The Miocene Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault in Austria allows study of the internal structure of a fault zone from the near surface to ∼30 km depth. As it enters the Tauern Window along the Rinderkarsee shear zone, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The shear zone consists of three 1- to 100-m-wide zones of brittle-ductile and ductile deformation separated by 500-m-wide zones of less deformed rocks. The southern shear zone is mylonitic, with ductile amphibole and plagioclase; weak crystal preferred orientations imply that the main deformation mechanism was dislocation-accommodated grain boundary sliding. The northern and central shear zones are characterized by discrete millimeter-wide shear zones with ductile quartz, muscovite, and biotite and brittle feldspar. Shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite; strain then localized in muscovite-rich grain boundary shear zones that linked to form throughgoing shear zones.
 Fault mechanics and earthquake rupture and termination processes are inexorably linked to the physical properties of fault zones. The brittle-ductile transition plays a key role in all these processes, such that understanding active deformation mechanisms at midcrustal depths is a critical component to our understanding of earthquake physics.
 In the early 1980s a paradigm for crustal rheology was developed that considered the crust to be composed of two distinct layers: a seismogenic upper crust deforming via pressure-dependent/temperature-insensitive brittle frictional sliding, and an aseismic lower crust deforming via temperature-dependent/pressure-insensitive ductile processes such as dislocation creep and diffusion creep [Brace and Kohlstedt, 1980; Sibson, 1982]. This idea gained credibility because the predicted crossover from frictional sliding to creep derived from extrapolation of laboratory constitutive relations for quartzofeldspathic crust is ∼10–15 km deep, a depth that accords with observed seismicity [Meissner and Strehlau, 1982; Ito, 1999]. This paradigm continues to serve as the basis for modern geodynamic models [e.g., Ellis and Stöckhert, 2004] and interpretations of the strength of the crust [Jackson et al., 2004].
 For an improved understanding of the brittle-ductile transition and earthquake physics, it is important to know how accurately this paradigm describes the Earth and in what major ways the different variables listed above interact to influence seismic and interseismic processes. This paper addresses several specific questions of interest in this regard. (1) How does strain partitioning change with depth across the brittle-ductile transition; is there a single fault/shear zone that simply gets progressively wider with depth? (2) How do deformation mechanisms change across the brittle-ductile transition; how common is the combination of quartz crystal plasticity, feldspar brittle failure and solution transfer creep? (3) By what means can transformational softening affect the rheology of Earth materials within the brittle-ductile transition?
 This study focuses on these questions through macroscopic to microstructural observations of the Miocene strike-slip Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault in central and eastern Austria. The SEMP fault has been differentially exhumed along strike, allowing one to study the detailed structure of the fault rocks from the Earth's surface down to ∼30 km depth, simply by moving along strike from the Vienna basin to the Tauern Window (Figure 2). Coincident with its entry into the Tauern Window, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The Tauern segment of the SEMP fault therefore represents a key location for studying earthquake nucleation and midcrustal rheology.
 There is little modern geochronology that dates displacement along the SEMP and its continuation into the Tauern Window, and along the other large-scale shear zones in the Tauern Window. One strand of the Greiner shear zone (Figure 2) of the western Tauern Window yielded electron microprobe U-Th-Pb monazite ages of 29–20 Ma [Barnes et al., 2004]. Muscovite from a chlorite-muscovite schist from the core of the Ahrntal shear zone and biotite from the central zone of the Rinderkarsee shear zone (Figure 2), the latter studied in this paper (see below), gave latest Oligocene (∼24 Ma) 40Ar/39Ar cooling ages (Table 1). In contrast, muscovite and biotite from an annealed garnet-hornblende schist from the Greiner shear zone yielded middle Miocene (∼15 Ma) 40Ar/39Ar ages (Table 1). Along the eastern SEMP, the Vienna basin and relict pull-apart basins demonstrate faulting during the early and middle Miocene [e.g., Peresson and Decker, 1997b], leaving the possibility that the eastern portion of the SEMP is younger (15 Ma?) than the western portion (25 Ma?). The absence of a younger, brittle overprint on the ductile shear zones of the Tauern Window may be because deformation migrated northward onto the strand of the SEMP that runs along the northern margin of the Tauern Window and the many faults in the northern Calcareous Alps.
