between these fragments, biotite, epidote and/or quartz precipitated (Fig. 6b and c).

## 4.2. Deformed corona microstructures around monazite

In the mylonitic pegmatites, monazite grains are surrounded by fluorapatite, allanite and epidote forming elongate corona microstructures (Fig. 7). The monazites were investigated by EMP (± 10 measurements per grain, depending on the size of the monazites). The corona microstructures show the same characteristics as those investigated in detail by Hentschel et al. (2020). For four monazites, ages from 70 to 286 Ma are inferred by EMP dating (Fig. 7f). The ages represent the Permian magmatic crystallisation age (250–287 Ma) as well as mixed
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** BSE images, sample FH36. (a, b) Orientation contrast of grain with high aspect ratio (bottom) and partly recrystallised grains affected by shear bands and grains within the shear bands. The shear band boundaries are indicated by dashed black lines. Note that the larger pores probably result from polishing. (c, d) BSE images of isometric quartz grains within the shear bands with grain boundary porosity.</figcaption></figure>

ages due to Alpine metamorphic reactions. These reactions are associ-
ated with the incomplete replacement by secondary monazite being
enriched in U, Pb, Y and Ca (Fig. 7e), during formation of the corona
microstructure (Hentschel et al., 2020).

The corona microstructures are elongate and aligned along the foli-
ation. The mylonitic foliation is deflected around the corona micro-
structures, which are in turn affected by the shear bands (Fig. 8a). While
bent elongate quartz grains have [c]-axes subparallel or slightly rotated
clockwise to Z (Fig. 8b and c), quartz within the shear bands show [c]-
axes at a high angle to the shear band boundaries (Fig. 8d and e). The
monazite in the core is a single crystal with an internal misorientation
below 10° (Fig. 9a and b). The surrounding fluorapatite and allanite
grains are polycrystalline with random texture, except for a slight
clustering of the fluorapatite c-axes (Fig. 9c and d).

4.3. *Cataclasite with mylonitic components*

The cataclasite consists of components of mylonitic pegmatites of up
to several cm in diameter within a fine-clastic matrix (Fig. 10). The
mineral assemblage and microfabric of the components is very similar to
that described for the mylonitic pegmatite (compare Figs. 2f and 10d).
The mylonitic foliation is likewise represented by elongate quartz grains
of a few tens of micrometer wide and a length on the order of several
hundred microns to a millimetre (Fig. 10d and e). Also, the most elon-
gate grains are commonly nearly isotropic in our X-Z thin sections, thus
indicating a crystallographic orientation close to the Y-axis of the
mylonitic component. Yet, a comparably well-developed second shear
band foliation has not been observed in the cataclasites. Instead, the
elongate quartz grains can be cut-off by irregular, branching cataclastic
zones (Fig. 10e). The fine-clastic matrix consists mostly of mica, feld-
spar, quartz and Fe-hydroxides as well as heulandite, as identified by
EDS and Raman spectroscopy. Heulandite is present in strain shadows of
mylonitic components, indicating precipitation from a pore fluid during
deformation (Fig. 10f).

5. Discussion

5.1. *Quartz deformation and shear band formation*

The influence of crystallographic orientation on the flow strength of quartz during dislocation creep has been investigated by deformation experiments of Muto et al. (2011) and in naturally deformed quartz in veins of the Rieserferner pluton just to the east of the study area by Ceccato et al. (2017). That the relative rates of dislocation glide, recrystallisation and microfracturing depend on the crystallographic orientation of quartz is well reflected also by the systematically varying textural and microstructural properties of the quartz grains distinguished here in four different microstructures:

