microstructural and textural development of calcite …...is related to the top-to-the-n...

Tectonophysics 316 (2000) 327–342www.elsevier.com/locate/tecto

Microstructural and textural development of calcite marblesduring polyphase deformation of Penninic units within the

Tauern Window (Eastern Alps)

W. Kurz a,*, F. Neubauer b, W. Unzog a, J. Genser b, X. Wang ba Institut fur Geologie und Palaontologie, Karl-Franzens-Universitat Graz, Heinrichstraße 26, 8010 Graz, Austria

b Institut fur Geologie und Palaontologie, Paris-Lodron-Universitat Salzburg, Hellbrunner Straße 34, 5020 Salzburg, Austria

Received 11 March 1999; accepted for publication 22 September 1999

Abstract

The evolution of calcite microstructures and crystallographic preferred orientations (CPOs) is well understood dueto well constrained experimental studies. However, the interpretation of naturally deformed calcite marbles is moredifficult because of less constrained strain paths, a multiphase deformation history, and variable P–T conditions. ThePenninic units within the Tauern Window (Eastern Alps) have been affected by several deformation events andmetamorphic overprint. Generally, three major deformational events can be distinguished. D1 is related to underthrust-ing of Penninic units beneath the Austroalpine nappe complex, and top-to-the-N nappe stacking within the Penniniccontinental units. Deformation stage D2 is interpreted as reflecting the subsequent continent collision between thePenninic continental units and the European foreland. D3 is related to the formation of the dome structure of theTauern Window. This polyphase deformation history can be partly reconstructed by the evolution of calcitemicrofabrics and CPOs.

Three types of calcite-fabrics are distinguished within the Penninic units of the Tauern Window and the LowerAustroalpine unit. D1-fabrics are characterized by equilibrated microstructures and LT-CPOs. The CPOs are generallystrong and symmetric, with one well developed cluster near the Z-axis of the finite strain ellipsoid. These fabrics havelocally been overprinted by subsequent amphibolite to greenschist facies metamorphism. Generally, the occurrence ofLT-fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central partof the Tauern Window, while HT-fabrics have been observed outside this area. Fabrics from the central part of theTauern Window have likely been strengthened during subsequent thermal equilibration, while the fabrics that havebeen observed at the peripheral parts have been less affected by subsequent metamorphic overprint. Therefore,D1-fabrics do not reflect D1 conditions, but subsequent thermal equilibration. Similar observations have been madefor the evolution of D2-fabrics. LT-fabrics dominate inside the amphibolite facies isograde, HT-fabrics occur outside(greenschist facies metamorphic conditions). The fabrics are characterized by high finite strains near the margins ofthe Tauern Window, the intensity of which decreases towards the central parts, where microstructures are characterizedby recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint. The strong CPOsdocument this influence. The HT-fabrics and microstructures within peripheral areas indicate that they have less beenaffected by this thermal event, and, therefore, are more indicative for the deformational conditions during D2. D3 isrestricted to distinct shear zones along the tectonic boundaries of the Tauern Window. From the central parts to the

* Corresponding author. Tel. : +43-316-380-8725(8734); fax: +43-316-380-9870.E-mail address: [email protected] ( W. Kurz)

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328 W. Kurz et al. / Tectonophysics 316 (2000) 327–342

shear zone boundaries, a clear evolution of microfabrics can be observed. In internal parts, microstructures arecharacteristic for intracrystalline plasticity with a dominating activity of r−-glide. Twinning was less important duringthe final phases of deformation. On approaching the shear zone boundaries, the grain size decreases due to dynamicrecrystallization and secondary grain size reduction until ultramylonites are formed. Within these domains grainboundary sliding seems to have been dominant.

In conclusion, calcite CPOs from polyphase areas do not only include information on the deformation conditions,but also bear information about the thermal overprint subsequent to the main deformational event. © 2000 ElsevierScience B.V. All rights reserved.

