variscan deformation, microstructural zonation and extensional exhumation of the moravian cadomian...

Geodinamica Acta (Paris) 1998, 11, 2-3, 119-137

Variscan deformation, microstructural zonation and extensional exhumation

of the Moravian Cadomian basement Michal Lobkowicz a, Karel Schulmann b* and Radek Melka b**

a Czech Geological Survey, Podbelohorska 49, Praha 5, CS- 150 00, Czech Republic b Charles University, Institute of Petrology and Structural Geology, Albertov 6, 12843 Praha 2,

Czech Republic

(Received 3 November 1995; accepted 27 February 1997)

Abstract - The Cadomian Dyje Batholith, in the foot-wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression.

Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz-diorites show a characteristic zonality marked by relatively high temperature flow in the west (550-580 “C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and sub- grain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Molda- nubian nappe.

The main ductile tectonic event D, is followed by a retro- gressive brittle-ductile and brittle deformation D,. D, results in the development of shear zones and faults superimposed on the D, mylonite fabric. D, is related to extension oblique to the D, fabric, associated with detachment and the westward movement of the Moravian nappes. 0 Elsevier, Paris

granite deformation / microstructural zonation I paleopiezometry I oblique underthrusting I extension

Resume - Deformation Varisque, zonalite microstructurale et exhumation extensionnelle du socle cadomien Mora- vien. Le batholite cadomien de Dyje, localist ?I la base des nappes varisques moraviennes, a et6 implique dans la deforma-

* Correspondence and reprints ** Deceased

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tion ductile varisque. Les roches du socle Brunovistullien on ttC sous-charrites dans les conditions du facies schiste vert suptrieur, le long d’une rampe oblique dextre active a la fin de l’episode varisque.

L’intensite de la deformation est inversement proportion- nelle a la distance par rapport au contact de front du chevauchement varisque. Les microstructures des granodiorites et des quartz diorite dtformtes montrent une zonation caracteristique marquee par une temperature de fluage relativement ClevCe a I’ouest (550-580 “C) et soulignte par la recristallisation dyna- mique des feldspaths et la recristallisation par migration des joints du quartz. La taille des grains de quartz diminue vers l’est, en relation avec la diminution de I’intensitC de la deformation. Dans la par-tie la plus orientale du massif autochtone de Dyje, la fracturation des feldspaths et la recristallisation par rotation des sous-grains de quartz predominent. Cette zonalite microstructurale est oblique au front du chevauchement majeur et elle correspond pratiquement aux isogrades metamorphiques deduits de la petrologic des metapelites. Elle traduit un stade de sous-charriage homogene du domaine Brunovistulien sous la nappe moldanubienne.

L’tvenement ductile principal D, est suivi d’une deformation fragile-ductile, puis fragile, Dz. La deformation Dz resulte du developpement de zones de cisaillement et des failles super- pastes a la fabrique mylonitique D,. Cette phase tardive est lice au bloquage des mouvements compressifs et a l’exhumation du massif autochtone par une extension oblique par rapport a la deformation D, 0 Elsevier, Paris

deformation de granite / zonalite microstructurale I paleo- piezometrie I sous-charriage / exhumation oblique

1. Introduction

The geology of the eastern margin of the Bohemian massif is marked by the collision of the pan-African Brunovistulian microplate [l] with the Modanubian terrane. Underthrusting of the Brunovistulian continental crust below the high-grade Moldanubian complex led to

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M. Lobkowicz, K Schulmann, R. Melka

the development of two Brunovistulian basement- derived thrust sheets, the upper Bites nappe and the lower Pleissing nappe [2, 31 and to strong deformation of autochthonous granitic rocks. This metamorphosed and deformed belt of the Brunovistulian continent has been referred to as the Moravian zone [4, 51. Rocks of the Moravian zone form two large scale windows, namely the Svratka Dome to the north and the Dyje dome to the south, below the Moldanubian terrane. Intense shear deformation and inversion of metamorphic isogrades is typical of the whole Moravian zone [6]. Deformed orthogneiss of Moravian nappe pile recently dated at 328.7 2 3.3 Ma and 325.5 + 0.7 Ma (40Ar/“9Ar cooling ages [7]) indicate that deformation is Visean in age.