TFA is total fusion age; IA is isochron age; WMA is weighted mean age (preferred age in italics); J is the irradiation parameter; isochron and weighted mean ages are based on fraction of 39Ar listed. Sample descriptions are 50801B1 low-grade phyllitic chlorite-muscovite schist; 50805B1 posttectonically annealed garnet-muscovite-hornblende schist; 50729A1 Zentralgneis granodiorite. Dating specifications are sample weights between 10 and 28 mg; the grain size was ≥0.25 mm. Ar/Ar dating is by W. Frank in the CEAL-Laboratory of the Slovak Academy of Sciences, Bratislava, Slovenia.
25.5 ± 1.0
24.6 ± 0.6
415 ± 62
25.0 ± 0.7
15.4 ± 0.6
15.4 ± 0.3
328 ± 27
15.0 ± 0.4
14.0 ± 0.3
15.3 ± 0.3
286 ± 22
14.6 ± 0.3
23.6 ± 0.8
23.5 ± 0.7
316 ± 32
23.4 ± 0.8
2.1. Strike-Slip Deformation at and Beneath the Brittle-Ductile Transition
 To investigate deformation along the SEMP at depths where the deformation was changing from brittle-ductile to dominantly ductile, we chose a shear zone in the NE corner of the Tauern Window near Rinderkarsee (a lake), south of Krimml (inset, Figure 2). The steep fabric of the Rinderkarsee shear zone implies that it formed late during the doming of the Tauern Window; otherwise, it would have been rotated to a shallower angle. The Rinderkarsee shear zone is hosted within the relatively homogeneous millimeter- to centimeter-scale granitic Zentralgneis and its much more diverse amphibolite-facies metasedimentary host rocks, the Habach Group (Figures 23–4). In detail, the shear zone consists of a series of three main high-strain zones (a northern shear zone, a central shear zone, and a southern shear zone) separated by low-strain panels. The bulk of the Zentralgneiss in the low-strain panels enjoyed a bulk strain estimated from grain shapes of γ ≈ 1 (Figure 3a).
2.1.1. Northern Shear Zone
 Immediately south of Rinderkarsee, a decameter-thick zone within the Zentralgneis contains steeply dipping, NE striking, chiefly meter-scale shear zones. At the grain scale the deformation within the shear zones was brittle-ductile (Figure 3b): ductilely stretched quartz grains have high aspect ratios, whereas feldspars are brittlely deformed. The angular difference between the schistosité (S) and cisaillement (C) planes of a weak S-C fabric [Berthé et al., 1979] suggests γ > 2 [Ramsay and Huber, 1983]. Sinistral shear is indicated by an asymmetrically boudinaged aplite dike and the S-C fabric.
2.1.2. Central Shear Zone
 The central shear zone is ∼450 m farther south (closer to the Habach Group contact) and also hosted in the Zentralgneis. It has a width of ∼50 m and consists of at least three generations of structures: the earliest are en echelon arrays of long (decimeter-scale) and narrow (centimeter- to millimeter-scale) subvertical sinistral and dextral shear zones that strike E-NE and are filled with muscovite, biotite, or chlorite. Like the northern shear zone, quartz was ductile and feldspar brittle. These features are truncated by a younger, wider (m scale), and more numerous set of brittle-ductile shear zones that form conjugate sinistral and dextral sets (Figure 3c); a few are brittle and bear chlorite. The youngest structures are sheared quartz veins, tension-gash arrays, and minor brittle faults (Figure 3a).
2.1.3. Southern Shear Zone
 Shear zone width and strain magnitude indicate that the southern shear zone is the main strand of the Rinderkarsee shear zone at our study site. It is separated from the central shear zone by a low-strain panel ∼600 m wide. It straddles the Zentralgneis-Habach Group contact: the Zentralgneis is phyllonitic over a 3-m-wide zone, whereas the less competent rocks of the Habach Group are mylonitic over a zone ≥100 m wide (Figure 3d). Like the northern shear zone, it consists of sinistral NE striking subvertical meter-scale shear zones. Strain, estimated from the relative amount of grain size reduction, is greatest in this southern shear zone, likely because of the competence contrast across the contact. At the outcrop scale, the rock has the typical “ductile” appearance of a mylonite: fabrics are homogeneous at the meter and smaller scale, except for localized slip along boudin surfaces.