Keywords: calcite; crystallographic preferred orientations (CPOs); Eastern Alps; microstructures; polyphase deformation; TauernWindow

1. Introduction normal faults along the margins of such domestructures. Consequently, deformational structuresin the inner parts of exhumed domes often docu-The evolution of textures and crystallographic

preferred orientations (CPOs) of calcite is well ment the evolution that was related to theunderthrust history. Microstructural studies com-constrained by experimental studies (e.g., Dietrich

and Song, 1984; Schmid, 1982; Schmid et al., bined with the evaluation of CPOs are importanttools in obtaining information on the deformation1987; Wenk et al., 1987; Tome et al., 1991;

Burkhard,1993; De Bresser and Spiers, 1997; and mechanisms operating in naturally deformedrocks. Due to the plastic behaviour of calcite atreferences cited therein). Wenk et al. (1987) distin-

guished between low-temperature (LT) and high- rather low-temperature conditions (e.g., Wenk,1985), data on the microstructural evolution oftemperature (HT ) CPOs. LT-fabrics are charac-

terized by single maxima near the Z-axis of the calcite are very useful for the documentation ofthe final phases of an orogenic event. This studyfinite strain ellipsoid and HT-fabrics form two

small maxima with the tendency to be distributed presents data on calcite microstructures in theTauern Window (Eastern Alps), which have beenalong a small circle centred on Z and a great circle

at small angles to the Y–Z-plane. The interpreta- affected by a polyphase deformation and metamor-phic history.tion of naturally deformed calcite marbles is

difficult because of under-constrained strain paths,multiphase deformation history and variable P–Tconditions. Calcite fabrics generally reflect the final 2. Geological settingphases of crustal deformation at decreasing tem-peratures (e.g., Burkhard, 1993). As calcite is an The Tauern Window exposes the Penninicimportant rock-forming mineral, the investigation nappe complex in the footwall of the Austroalpineof its microstructures and CPOs give important nappe complex (Fig. 1). The Austroalpine nappeinformation on the deformation mechanisms complex formed the hanging-wall plate duringwithin the crust. Late Cretaceous and Early Tertiary continent–

Underthrust continental crust is often exposed continent collision in the Alps. The Penninicwithin tectonic windows in internal zones of colli- nappe-stack includes both passive and active conti-sional orogens. Generally, underthrust crust will nental margin sequences ( Kurz et al., 1998b),be exhumed during the final stages of continent– which have been deposited on continental base-continent collision, in an extensional regime (e.g., ment units and ophiolitic sequences. From bottomPlatt, 1986, 1993; Dewey, 1988), and exposed as to top the following tectonic units are distinguishedmetamorphic domes within the internal zones of (Tollmann, 1975, 1977; Frank et al., 1987; Kurzcollisional orogens (Selverstone, 1988; Genser and et al., 1996, 1998b) (Fig. 1b and c):Neubauer, 1989). Removal of hanging-wall crust 1. The Venediger and Wolfendorn Nappes (Frisch,

1974, 1975), which comprise a pre-Variscanis achieved by detachment along ductile low-angle

329W. Kurz et al. / Tectonophysics 316 (2000) 327–342

Fig. 1. (a) Tectonic sketch map of the Tauern Window; S, Sonnblick Dome; HA, Hochalm–Ankogel Dome; H, Holltor–RotguldenDome; G, Granatspitz Dome; ZV, Zillertal–Venediger Domes; TA, Tux–Ahorn Domes; A–A∞, location of cross-section in (b) (afterKurz et al., 1998b). (b) Section across the central part of the Tauern Window [for location see (a)].

basement (e.g., the Riffl Nappe) intruded by metapelites and graphitic quartzites (MurtorlGroup);Variscan granitoids (the Zentralgneis), and

Jurassic to Cretaceous cover sequences; 3. The Eclogite Zone, restricted to the centralsouthern Tauern Window, characterized by a2. The Storz Nappe comprising Variscan and

Alpidic polymetamorphic basement rocks cov- Mesozoic volcano-sedimentary sequence of adistal continental slope;ered by late Paleozoic (?) or Cretaceous (?)