1) Bent high aspect ratio grains with the [c]-axis oriented preferentially subparallel to the Y-axis of the sample (Fig. 2c and d; 3a, b, f) indicate rhombohedral and prism-<a> dislocation glide (e.g. Linker et al., 1984; Passchier, 1985; Schmid and Casey, 1986; Law, 1990; Toy et al., 2008; Morales et al., 2011; Muto et al., 2011; Kilian and Heilbronner, 2017; Ceccato et al., 2017). They show no deformation lamellae and no subgrains; they are not truncated by shear bands and do not show sutured grain boundaries. These bent high aspect ratio grains indicate dislocation glide-controlled deformation. They are interpreted to have been in a favourable crystallographic orientation to activate dislocation glide with respect to the stress-strain field during deformation. Similar elongate quartz grains in mylonites that occur associated with pseudotachylytes in the Saint-Barthélemy massif, French Pyrenees, have been attributed to high differential stress conditions (Passchier, 1982, 1985).

2) Grains with the [c]-axis oriented preferentially at about 10° counterclockwise (synthetically) to Z, normal to the foliation, commonly show sub-basal deformation lamellae (Figs. 2d and 3a). They are surrounded by subgrains and new grains of similar shape and similar crystallographic orientation (Fig. 3a–c, g), indicating dislocation glide and subgrain rotation recrystallisation, i.e. dislocation creep (e. g. Drury and Urai, 1990; Hirth and Tullis, 1992; Poirier, 1985; Stipp and Kunze, 2008; Trepmann and Stöckhert, 2009). Deformation lamellae are known to occur together with microfractures, short wavelength undulatory extinction (SWUE), and dislocation tangles.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Cataclastically deformed garnet (see Fig. 2a), sample FH36. (a) BSE image of garnet with bookshelf structure. (b) Polarised light micrograph of the cataclastic garnet shown in (a) and (c) EBSD map, colour coded by relative misorientation. (d) Pole figures showing crystallographic orientation of fragment colour coded corresponding to the map in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

They are characteristic of dislocation glide with limited dislocation climb at high stress crystal plasticity (e.g. McLaren and Hobbs, 1972; Drury, 1993; Trepmann and Stöckhert, 2003, 2013; Derez et al., 2014; Palazzin et al., 2018). Here, the grains with deformation lamellae are interpreted to be in an orientation not as effective for dislocation glide compared to those described in 1), causing dislocation pile up, consistent with the observation that they have a lower aspect ratio and a systematically different crystallographic orientation (Fig. 3). Strain concentration at the boundary of these grains further caused some recrystallisation by subgrain rotation recrystallisation.

3) Grains with the [c]-axis oriented preferentially at about 10° clockwise (antithetically) to Z are truncated by C'-type shear bands (Fig. 2e; 3a-c, h; 4a, b). The grains also contain subgrains and recrystallised grains of similar size and crystallographic orientations, indicating a combination of dislocation glide, subgrain rotation recrystallisation and brittle deformation. These grains are interpreted to have been in an unfavourable crystallographic orientation to activate dislocation glide with respect to the stress-strain field during deformation, consistent with the findings of Ceccato et al. (2017) and Papeschi and Musumeci (2019). They deformed by a
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Fractured accessory minerals, sample FH36. (a) Polarised light micrograph of a tourmaline (Tur) bookshelf structure, crossed polarisers. (b) BSE image showing a fractured allanite (Aln) sealed with biotite (Bt) and epidote (Ep). (c) BSE image of fractured zircon (Zr) sealed with biotite (Bt, white arrow). Note also fractured allanite (Aln) sealed with biotite (black arrow).</figcaption></figure>

combination of fracturing and dislocation glide with recrystallisation at sites of high dislocation densities.

The quartz microstructures 1) to 3) are interpreted to represent a sequence of decreasing strain by dislocation glide and increasing strain by microfracturing depending on crystallographic orientation. Recrystallisation is concentrated at sites of increased accumulated dislocation densities, e.g. grain boundaries, in agreement with the findings of Cecato et al. (2017).