2.1.4. Shear Zone Kinematics
 We used fault striae analysis to analyze the kinematics of the Rinderkarsee shear zones. The sense of motion on faults and shear zones was determined using Riedel shears, S-C fabrics, drag of foliation, and fibrous mineral growth [e.g., Petit, 1987]. A confidence level, based on the type, abundance, and clarity of the indicator, was assigned to each sense of motion determination, and used as a weighting factor in our kinematic analysis. We used the computer program package of Sperner et al.  for fault slip analysis to calculate the orientation of the principal stress axes [Angelier, 1984]. Our brittle-ductile fault data are best interpreted by using a fracture angle of 45° rather than 30°; this higher angle is in agreement with experimental data that links higher fracture angle with higher confining pressures [Mogi, 1974].
 The simple NE striking sinistral shear zones in the northern and southern shear zones accommodated ∼NNW and ∼ENE trending shortening and extension, respectively (Figure 4), accommodating orogen-parallel extension and orogen-perpendicular contraction along with eastward extrusion of Tauern Window. The central shear zone is more complicated. The orientations of the early sinistral and dextral en echelon shear zones shear zones, the significant overlap in their orientations, and their millimeter- to centimeter-scale offsets, suggest that rather than being simple conjugate pairs, these early shear zones may be related to lithologic heterogeneities in the Zentralgneis. While these shear zones share an extension direction with earlier structures, both the principal extension direction, e1, and the intermediate strain axis, e2, vary by as much as 30°. This variation may be related to the interaction of a vertical shortening component with the dominant NNW-ESE shortening. However, the main brittle-ductile faults of the central shear zone accommodated ∼NW directed shortening and ∼NE directed extension, similar to the northern and southern shear zones. The youngest structures (sheared quartz veins, tension gash arrays, and minor brittle faults) are kinematically compatible with this.
 Although overprinting relationships within each of the northern, central, and southern shear zones constrain the relative timing of structures within each of those zones, we cannot constrain whether these three shear zones were active together or in sequence. If the general change from brittle-ductile deformation to brittle deformation observed in the central and southern shear zones is indicative of the history of the entire Rinderkarsee shear zone, we infer that the earliest deformation was the ductile amphibolite-facies (see below) deformation typified by the southern shear zone, midstage deformation was brittle-ductile typified by the bulk of the structures in the central and northern shear zones, and the youngest deformation was brittle and typified by the minor cracks/veins of the central shear zone.
2.2. Optical Microstructures
 To evaluate the grain-scale deformation mechanisms within the Rinderkarsee shear zone, we studied three primary microstructural types: (1) an early(?) high-strain ductile mylonite from the southern shear zone, (2) a moderate-strain brittle-ductile shear zone from the central shear zone, and (3) the late stage(?) tip of a millimeter-scale brittle-ductile shear zone from the central shear zone. The undeformed host gneiss is a biotite-bearing granite with ∼1 mm grains of 5% biotite, 35–55% quartz, 25–40% alkali feldspar, and 10–25% plagioclase. Plagioclase includes ∼5 vol % of ∼10 μm diameter muscovite + quartz grains, presumably formed from decomposition of the K2O component in the plagioclase (see below). All the alkali feldspar is “interlocking” antiperthite [e.g., Alling, 1938] (average bulk grain composition of an00ab52or48; measured by electron probe) composed of 10–100 μm wide intergrowths of an00ab99or01 (albite) and an00ab05or95 (microcline) and is referred to as “K-feldspar.”
2.2.1. High-Strain Ductile Mylonite
 The highest-temperature and highest-strain rocks are the mylonitic rocks of the Habach Group from the southern shear zone (Figure 5a). Mylonitic amphibolites are composed chiefly of prismatic 50-μm-long hornblende with minor undulatory extinction and equant 20 μm plagioclase. Mylonitic quartzofeldspathic rocks consist predominantly of subequal amounts of plagioclase, quartz, and biotite. Rare plagioclase porphyroclasts up to 1 mm in diameter have patchy undulatory extinction, subgrains, deformation twins, and core-and-mantle recrystallization microstructures; the bulk of the plagioclase are 20–50 μm, have axial ratios of ∼2:1, sweeping undulatory extinction, and straight to curved grain boundaries. Where not pinned by other phases, quartz reaches 200 μm and has sweeping undulatory extinction, well-defined subgrains, and dentate grain boundaries; most of the quartz grains are pinned to <50 μm.