4. The Rote Wand–Modereck Nappe, formed of part of the Tauern Window, D1 fabrics have beenpreserved. D1 and D2 are separated by a phase ofbasement rocks, covered by Permian to Triassiclower amphibolite to greenschist facies metamor-quartzites and Triassic metacarbonates, Jurassicphism (‘Tauern Crystallization’), which was relatedbreccias, calcareous micaschists and metatuffs,to the thermal equilibration subsequent to the sub-and Cretaceous metapelites and metapsam-duction of the Penninic unit and collision with themites. All these units were part of the southernAustroalpine unit. Locally, D2 was contemporane-margin of the Penninic continental unit priorous with this metamorphism phase. D3 is related toto collision with the Austroalpine unit.the formation of the dome structure of the Tauern5. The Glockner Nappe, comprising a former oce-Window. This D3 phase is characterized by theanic basement (serpentinites and other ultra-interference of multiply developed structures, andmafic rocks), and an incomplete ophioliticby deformation partitioning and shear localizationsequence, containing micaceous marbles andalong the dome margins (Behrmann and Frisch,calcareous micaschists;1990; Kurz and Neubauer, 1996), where a new6. The Matrei Zone, including the Klammkalkfoliation S3, associated with a stretching lineationunit, which represents an accretionary wedgeL3, was developed. The exhumation history of the(Frisch et al., 1987).Penninic units (Fig. 1) is constrained by petrologicalThese Penninic units are surrounded by Austroalpineand geochronological (Cliff et al., 1985; Droop,units, all together affected by Cenozoic amphibolite1985; Selverstone, 1985, 1988, 1993; Christensento greenschist grade metamorphism subsequent toet al., 1994), and by structural data (Behrmann,nappe stacking. The metamorphic grade decreases1988; Genser and Neubauer, 1989; Kurz andcontinuously from the central to the peripheral partsNeubauer, 1996; Selverstone, 1988). Althoughof the Tauern Window.modern microstructural studies combined with theThe Tauern Window is bordered along its north-investigation of CPOs exist for many areas withinern and southern margins by sinistral strike–slipthe Tauern Window, these data have only beenfaults, linked with several major dextral faultsused for kinematic interpretations in terms of shear(Ratschbacher et al., 1991a; Kurz et al., 1994;criteria (e.g., Behrmann and Ratschbacher, 1989;Wang and Neubauer, 1998), and by low-angleBehrmann, 1990; Behrmann and Frisch, 1990;normal faults along its eastern and western marginsWallis et al., 1993; Wallis and Behrmann, 1996).(Selverstone, 1988; Behrmann, 1988, 1990)However, these data only exist for limited areas. In(Fig. 1). The arrangement of these major faultsorder to constrain the deformational behaviour ofwas interpreted in terms of a pull-apart structure,several units during distinct phases of deformation,which triggered unroofing and subsequent exhu-and to constrain the relation between deformationmation and uplift of underplated lithosphere inand metamorphism, representative examples ofthe area of the Tauern Window (Genser andcalcite microfabrics and CPOs have been chosenNeubauer, 1989).for this presentation. The entire set of data will beGenerally, three major phases of deformationpublished separately ( Kurz et al., in press).(D1, D2, D3) are distinguished (Bickle and

Hawkesworth, 1978; Droop, 1981; Lammerer, 1988;Behrmann, 1990; Genser, 1992; Kurz et al., 1996, 3. Methods1998a, b). D1 is related to the top-to-the-N emplace-ment of several nappes, which resulted in the devel- In this section we discuss the evolution of calciteopment of a first foliation (S1), and a N-trending microstructures and CPOs along several structuralstretching lineation (L1). D2 is characterized by the sections across the Tauern Window. The representa-development of a second foliation S2, and a W- to tive samples have been chosen from areas where theNW-trending stretching lineation L2. D2 is related distinction between D1, D2, and D3 is clear. X-rayto the emplacement of the Penninic nappe stack texture analyses of calcite-mylonites were carried outonto the European foreland. Especially the western with a Siemens D500 X-ray texture goniometer atand southeastern parts of the Tauern Window have the University of Graz (Austria). We derived incom-

plete pole figures of <f>[102], <r>[104],been pervasively affected by D2; within the central