4) C'-type shear bands are usually interpreted to develop as a second foliation in a primarily foliated mylonite with strong anisotropy at a late stage of shear zone development (Platt and Vissers, 1980; Passchier and Trouw, 2005; Gillam et al., 2014; Bukovská et al., 2016; Finch et al., 2020). In our case the most frequently observed orientation is with in average 40° to the foliation slightly steeper than the most commonly

observed angles of 15–30° for C'-type shear bands in mica-rich rocks (Passchier and Trouw, 2005). Bukovská et al. (2016) also observed shear bands at relatively high angles to the foliation, which initiated by fracturing at high differential stresses of 250 MPa. Finch et al. (2020) showed in numerical models that depending on the viscosity contrast >5–15% weak phases are required to form C'-type shear bands. There is only a low amount of micas (ca. 5%), which, however, appears here to be not the weak phase controlling the formation of the shear bands. In contrast, we observe that the C'-type shear bands are restricted to the relative high strength quartz layers. Similar to the findings of Bukovská et al. (2016), we interprete that the shear bands were initiated by microcracking of the quartz grains in unfavourable orientation for dislocation glide. Once fractured, these planes act as weak planes. Notably, the orientation is close to that of extensional T-fractures in brittle fault zones (Petit, 1987; Passchier and Trouw, 2005). Thus, the C'-type shear bands represent planes of low normal stress as opposed to high resolved shear stress, consistent with the relative low amount of shear offset and in accordance with the numerical experiments by Finch et al. (2020). Together with the occurrence of small isometric quartz grains with high grain-boundary porosity within the shear bands (Fig. 4), we suggest that they correspond to vein-like, tensile structures: The quartz in unfavourable orientation for dislocation glide fractured along planes of low normal stress and new quartz precipitated from a pore fluid, similar to the model proposed by Kjøll et al. (2015). Consistently, mesoscopic-scale C'-type shear bands are observed associated to quartz veins (e.g. Trepmann and Seybold, 2019). The shear experiments on polycrystalline quartz by Palazzin et al. (2018) report on very similar C'-type shear bands at 35° to the foliation. They found that the water content of initially fluid-inclusion-rich porphyroclasts decreases during recrystallisation and dilatant shear bands act as sinks for H₂O during deformation allowing for precipitation of quartz. The isometric shape and the grain boundary porosity observed for the shear band grains are well consistent with this interpretation and similar to the findings reported on naturally deformed quartz (Kjøll et al., 2015). Furthermore, during recrystallisation, water can be drained out along the migrating grain boundaries (e.g. Urai et al., 1986; Jessell et al., 2003; Schenk and Urai, 2005; Trepmann et al., 2010), with the shear bands acting as sites of quartz precipitation. Quartz in pegmatites is usually containing a high amount of submicroscopic fluid inclusions and can therefore be considered volatile-rich. In this way, recrystallisation of the porphyroclasts served as a local (and transient) source for fluids.

The crystallographic orientation of the grains within shear bands with [c]-axes at a high angle to the shear band boundaries (Fig. 3c) might be explained by a combination of epitactic precipitation from the pore fluid and subsequent rigid body rotation during progressive shearing consistent with the sinistral sense of shear in the shear bands, similar to the model proposed by Kjøll et al. (2015). Growth of grains with the [c]-axis perpendicular to the substratum is also known for quartz vein growth (e.g. Bons et al., 2012 and references therein). Furthermore, experimentally deformed quartz with peripheral [c]-axis maxima has been suggested by Kilian and Heilbronner (2017) to rather result from nucleation and growth than to dislocation creep with basal-<a> dislocation glide. Consistently, we interpret the strain-free isometric quartz grains in the shear band with high grain boundary porosity and peripheral [c]-axis maximum to be due to nucleation and growth with major influence of precipitation from the pore fluid in dilational sites as opposed to dislocation creep at sites of high shear stress. The straight or smoothly curved grain boundaries of the strain-free new grains in the shear bands furthermore suggest that the interfacial free energy controlled the grain boundary microstructure, representing already an annealed stage at low stresses (Küster and Stöckhert, 1999; Trepmann and Stöckhert, 2003; Trepmann et al., 2010).
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** BSE images of corona microstructures around monazite used for EMP-dating, sample FH36 (yellow numbers represent range of measured EMP ages). (a) Large remnant monazite with small rim of fluo-rapatite, allanite and epidote. (b) Asymmetric shape of corona microstructure aligned in the foliation. (c) Largely replaced monazite in elongate corona microstructure aligned along the foliation deflected in a shear band. (d) Heterogeneous corona microstructure. (e) Uranium distribution map for the monazite from fig. c. (f) Histogram showing EMP-ages of monazite, partly preserving their Permian magmatic age as well as mixed ages due to the replacement reactions of monazite during the Alpine tectonometamorphism. The distribution data show the ages of monazite from other pegmatites from the studied area (Hentschel et al. 2020). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