2.2.2. Moderate-Strain Brittle-Ductile Shear Zone
 Deformation within the moderate-strain central shear zone was accommodated by mixed brittle and ductile deformation (Figure 5b). Millimeter-scale quartz grains in the host gneiss have been deformed into polycrystalline clots of ∼200-μm-scale grains with embayed boundaries, pervasive undulatory extinction, island grains, and numerous subgrains, all features typical of regime 3 of Hirth and Tullis . K-feldspar and plagioclase porphyroclasts have brittle fractures filled with epidote, muscovite, and quartz, deformation lamellae, deformation twins, patchy undulatory extinction, and local core-and-mantle microstructures. Strain is concentrated in quartz- and mica-rich zones that anastomoze around brittlely deformed K-feldspar and plagioclase porphyroclasts.
2.2.3. Brittle-Ductile Shear Zone Tip
 The millimeter-scale tips of several of the late, long, narrow shear zones of the central shear zone show unexpected features. The rock hosting the shear zones shows only brittle-ductile deformation: quartz shows microstructures typical of grain boundary migration (i.e., regime 3 of Hirth and Tullis ), and feldspars have brittle fractures, mechanical twins, deformation lamellae, undulatory extinction and some subgrains (Figure 5c). The feldspar cataclasis was accompanied by the growth of muscovite. This breakdown localized deformation in muscovite-rich grain boundaries, which eventually connected to form throughgoing, discrete shear zones (Figure 5d). Within these discrete shear zones, all grains (K-feldspar, plagioclase, and quartz) are pervasively recrystallized.
2.3. Electron Backscatter Diffraction Microanalysis
 Critical additional information regarding the grain-scale deformation was obtained from electron backscatter diffraction (EBSD; see Appendix A). We used two spatial scales of EBSD to characterize the microstructural evolution of the Rinderkarsee shear zone: (1) sample-scale determinations of lattice preferred orientations (LPO; same as crystallographic preferred orientation) and (2) finer-scale maps of selected smaller areas to determine LPOs, grain shapes, grain sizes, intragrain lattice rotation, and intergrain orientation relationships.
2.3.1. High-Strain Ductile Mylonite
 The microstructure of a typical mylonitic sample from the southern shear zone is exhibited in Figure 6; this sample (50729G6) is a metaconglomerate from the Habach Group a few meters from the contact with the Zentralgneis. The sample grain size is fairly uniform although the plagioclase are the finest (Figure 6a); the feldspar and quartz grains are all isolated, but both, particularly quartz, which is more abundant and coarser, show subgrains. Plagioclase, K-feldspar, and quartz all have aspect ratios of ∼2:1, determined by processing of EBSD data. Rare plagioclase porphyroclasts have maximum dimensions of 500–700 μm and well-developed strain shadows filled with recrystallized quartz (Figure 6b). These plagioclase porphyroclasts show 5°–8° of misorientation from core to rim (Figure 6c), and the more abundant 50–100 μm matrix plagioclase grain show 3°–6° misorientation from core to rim. The matrix feldspar grains have weak LPOs (multiples of uniform distribution (MUD) ≤ 2), with  subparallel to the X strain axis and poles to (010) subparallel to the Z strain axis (Figure 6e). All the quartz grains, regardless of whether they are matrix grains or hosted in plagioclase strain shadows, show a random distribution of grain orientations (MUD < 2) (Figure 6c). Individual quartz grains within strain shadows have <5° of misorientation from core to rim that are compatible with rotation around the c axis. Quartz matrix grains yield misorientation angle distributions for neighbor pairs and random pairs that are equivalent at the 80% confidence level (using the Kolmogorov-Smirnov test, d = 1.08) [Wheeler et al., 2001] (Figure 6f). Recrystallized quartz grains in the strain shadow of a plagioclase porphyroclasts show a greater correlation between neighbor and random pairs (d = 0.75; confidence interval <80%), and have a peak of misorientations between 2° and 10° (Figure 6g).
 To assess whether these weak LPOs are typical of the mylonite zone, two other high-strain rocks of different composition were also measured. Sample 50729G4 is a biotite-rich gneiss from the Habach Group and 50729G5 is an aplitic Zentralgneis dike intruding the Habach Group. Both have weak LPOs (Figures 7a and 7b) similar to mylonitic sample 50729G6.