<a>[110], [113], [202], and [116]. <c>[006] pole generally not exceed 0.2 mm (Fig. 2b). The grainsare slightly elongated subparallel to the S1 foliationfigures are difficult to be measured directly.(Rf=1.5–2.0; Rf defines the ratio between the longTherefore, Orientation Distribution Functionsand short axis of single grains in the X–Z-section).(ODFs) have been calculated from the X-ray poleThe grain boundaries are generally straight andfigures. <c>[001], <m>[100], [101] and [011]meet in triple junctions, whereas the serrate grainhave been recalculated from the ODF. The evalua-boundaries are less developed. Few grains aretion of pole figures and recalculation was done withtwinned, but these grains show only one set ofthe program TAT v. 2.2c/ODF AT v.1.1a providedtwins. Only within the Matrei Zone along theby Siemens Co. (Harmonic Method, Bunge, 1981,northern margin of the Tauern Window dolomite1985; Bunge and Esling, 1985), and with the programmarbles occur as massive, coarse-grained dolomites (Vector Method, Schaeben et al., 1985;without internal foliation.Schaeben, 1994). Both program packages include

Locally, especially in the southeastern and east-corrections for background and beam defocusing.ern parts of the Tauern Window, coarse, isometricSeveral packages use Bravais crystallographic indicesor slightly elongated calcite grains with a size offor the description of crystallographic directions,up to 0.5 mm, show serrated grain boundaries andleaving the index {i} of the {hkil} notation. Optical the development of polysynthetic twin lamellae.

CPOs were measured with standard universal stage These grains are surrounded by dynamicallymethods. recrystallized grains with a size of ca. 0.2 mm

(Fig. 2c), which indicates a different deformation–recrystallization mechanism.

4. D1

microstructures

The central part of the Tauern Window has 5. D1

crystallographic preferred orientationsgenerally not been affected by D2. Therefore,microstructures and CPOs developed during or Calcite CPOs related to D1 are generally charac-

terized by LT-fabrics (Fig. 3). The c-axes [001]subsequently to D1 are well preserved. The micro-form clusters near the Z-axis of the finite strainstructures of calcite marbles are very similar withinellipsoid (Fig. 2a). In places, these clusters areall tectonostratigraphic units. Calcite is charac-asymmetrically arranged; the fabric asymmetryterized by the formation of isometric, polygon-documents top-to-the-N sense of shear. Withinshaped grains ca. 0.5 mm in size (Fig. 2a). Thesecalcite-dolomite marbles, the calcite c-axes formgrains mainly show straight grain boundaries, rarelysingle girdle distributions within the Y–Z-planeslighty serrated grain boundaries. The grains con-(Fig. 2b). However, this might be interpreted intain e-twins, in particular in coarse-grained rocksterms of an effect of a second phase, for example,(Fig. 2a) with a grain size of up to 1 mm. Mostdeformation partitioning between calcite and dolo-twins show moderately thick to thin straight lamel-mite. In the central southern part of the Tauernlae with straight boundaries, similar to the geomet-Window, the c-axis distributions are characterizedric types I and II described by Burkhard (1993).by HT-fabrics (Fig. 3). The a-axes [110] and theWithin the distinct domains thick patchy twins withpoles to the prism planes [100], as well as thesutured boundaries, similar to type IV, occur. Thepoles to the rhombs [101] [011], form a girdlemajority of grains displays two sets of conjugatewithin the X–Y-plane; in places, this girdle is

e-twins (Fig. 2a), both oblique to the S1 foliation oblique to the foliation plane; the angle of thetraced by layers of white mica. The geometrical fabric asymmetry is 1–10°.arrangement of these twins is related to a simpleshear component indicating a top-to-the-N sense ofshear. Most of the calcite marbles are intercalated 6. D

2microstructures

with dolomite marbles on a centimetre, decimetreand metre scale. Dolomite marbles occur as fine- D2-related calcite microstructures are very

homogeneous within the Penninic units over thegrained rocks with a uniform grain size that does