## 5.2. Quasi-instantaneous distributed fracturing

The fracturing of quartz in unfavourable orientations can be interpreted as the initial process during formation of the shear band foliation. The even spacing and homogenous angle of the shear bands (Fig. 2 a-c) are suggesting that the fracturing is a unique, single event as opposed to have occurred continuously during creep. If fracturing would have occurred at different sites continuously during creep, a larger spread in angles and spacings is expected, as discussed for mid-point fracturing of rigid particles in a viscous matrix (e.g. Ji and Zhao, 1994; Zhao and Ji, 1997). The same holds true for the bookshelf structures of, both, garnet (Fig. 5) and tourmaline (Fig. 6a). If fracturing of both minerals would have been due to progressive mid-point fracturing in a continuously creeping matrix, a systematically different spacing of cracks would be expected, which is not observed. Consistently, Le et al. (2005) found a theoretical solution for distributed instantaneous fracturing of brittle inclusions in a ductile matrix, which requires high stress-loading rates. Cataclastic deformation of garnet in an otherwise viscously flowing matrix has been related to transient high stresses, commonly associated with coseismic deformation (e.g. Trepmann and Stöckhert, 2002; Austrheim et al., 2017; Hawemann et al., 2019; Song et al., 2020).

We suggest that the cataclastic deformation of garnet and tourmaline was coeval with distributed fracturing of quartz in unfavourable orientation for dislocation glide at transient high stresses (Fig. 11). Subsequently at decreasing stresses, quartz underwent further dislocation creep and new grains precipitated along the shear bands, whereas the rigid garnet and tourmaline fragments rotated with a sinistral sense forming the antithetic bookshelf structures. The distributed microcracking occurred at a discrete stage of transiently high stresses and strain rates.

## 5.3. Relation between the deformation microstructures in mylonites and cataclasite

Before and after the discrete stage of distributed microcracking representing the initiation of the shear band foliation, creep occurred at lower stresses and greenschist facies conditions. The epidote-allanite-fluorapatite coronae (Figs. 7–9) represent valuable reference microstructures and can be used to further elucidate the deformation history. Hentschel et al. (2020) revealed an age of $60 \pm 6$ Ma for the formation of very similar corona microstructures in pegmatites from the same area by U–Th–Pb in situ dating of allanite by laser ablation inductively coupled mass spectrometry, which is consistent with the youngest of the monazite EMP ages (Fig. 7). As the corona microstructures are aligned within the foliation, some deformation is coeval and likely also predated corona formation (Hentschel et al., 2020). However, given that the mylonitic foliation is deflected by the corona microstructure and the shear bands are affecting the outer epidote-allanite zone of the corona (Fig. 8; 9), both, mylonitic main foliation as well as the shear band foliation are interpreted to postdate the corona formation at 60 Ma. It therefore took place after the Eoalpine tectonometamorphic event in the Cretaceous, but at prevailing greenschist facies conditions, as indicated by biotite and epidote precipitation in fractures of the corona minerals (Fig. 6b and c). The Austroalpine basement to the south of the western Tauern Window was at temperatures between 300 °C and 350 °C from the Eoalpine tectonometamorphic event in the Cretaceous until uplift and cooling to below greenschist facies conditions in the Oligocene (Stöckhert, 1984), consistent with other geochronological data from this area (Klotz et al., 2019; Steenken et al., 2002 and references therein). Thus, the recorded deformation took place in the time between 60 and 30 Ma at more or less constant greenschist facies conditions. Only
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** EBSD data, corona, sample FH36. (a) EBSD-map of corona microstructure, colour coded by Euler angles, around monazite surrounded by fluorapatite, allanite and epidote. (b, c) Corresponding pole figure and density plot of quartz grain orientations at the lower left and upper right quadrants around the corona microstructure. (d, e) Corresponding pole figure and density plot of quartz grain orientations at the upper left and lower right quadrants around the corona microstructure. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** EBSD data, corona, sample FH36. (a) EBSD-map of corona microstructure. Monazite is colour coded by relative misorientation (10°), fluorapatite is in blue and allanite in green colour. (b–d) Corresponding pole figures of monazite, fluorapatite and allanite orientations, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