2.3.2. Moderate-Strain Brittle-Ductile Shear Zone
 Moderate-strain brittle-ductile shear zones (e.g., sample 50729F3) contain quartz augen up to 2 mm, recrystallized quartz grains, and mica-rich foliation planes. Individual quartz augen larger than ∼50 μm show a strong “single-crystal” fabric with their c axes subparallel to the Z strain axis (Figure 8b) and outward increasing misorientations of up to 5° between core and rim that show rotation about poles to prism planes. Matrix quartz grains show a weaker, but still significant LPO with c axes subparallel to Y and dispersed 〈a〉 axes (Figure 8c). The presence of distinct fabrics in the quartz augen and matrix quartz suggests that the quartz augen are relicts of an older fabric and are unfavorably oriented for slip. In contrast to the quartz grains, the feldspar porphyroclasts contain no optically visible subgrains and are transected by fractures filled with neoblastic quartz and muscovite.
2.3.3. Brittle-Ductile Shear Zone Tip
 Late, millimeter-scale brittle-ductile shear zone tips are typified by Zentralgneis sample 50729H1b (Figure 9). The feldspar grains have a strong LPO (Figures 9d and 9e) characterized by  subparallel to the X strain axis and poles to (001) subparallel to the Z strain axis. Quartz grains have a modest LPO (MUD ≤ 6) with c subparallel to Z and distributed 〈a〉 axes (Figure 9e). Individual grains of feldspar and quartz have aspect ratios of ∼2:1 and show core-rim misorientations of 4°–8° (Figure 9f); the intragrain lattice rotations in quartz are compatible with rotation about the c axis. Quartz neighbor pairs show an abundance of low misorientation angles compared to random pairs of grains, suggesting subgrain rotation recrystallization (Figure 9g).
3.1. Quartz Deformation Mechanisms at Rinderkarsee
 By what grain-scale mechanisms did the quartz grains in the Rinderkarsee shear zone deform? Clear evidence of crystal plasticity mandates some combination dislocation glide or creep, solution transfer creep, and grain boundary sliding.
 A group of microstructural observations argue in favor of dislocation creep: (1) All the Rinderkarsee shear zone samples show optical quartz microstructures indicative of high grain boundary mobility and regime 3 dynamic recrystallization [e.g., Hirth and Tullis, 1992; Stipp et al., 2002]. (2) The medium-strain shear zone (Figure 8) and the shear zone tip (Figure 9) have strong quartz LPOs. (3) Individual quartz grains show clear intragrain lattice rotation. (4) The statistically significant difference between the distribution of misorientation angles between neighbor pairs and random pairs of quartz grains in the plagioclase strain shadow of mylonitic sample 50729G6 and in the shear zone tip sample 50729H1b implies that these neighboring quartz grains are related through subgrain rotation recrystallization [Lloyd et al., 1997; Wheeler et al., 2001].
 In contrast to the above observations that speak in favor of dislocation glide or creep, other microstructural observations indicate the activity of other deformation mechanisms: (1) The similarity in the misorientation distributions of the neighbor pairs and random pairs in the matrix quartz of mylonitic sample 50729G6 implies that those grains are not related through dislocation creep and (2) the quartz LPOs in the mylonite samples are weak to random (Figures 6 and 7).
 These microstructural observations, combined with the fine grain size and equant shapes of the quartz grains, render it most plausible that the dominant quartz deformation mechanism in the samples with weak LPOs was dislocation creep-accommodated grain boundary sliding [Boullier and Gueguen, 1975; Rutter et al., 1994]. Cathodoluminescence microscopy was used to look in the moderate-strain, mylonitic and shear zone tip samples for evidence of cracking, dissolution, or precipitation of quartz that might be correlated with the fluid required for muscovite formation from feldspar; none was observed.
3.2. Plagioclase Deformation Mechanisms at Rinderkarsee
 Plagioclase in mylonitic samples show optical microstructures and intragrain lattice orientation indicative of dislocation creep. The absence of microcracks and the fine equant grain size are compatible with dislocation creep-accommodated grain boundary sliding in feldspar in the high-strain mylonite; this is in spite of the fact that feldspar does not form isolated, pinned grains in the mylonitic rocks.