Fig. 2. D1-related calcite microstructures and textures. (a) Slightly elongated grains, Rote Wand–Modereck Nappe, central TauernWindow; LT-c-axes distributions. (b) Calcite–dolomite marble, Rote Wand–Modereck Nappe, central Tauern Window; single girdledistribution of the c-axes. (c) Dynamically recrystallized grains along the grain boundaries of coarse calcite. X-ray textures (equalangle projections; logarithmic gradation of isolines; first isoline: uniform distribution; fifth isoline: 85% of maximum).

entire Tauern Window. Calcite grains are strongly foliation plane (Fig. 4). The mesoscopically high-strain appearance is reflected by the elongation ofelongated (Rf ca. 5–8), and form a shape-preferred

orientation either parallel, or oblique to the S2 the grains. Therefore, the macroscopic shear plane


Fig. 3. (a) Simplified tectonic map of the Tauern Window (for explanation see Fig. 1a) displaying the distribution of D1-related CPOs.L, LT-fabrics; H, HT-fabrics; I, single girdle distributions. (b) Classification of LT- and HT-fabrics (from Wenk et al., 1987).

is assumed to be in close coincidence with the the grain boundaries commonly meet in triplejunctions. Grain size is variable and stronglymesoscopic foliation. In single domains the grains

define a plane of mean elongation which is slightly depends on the presence of additional mineralphases, especially white mica and graphite, thatinclined (10–15°) to the mesoscopic foliation.

However, the elongation of the calcite grains is take influence on the size of recrystallized grains.The average grain size of pure marble mylonitesstronger near the margins of the Tauern Window

(Rf 5–8; Fig. 4a) than within the central parts (Rf is 0.5–1 mm. Generally, the grain size increasestowards the inner parts, according to the general1–3; Fig. 4b). In the peripheral parts of the Tauern

Window the calcite grains show either straight, or trend of increasing metamorphic grade. In thecentral parts the grain size reaches up to 1.5 mm,slightly sutured boundaries. In the central parts,


Fig. 4. D2-related calcite microstructures and textures. (a) Highly elongated calcite grains with lobate grain boundaries, GlocknerNappe, southwestern Tauern Window; HT-fabrics with two maxima. (b) Slightly elongated calcite grains, Wolfendorn Nappe,northwestern Tauern Window; LT fabrics. (c) Dynamically recrystallized grains surrounding coarse grains, Lower Austroalpine unit,northeastern Tauern Window; LT-fabrics.

in the peripheral parts 0.5 mm at maximum. The calcite twins show moderately thick to thick twinsand straight lamellae, similar to the geometriccalcite grains show multiple sets of twins; however

one set of e-twins, generally oriented subparallel types I and II described by Burkhard (1993). Ingeneral, twin widths are larger near the marginsto the S2 foliation plane, dominates, especially

along the margins of the Tauern Window. Most of the Tauern Window and decrease towards the


Fig. 5. Simplified tectonic map of the Tauern Window (for explanation see Fig. 1), displaying the distribution of D2-related calcitemicrostructures and CPOs. L, LT-fabrics; H, HT-fabrics; I, single girdle distributions.

central parts, from 0.02 to 0.1 mm. In most cases, a-axes [110] and the poles to the prism planes[100] are distributed along a girdle within the X–the grains are not twinned in the innermost parts

of the Tauern Window. Within the Lower Y-plane, with one maximum near the Y-axis. Thepoles to the rhombs [202] plot along girdles sub-Austroalpine unit of the northeastern Tauern

Window calcite forms dynamically recrystallized parallel to the X–Y-plane, too. The LT-fabricsform clusters near Z. The CPOs are commonlygrains with a maximum size of 0.1 mm, that are

surrounding coarse grains a maximum of 0.5 mm characterized by rather symmetric fabrics. Only indistinct domains, they are asymmetricallyin size (Fig. 4c). These grains may show multiple

sets of twins. arranged; however, the fabric asymmetry reaches5° at maximum and documents a top-to-the Wsense of shear. The a-axes [110] and the poles tothe prism planes [100] are distributed along a7. D