heulandite present in strain shadows (Fig. 10e) and veins of the cata-
clasites indicates precipitation from the pore fluid during deformation at
already slightly lower temperature conditions (e.g. Weisenberger and
Bucher, 2010). This deformation therefore may have occurred during
uplift to below 300 °C at 30 Ma, the main activity of the DAV shear zone
(e.g. Stöckhert, 1984; Müller et al., 2000; Mancktelow et al., 2001; Klotz
et al., 2019). The components show very similar microfabrics compared
to the mylonitic pegmatites with strongly elongate quartz grains
(Fig. 10d–f). Yet, no equally well developed shear band foliation has
been observed in any component. Therefore, it cannot be excluded that
the formation of the cataclasites might be coeval with the formation of

the shear bands. However, in the cataclasites, unlike the shear bands,
very little creep strain postdated brittle fracturing.

5.4. *Changing deformation mechanisms at greenschist facies conditions*

Changing deformation mechanisms at unvarying PT-conditions
require a change of other factors, which can be fluid availability,
microstructure (crystallographic orientation, grain size, phase distribu-
tion, etc.) and/or stress and strain rate. These factors are not indepen-
dent of each other and change during deformation and metamorphic
reactions within the rock system. Furthermore, stresses at the base of the
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Polarised light micrographs of cataclasites, Weitental, sample FH123. (a) Sheared components of mylonitic pegmatite in foliated matrix. (b) Close up of the mylonitic fragment from the area indicate in (a) with yellow arrows indicating feldspar porphyroclasts. (c, d) Mylonitic component with elongate quartz grain with [c]-axis at high angle to the thin section (red arrow) and micas along the main foliation (yellow arrows). (e) Mylonite component with elongate quartz grains cut off by cataclastic zones (red arrow) (crossed polarisers). (f) Heulandite (yellow arrow) in a strain shadow around mylonitic pegmatite-component. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Synoptic sketch of deformation history inferred from the mylonitic pegmatite at systematically changing stress and more or less constant greenschist facies conditions. Question marks refers to the uncertainty that earlier variations in stress are likely but not equivocally recorded, d.l. refers to deformation lamellae in quartz. See text for discussion.</figcaption></figure>

seismogenic zone can change “externally” during seismic activity in the overlying seismogenic crust (e.g. Ellis and Stöckhert, 2004a, 2004b; Nüchter and Ellis, 2010; Scholz, 2002). These “internal” and “external” stress/strain variations do not exclude each other. On the contrary, an increase in stress and strain rate below hypocentral depth due to seismic

slip in the overlying seismogenic zone can be expected to amplify the plastic anisotropy: the higher the stresses and strain-rates, the more important are local heterogeneities. Continuous dislocation creep of quartz at greenschist facies would result in homogeneous microstructures (e.g. Trepmann and Stöckhert, 2003). The importance of local
stress-strain heterogeneities during creep are evident by the strong control of crystallographic orientation on deformation behaviour of quartz in the mylonitic pegmatites described here. The heterogeneity of the microstructure with evidence of quasi-instantaneous microcracking of quartz with unfavourable orientation for dislocation glide, coeval with cataclastic deformation of garnet and tourmaline requires fast stress built up and reflects the short-lived character of deformation (Küster and Stöckhert, 1999; Nüchter and Stöckhert, 2007; Trepmann and Stöckhert, 2001, 2002, 2003).