 Plagioclase in shear zone tips show optical microstructures and intragrain lattice orientation (e.g., Figure 9) indicative of dislocation creep. These features, combined with the thin section scale microcracks discussed above, imply that feldspar deformation in the shear zone tips involved a combination of microcracking, dislocation creep, and grain boundary sliding. Outside the millimeter-scale shear zones, there is a weak to random LPO in all phases. Within these shear zones, however, all phases have strong LPOs. There are also significant misorientations across grains of all phases, and a statistically significant difference between neighbor pair and random pair misorientations. This reveals that the active recrystallization mechanisms within nascent shear zones included subgrain rotation and grain boundary migration.
3.3. Dislocation Glide Systems at Rinderkarsee
 Our data provide two means of determining which slip systems were active during the formation of the Rinderkarsee shear zone: multigrain LPOs and intragrain rotations. In the mylonitic rocks (Figures 6 and 7), the feldspar and quartz LPOs are weak, but quartz intragrain rotation in 50729G6 suggests basal slip in the 〈a〉 direction. In the moderate-strain, brittle-ductile central shear zone (Figure 8), the LPOs and intragrain lattice rotations suggest prism 〈a〉 slip in quartz matrix grains and basal 〈a〉 slip in quartz porphyroclasts. Feldspars in the same shear zone yielded no LPOs, have no subgrains, and are transected by fractures, indicating brittle deformation. The LPOs and intragrain rotations from the shear zone tips (Figure 9) indicate a combination of basal and prism 〈a〉 slip in quartz and (001) slip in feldspar.
3.4. Summary of Deformation Mechanisms at Rinderkarsee
 In summary, the shear zone tips are interpreted to have formed by dislocation creep in quartz and microcracking, dislocation creep, and grain boundary sliding of feldspar. The moderate-strain samples deformed via quartz dislocation creep and plagioclase cataclasis in quartz-rich samples and dislocation creep-accommodated grain boundary sliding of quartz and cataclasis + dislocation creep of plagioclase in quartzofeldspathic samples. The high-strain samples formed through dislocation creep-accommodated grain boundary sliding of quartz and feldspar.
3.5. Transformational Plasticity at Rinderkarsee
 The microstructures described above indicate that the late, small-scale shear zones at Rinderkarsee formed where fluid influx converted K-feldspar into muscovite. The neoblastic muscovites then served to localize deformation, forming throughgoing, discrete shear zones. K-feldspar, plagioclase, and quartz in these zones show optical and EBSD evidence of dynamic recrystallization even though the feldspars outside the shear zones remained brittle.
3.6. Regional Extent of Rinderkarsee Shear Zone
Rosenberg et al.  mapped the Rinderkarsee shear zone 80 km farther west from our study area as part of their Ahorn shear zone (Figures 2 and 10). They report a westward reduction in strain partitioning: the one to hundreds of meter-wide shear zones separated by ∼500 m low-strain panels observed at Rinderkarsee gradually give way to a more distributed ductile shear zone as much as 2 km wide. They also document a temporal transition from early amphibolite-facies deformation at ≥500°C in which quartz and plagioclase were ductile, to late greenschist-facies deformation at ≤300°C in which quartz was undergoing dislocation glide, but not creep.
3.7. Deformation Depth at Rinderkarsee
 Constraining the depth of deformation at Rinderkarsee is not straightforward because of an absence of rock types suitable for barometry, but can be estimated crudely. If the deformation conditions of the Greiner shear zone, 6–8 kbar and 500–570°C [Selverstone et al., 1984], represent a thermal steady state apropos to the entire Tauern Window, the deformation temperature range of 500–300°C inferred for the Rinderkarsee area corresponds to a depth range of ∼24–14 km (assuming a density of 2.75 g/cm3). If our biotite 40Ar/39Ar age from the Rinderkarsee shear zone of ∼24 Ma represents cooling through the standard closure temperature of ∼350–300°C [Dunlap, 2000], this was at a depth of ∼16–17 km. This agrees with the overall exhumation history of the Tauern Window noted earlier: ∼25 km depth at 30 Ma, ∼10–15 km at 25–17 Ma, and ∼4 km by 7 Ma [Oxburgh et al., 1966; Cliff et al., 1985; Spear and Franz, 1986; von Blanckenburg et al., 1989; Christensen et al., 1994].
3.8. Implications for the Brittle-Ductile Transition in Continental Crust
 Let us now return to the three questions posed in the introduction about the general applicability of the brittle-ductile transition model in Figure 1.