2crystallographic preferred orientations

girdle within the X–Y-plane, with one maximumnear the Y-axis. Within the Lower AustroalpineD2-related CPOs are similar over the entire

Tauern Window, and within several tectonostrati- unit the calcite c-axes [001] form well definedclusters near Z. The a-axes [110] and the poles tographic units (Figs. 4 and 5). c-axes distributions

that are typical for HT-fabrics along the margins the prism planes [100] are distributed along agirdle within the X–Y-plane, with one maximumof the Tauern Window, and by LT fabrics in the

central parts. The HT-fabrics generally show two near the Y-axis. The poles to the rhombs [101][011] plot along girdles subparallel to the X–Y-maxima near Z with the tendency to be distributed

along single girdles near the Y–Z-plane of the plane, too. In the eastern part of the TauernWindow, twin-c-axis-pairs indicate an orientationfinite strain ellipsoid. One maximum is centred

near the Z-axis, two maxima are situated between of the maximum principal stress (s1) oblique tothe S2 foliation ( Kurz and Neubauer, 1996).the Y- and Z-axis of the finite strain ellipsoid. The


However, these s1 directions are more asymmetric Tauern Window, ultramylonites with opticallythan the c-axis pole figures. indiscernible calcite grains are developed within

the shear zones (Fig. 6c).

8. D3

microstructures9. D

3crystallographic preferred orientations

In zones that are affected by low-angle normaland strike–slip faults bordering the Tauern Calcite c-axes [001] fabrics from D3-shear zonesWindow, calcite displays uniform grain size are characterized by clusters close to the Z-axis ofbetween 0.3 and 1 mm. Within these shear zones the finite strain ellipsoid (Fig. 6). Locally, especi-a new S3 foliation has been formed. Calcite is ally along the northern margin of the Tauernhomogeneously twinned by single sets of e-twins Window (Salzach–Ennstal fault, Fig. 7), c-axesthat are oriented either subparallel or slightly distributions are typical for HT-fabrics. c-axes tendoblique to the penetrative foliation (Fig. 6a). Most to form single girdles within the Y–Z-plane of thetwins are thick (0.1–0.2 mm) and curved, and finite strain ellipsoid (Fig. 6b). One maximum islocally show features of polyphase twinning (‘twins centred near the Z-axis, two maxima are situatedin twins’), similar to type III of Burkhard (1993). between the Y- and the Z-axes. The a-axes [110]Such fabrics are typical for synmetamorphic and the poles to the prism planes [100] are distrib-intracrystalline deformation (r- and f-glide). Twins uted along a girdle within the X–Y-plane, with oneare often bent due to intracrystalline plasticity of maximum near the Y-axis. The poles to the rhombscalcite within micro-scale shear zones, while [101] [011] plot along girdles subparallel to thedomains between conjugate sets of shear bands X–Y-plane. At a further distance from the mainare less deformed (Fig. 6a). The grains show ser- fault, LT-fabrics are developed. HT-fabrics domi-rated and sutured, irregular boundaries; in places nate along the eastern margin of the Tauernthey are surrounded by fine recrystallized grains Window. LT-fabrics dominate along the southern(<0.1 mm). Core-mantle textures are occasionally and southeastern margins of the Tauern Windowrecognized. Twin-c-axis-pairs indicate an orienta- (Fig. 6a and c). The asymmetry of the CPO-fabricstion of the maximum principal stress (s1) subparal-