A fast stress-built up to high stresses required for the quasi-instantaneous distributed microcracking is not compatible with high pore fluid pressures, which would result in low effective stresses (e.g. Küster and Stöckhert, 1999; Nüchter and Stöckhert, 2008). Consistently, the mylonitic pegmatites described here and comparable rocks from the area, show relatively little evidence of fluid-induced alteration, e.g. replacement of garnet by chlorite and of feldspar by sericite (Hentschel et al., 2019, 2020). Transient availability of fluids, however, is documented by the corona microstructures that result from incomplete breakdown reactions of monazite (Gaidies et al., 2017; Hentschel et al., 2020). Locally and transiently increased availability of fluids is also represented by the shear band foliation. The fluid stored in (sub-) microscopic inclusion in the original quartz from the pegmatites was expelled along the migrating grain boundaries during recrystallisation, where the shear bands acted as sinks, consistent with the findings from Palazzin et al. (2018) and Kjøll et al. (2015).

As the relevant depths are not directly accessible for stress measurements, absolute constraints on stress mostly rely on a comparison of experimentally calibrated and natural microstructures as well as experimentally derived flow laws (e.g. Poirier, 1985), assuming that the steady state microstructures are not significantly modified during the later geological history. Although microstructural development at non-steady state conditions and varying stress is rather the rule than the exception, paleopiezometers are still widely used (e.g. Hawemann et al., 2019; Papa et al., 2020), which might reflect the stress conditions at a certain but not constrained stage of the geological history. Especially, quartz micro-fabrics are known to be largely modified during deformation at the systematically changing stresses during the seismic cycle well-below hypocentral depths (Trepmann and Stöckhert, 2003). Even experiments performed at similar PT-conditions, though different initial microstructures and water contents, largely yield different stress conditions during deformation (Palazzin et al., 2018). Given the quartz-rich lithology of the mylonitic pegmatites, they unfortunately do not provide meaningful quantitative constraints on the stress. However, the microfabric can be used to qualitatively analyse the sequence of changing deformation conditions. They record that microstructure and stress changed episodically between Eoalpine tectonometamorphism and Oligocene uplift (Fig. 11).

## 5.5. Transient deformation and long-term tectonic activity

The model proposed by Nüchter et al. (2013) explained the unique imprint of deformation at peak stress after a preceding stage of creep at lower stresses by deformation at systematically decreasing distance to the tip of a propagating seismic active thrust fault at crustal depth corresponding to the base of the seismogenic zone (i.e. at constant greenschist facies conditions). Their model, was developed for the Preveli crystalline nappe on Crete, Greece, where pseudotachylytes represent the final imprint before the nappe got detached from its substratum.

The DAV, however, represents a strike-slip fault with a vertical component, where the northern block moved upwards in the Oligocene (e.g. Kleinschrodt, 1987; Klotz et al., 2019; Mancktelow et al., 2001; Klotz et al., 2019). There, several discrete stages of transiently high stresses during the tectonically active time at greenschist facies conditions between the Eoalpine tectonometamorphic event in the Cretaceous and final uplift in the Oligocene are expected.