 1. The Rinderkarsee shear zone is not a single strand but is composed of 1- to 100-m-wide high-strain shear zones separated by ∼500-m-wide low-strain panels; macroscopic deformation in the shear zones varied from ductile to brittle-ductile to brittle. The bulk of the strain was accommodated by the southernmost, ∼100 m thick ductile shear zone.
 2. A broad range of deformation mechanisms were active, depending on the type of crystal, the type of rock, and the style of the shear zone. All grains in the largest, ductile shear zone deformed via dislocation creep-accommodated grain boundary sliding. The moderate-strain shear zones are similar, but feldspar deformed principally by microcracking. The thinnest, incipient shear zones developed in muscovite-rich zones formed from hydration of feldspar; all phases within deformed by dislocation creep. Addressing this variation would improve computational models of the brittle-ductile transition.
 3. The alteration of feldspars to phyllosilicates is a recognized form of transformational plasticity in fault zones that has two principal effects: the growth of weak phyllosilicates weakens the entire rock, promoting deformation of feldspar and quartz, and the reduction in grain size can promote diffusion-accommodated grain boundary sliding or solution transfer creep [Evans, 1988; Janecke and Evans, 1988; Wintsch et al., 1995; Hippertt, 1998; Imber et al., 2001; Ingles et al., 1999; Gueydan et al., 2003]. What is novel about the Rinderkarsee shear zone is that the growth of weak phyllosilicates promoted the formation of shear zones in which the dominant deformation mechanism was dislocation creep.
 The Rinderkarsee shear zone provides an excellent opportunity to study fault structure at the transition from brittle-ductile to ductile deformation. It is overall a brittle-ductile shear zone, but is composed of narrow, highly localized (1–100 m wide) zones of concentrated strain that are either brittle-ductile or ductile.
 The highest-strain zone is also the widest (100 m), and represents a discrete, downward continuation of the Salzach fault into the ductile midcrust. This zone is mylonitic, but shows weak lattice-preferred orientations, implying that the bulk grain-scale deformation of quartz and feldspar was dislocation-assisted grain boundary sliding. Moderate-strain brittle-ductile shear zones formed via quartz dislocation creep and plagioclase cataclasis in quartz-rich samples and via dislocation creep-accommodated grain boundary sliding of quartz and cataclasis + dislocation creep of plagioclase in quartzofeldspathic samples. Millimeter-scale incipient shear zones show strong lattice-preferred orientations and dynamic recrystallization of all phases, including plagioclase and K-feldspar, even though feldspars outside the shear zones were undergoing cataclasis. This unusual feldspar plasticity was accompanied by fluid-driven alteration of K-feldspar to muscovite that served to localize strain along muscovite-rich grain boundaries that developed into throughgoing grain-scale shear zones.
Appendix A:: EBSD Methods
 Thin sections were polished for 1 h with 1 μm and 0.25 μm diamond polishing compound, for 12–14 h in colloidal silica, and then carbon coated. They were analyzed in a JEOL 6300 scanning electron microscope fitted with an HKL Nordlys EBSD camera. They were inclined 70° to the incident beam. Working conditions were: 20 kV acceleration voltage, ∼4.5 nA beam current, <1 μm spot size, and 25 mm working distance. Diffraction patterns were collected and indexed using CHANNEL 5 HKL software. By carefully examining the effect of parameters such as Hough transform, number of reflectors, minimum and maximum number of bands detected, and the chosen match unit, we are confident that the CHANNEL software indexed the correct phase >95% of the time. Samples were analyzed in both coarse and fine mode. Coarse scans used a step size of 50 μm designed to gather the bulk LPO of the sample and covered an area of 1 cm2, whereas fine scans used a step size of 4 μm to examine textural attributes of samples and individual grains. We used the noise reduction system within the HKL software, carefully comparing our maps to the band contrast and fore scatter images to fill grains without artificially growing them.
 Funded by NSF grants EAR-0309995 and EAR-0309542 and DFG grant Ra 442/19. Kurt Decker (Universität Wien), Bernd Lammerer (LMU München), and Franz Neubauer (Universität Salzburg) graciously shared their SEMP expertise. Nicholas Barth was mountain goat 2. We thank Charles Sammis for helpful discussions and Claudio Rosenberg for a preprint of his Tauern shear zone paper. Joe White and Wolfgang Frisch provided helpful reviews.