depends on the orientation of the shear zoneslel to the pole of the penetrative foliation ( Kurzbordering the Tauern Window. Along the northernand Neubauer, 1996). The mesoscopically high-margin of the Tauern window (Salzach fault), thestrain appearance is reflected by the elongation ofasymmetry ranges from 5 to 20° (Fig. 7); this hasthe grains and by the rotation of initial twins intoalso been been observed along the E–W-strikingan orientation subparallel to the S3 foliation. Ifshear zones along the southern margin of thetwins would have formed during the last stages ofTauern Window ( Kurz and Neubauer, 1996;deformation, the s1 orientation should be at 45°Fig. 7). Along the NW–SE striking Moll Valleyto the shear plane in simple shear, and coincideFault, the asymmetry ranges between 0 and 5°with the foliation normal in pure shear( Kurz and Neubauer, 1996; Fig. 7), which implies(Ratschbacher et al., 1991b). However, if twinsa higher pure shear component according to Wenkform throughout the deformation history, oldet al. (1987). These observations are consistenttwins progressively rotate relative to the shearwith a NNE to NE orientation of the maximumplane and s1 should be <45° to the shear plane.principle stress (s1): the higher the angle betweenAlong the Salzach Fault, which forms the northerns1 and the shear zone, the higher the pure shearmargin of the Tauern Window, the calcite grainscomponent. Generally, CPOs from coarse-grainedare extremely elongated (Rf ca. 8–12). Generally,mylonites and ultramylonites do not differ signifi-the elongated grains show a shape preferred orien-cantly. Referring to Schmid (1983), the equanttation subparallel to the macroscopic foliation.fine-grained ultramylonites should indicate a grain-These grains show only one set of e-twins that areboundary-dominated flow mechanism with a weaksubparallel to the foliation plane (Fig. 6b).

Approaching the Austroalpine units around the CPO. However, this is not supported by the


Fig. 6. D3-related calcite microstructures and textures. (a) Dynamically recrystallized grains surrounding coarse grains, Moser Fault,southeastern Tauern Window; LT-fabrics. (b) Highly elongated calcite grains, Salzach Fault; HT fabrics. (c) Calcite ultramylonite,Moll Valley Fault, southeastern Tauern Window; LT-fabrics.


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presence of strong CPOs that have been observed 2. Similar observations have been made for thealong the D3 shear zones bordering the Tauern evolution of D2-fabrics. However, D2 was gen-Window. erally contemporaneous to the metamorphic

overprint. LT-fabrics dominate inside theamphibolite facies isograde (Fig. 5), HT-fabrics

10. Summary and discussion occur outside. Therefore, the CPOs within thecentral parts seem to reflect the influence of

Three types of calcite-fabrics can be distin- subsequent thermal overprint (similar to theguished within the Penninic units of the Tauern fabrics that have been described for D1), whichWindow and the Lower Austroalpine unit. Each has strengthened the CPOs, while thetype is characteristic for a distinct phase of defor- HT-fabrics and microstructures within the peri-mation and metamorphism: pheral areas of the Tauern Window are signifi-1. D1-fabrics are characterized by well equilibrated cant for the deformational conditions during

microstructures. The strong and symmetric D2. However, the occurrence of HT-fabrics onlyCPOs seem to indicate a dominating flattening indicates temperature conditions above 350° Ccomponent. However, the fabrics have been ( Wenk et al., 1987). In the central parts, theoverprinted by subsequent amphibolite to microstructures are characterized by recrystalli-greenschist facies metamorphism, which can be zation and thermal equilibration, coincidingobserved from the associated rock assemblages with amphibolite facies metamorphic conditions(e.g., Kurz et al., 1996). Subsequent twinning in the core of the Tauern Window. In this area,did not highly affect the microstructures and the CPOs are sharper. Approaching the marginsCPOs; <30% of the grains have been twinned, of the Tauern Window, microstructures arecommonly only by one set of twins (Fig. 2a). characterized by strong elongation and multipleThe occurrence of LT-fabrics dominates the twinning, defining high finite strain decreasingarea of amphibolite facies metamorphic condi- towards the central parts. Concerning thesetions in the central part of the Tauern Window strongly deformed calcite tectonites, we assume(Fig. 3; Hock, 1980; Droop, 1985). HT-fabrics that the foliation and lineation coincide withare restricted to the peripheral parts and