The mylonitic pegmatites from Weitental north of the DAV record one dominant event of transiently high stresses with distributed

microcracking of quartz, garnet and tourmaline at greenschist facies conditions between stages of creep, as indicated by the bookshelf structures and the shear band foliation offsetting the mylonitic foliation. The intersection of the two foliations is about normal to the sample's lineation (Fig. 1b), indicating that both foliations formed in a not markedly changed, similar tectonic setting. It is considered very likely that the first stage of mylonitic foliation represents a stage of accelerated creep, but cannot be unambiguously proven because of major modification during the subsequent deformation. Consistently, Passchier (1982, 1985), observed elongate quartz crystals, which are similar to those that characterise the first mylonitic foliation in our samples, to occur episodically and coeval with pseudotachylytes at the same metamorphic conditions, which requires changing stress- and strain rate-conditions. Therefore, formation of the corona microstructure dated to 60 Ma also in other pegmatitic mylonites from the same area (Hentschel et al., 2020) is considered to reflect an earlier transient stage at the same pressure-temperature conditions. The mylonitic foliation is also very well-developed within the components of the cataclasites, except without the presence of a clear shear band foliation (Fig. 10b, d, e). The cataclasites, however, might either be coeval or a subsequent final stage of transient deformation at slightly lower pressure-temperature conditions, as discussed above.

During the long-term tectonic activity from the Eoalpine tectonometamorphic event to the uplift in the Oligocene, discrete stages of transient deformation at high stresses after and before creep at lower stresses and greenschist facies conditions have occurred (Fig. 11). This episodic deformation most probably occurred distributed in space and time within the northern block and was probably related to activity of a precursor of the DAV, respectively, an evolving DAV. This interpretation is consistent with the observations of a high amount of smaller scaled shear zones within the rocks of the northern block (Mancktelow et al., 2001; Hentschel et al., 2019), as well as the recent geochronological data with evidence of Miocene activity of a successor DAV by Klotz et al. (2019).

## 6. Conclusions

The mylonitic pegmatites from Weitental just to the north of the DAV record discrete stages of transient deformation after and before creep at decreasing stresses at more or less constant greenschist facies PT-conditions between the Eoalpine tectonometamorphic event and uplift in the Oligocene (Fig. 11). The main foliation is coeval or postdates formation of corona microstructures around monazite (around 60 Ma) and predates the formation of the shear band foliation. The shear bands reflect the different mechanical behaviour of quartz dependent on crystallographic orientation: (1) rhombohedral and prism-<a> dislocation glide in the most suitable orientation, (2) dislocation glide, formation of deformation lamellae and localised recrystallisation in less well oriented grains and (3) distributed microcracking in unfavourable orientations for dislocation glide. Distributed microcracking of quartz is interpreted to be coeval with cataclastic deformation of garnet and tourmaline. It occurred quasi-instantaneously, as indicated by the homogeneous spacing of the fractures and the low spread in angles to the foliation for the respective microstructures. Quasi-instantaneous microcracking of quartz, garnet and tourmaline requires transient high stresses and low pore-fluid pressure. During subsequent creep at decreasing stresses, the rigid garnet and tourmaline fragments rotated in the creeping quartz matrix with quartz precipitating from the pore fluid along the shear band boundaries. The recrystallisation of the fluid-inclusion-rich original quartz grains acted as local source for fluids and the shear band boundaries as sinks. A coeval or subsequent high-stress event at slightly lower temperatures is recorded by cataclasites with mylonitic components and heulandite in strain shadows.

The mylonitic pegmatites from the Austroalpine basement document the importance of discrete events of transient high-stress deformation during long-term tectonic activity in the Eastern Alps from the Eoalpine tectonometamorphic event in the Cretaceous to the uplift below greenschist facies conditions in the Oligocene. Stress- and strain rate-
variations are the controlling factors, as opposed to changes in temperature.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This study was funded by the Deutsche Forschungsgemeinschaft (DFG grant TR534-4-1 and TR534-6-1). CT thanks Bernhard Stöckhert for his support and valuable discussions. Tarek Alatrach is acknowledged for assistance during the field work and data processing. Fabian Dellefant is acknowledged for the specification of the zeolites via Raman spectroscopy. The editorial handling by Virginia Toy and the comprehensive and constructive reviews by Samuele Papeschi and an anonymous reviewer are greatly appreciated.

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