the macroscopic shear plane and shear direc-approximately coincide with the occurrence oftion. However, since such high finite strainsgreenschist facies metamorphic assemblagescannot be achieved by twinning alone [only up(Fig. 3). Therefore, we assume that the defor-to 15% by e-twinning; Burkhard (1993)], plasticmation conditions during D1 were appropriatedeformation of calcite grains is assumed to beto produce HT-fabrics. However, these fabricsthe major deformation mechanism prior tohave been modified during subsequent meta-twinning. Since CPOs, especially the concen-morphism. Fabrics occurring inside this iso-tration of c-axes, generally remain constant atgrade seem to have been strengthened duringfinite strains of 20% and more, it can besubsequent equilibration [according to Schmidassumed that subsequent twinning did neitherand Casey (1986)] by forming single maximaaffect the CPOs nor the general microstructurenear Z [LT-fabrics of Wenk et al. (1987)].of these marble mylonites. Only within theAccordingly, the fabrics outside the isogradeLower Austroalpine unit the LT-fabrics seemhave less been affected because of the lowerto reflect the low-grade metamorphic conditionsmetamorphic grade (greenschist facies); withinduring D2.this area the deformational HT-fabrics have

3. D3 is restricted to distinct shear zones alongbeen preserved. In this case the CPOs are notthe tectonic boundaries of the Tauern Window.only significant for the conditions during defor-During exhumation and D3 doming, deforma-mation (D1) in terms of LT- and HT-fabrics,tion occurred during cooling ( Kurz andbut also bear information about the thermalNeubauer, 1996). Microstructures suggestequilibration subsequent to the main deforma-

tional event. intracrystalline plasticity with a dominating


r−-glide. Twinning was less important during Window, whereas HT-fabrics have been observedoutside this isograde, which is in contradiction tothe final deformation phases. However, curved

twins indicate that plasticity was an important the general classification of LT-and HT-fabrics byWenk et al. (1987). Accordingly, fabrics that havedeformation mechanism subsequent to twinning

too. Assuming that temperatures did not rise been observed in the central part of the TauernWindow have been modified and strengthenedduring this deformational event, this feature

may be interpreted in terms of fast strain rates. during thermal equilibration at amphibolite faciesmetamorphic conditions. Fabrics that have beenTwin occurrence decreases continuously

towards the shear zone boundaries of the observed at the peripheral parts have less beenaffected by overprint at greenschist facies meta-Tauern Window. The grain size is decreasing

due to dynamic recrystallization and secondary morphic conditions, and approximately reflect theoriginal deformation conditions. Generally,grain size reduction until the formation of ultra-

mylonites. Within these shear zones grain assumptions on deformational fabrics of calciteare only significant at low grade metamorphicboundary sliding, respect to the grain size sensi-

tive flow, seems to dominate. The ultramylon- conditions.ites show stronger CPOs and well defined singlec-axis maxima near Z, which is similar to theexperimental results described by Casey et al. Acknowledgements(1998). The calcite textures are very similar: thec-axes are either aligned subparallel to the folia- The authors appreciate discussions on the topicstion normal or are rotated away from this axis presented in this study with Harry Fritz, Nikoagainst the sense of shear. The fabric asymmetry Froitzheim, Neil Mancktelow and Robert Handler.of the CPOs depends on the orientation of the They gratefully acknowledge suggestions by Jean-distinct shear zones with respect to the external Pierre Burg and Karel Schulmann on an earlierstresses. Along the E–W-striking northern and version of the manuscript, and the formal reviewssouthern margins of the Tauern Window by Martin Burkhard and Karel Schulmann. The(Salzach fault), the asymmetry ranges from 5 authors also gratefully acknowledge the softwareto 20° (Fig. 7). Along the NW–SE-striking packages provided by Eckard Wallbrecher andMoll Valley Fault, the asymmetry ranges Helmut Schaeben for the documentation of severalbetween 0 and 5° (Fig. 7), which implies a orientation data. The studies presented in thishigher pure shear component according to paper have been carried out within the projectWenk et al. (1987). These observations are P12179-GEO of the Austrian Researchconsistent with a NNE to NE orientation of Foundation.the maximum principle stress (s1) during domeformation ( Kurz and Neubauer, 1996).

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microstructural and textural development of calcite …...is related to the top-to-the-n...

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