Autochthonous basement rocks in the footwall of the Moravian-Moldanubian nappe pile were intensely involved in Variscan tectonics and acted as a lateral ramp along which the whole nappe stack was transported to the north-east [8, 91. This dominantly granitic, but compositionally variable basement, shows a polyphase and heterogeneous deformation with a complex set of structures associated with thrust-wrench ductile and brittle tectonics. The syn-metamorphic deformation decreases from the west and is overprinted by later brittle-ductile deformation. The degree and style of the deformation is controlled not only by the thermal conditions of shearing, but also by the geometry of pre-Varis- can structures and the composition of granitic rocks.

Figure 1. a. Location map in European Variscides. Figure 1. a. Carte de localisation dans la chaine varisque.

Granitoids represent the most common rock type in the middle and supracrustal levels of most of erogenic belts. Hence, the deformational behaviour and rheology of quartz-feldspar material governs the brittle and ductile rheology of mid- and supra-crustal levels in collisional erogenic belts [lo]. The thermal conditions and composition of granitic rocks critically influences the strength of the quartz-feldspar crust [ 111. Therefore, numerous studies have been devoted in the last 20 years to the understanding of the microstructural evolution of sheared granitic rocks and of their deformational behaviour at different crustal levels [l l-141. The eastern margin of the Bohemian Massif with its classical Barro- vian metamorphic zonation offers an example of deformation of quartz-feldspar crust in a collisional zone.

2) the lower Moravian allochthonous unit (Pleissing nappe), composed of highly sheared Weitersfeld orthogneiss at the base and micaschists with intercalations of marbles and calcsilicate rocks at the top; and 3) the upper Moravian allochthonous unit (BfteS nappe) consisting of a thick sheet of mylonitic BiteS orthogneiss and an upper metapelitic sequence.

This paper discusses: 1) the geometry and kinematics of syn-metamorphic wrench deformation of the granitic basement, and their relations to the Barrovian metamorphic gradient; 2) the geometry of late brittle exhuma- tional tectonics and its relationship to the early compressional structures; and 3) the microstructural and rheological behaviour of compositionally different granitoid rocks under different thermal conditions.

2. Geology of the Dyje Massif

The Dyje window exhibits typical Moravian lithotectonic zonation (figure 1) characterized by: 1) basement granitoid rocks intruding metasedimentary host rocks;

The autochthonous domain of the Dyje window consists of Cadomian granitoids [ 151 in the core and their para-autochthonous metasedimentary host rocks. The Dyje granitic massif comprises several types of granitic rocks [ 161 that can be attributed to intrusions of succes- sive magmas $gure 2). The western part of the batholith is composed of medium grained biotite granite (Hauptgranite in Austrian terminology [17]). To the north, a porphyric granite prevails, and the northernmost termination of the massif is formed by medium grained granodiorite and quartz diorite. Lense-shaped bodies of dark quartz-diorite with abundant biotite and amphibole occur in the core of the batholith (figure 2). These are considered the oldest intrusive rocks [ 181. However, evidence of mixing between basic diorites and surrounding granodiorite is present, indicating coeval emplacement of both magmas. The eastern portion of the batholith is formed by K-feldspar megacrystic granodiorite. Rare xenoliths of schists and quartzites of mantle rocks occur in the central and northern parts of the Dyje batholith [19, 201. All magmatic members of the Dyje massif are crosscut by numerous aplitic or fine grained granitic dykes. Rb-Sr whole rock isotopic analysis of the medium grained biotite granite has yielded an age of 551 + 6 Ma [20].

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50km

Figure 1. b. Schematic geological map of the Dyje dome.

Figure 1. b. SchCma gtologique du Dame de Dyje.

Metasedimentary host rocks of the autochthonous Dyje massif consist of a monotonous sequence domi- nated by metagreywackes, metahornfels, garnet micaschists and quartzites [16, 181. These rocks are locally intruded by dykes and apophyses of the Dyje granitoids, producing an early HT/LP metamorphism characterized by relics of cordierite in migmatitic gneisses [3, 18, 21- 231. During the Variscan collision, the metasediments were partly detached from the rigid granitic basement [91.

In the eastern part of the Dyje massif, a small outlier of unmetamorphosed parautochthonous sedimentary Devonian cover rests on the granodiorites (figure 2). It is represented by basal monomict quartz conglomerates, quartz and feldspathic arkoses, and kaolinitic sand- stones [24]. Its original sedimentary transgressive contact with the granodiorite was tectonically modified during the Variscan orogeny [24].

The metamorphic grade increases systematically from the garnet zone in the para-autochthonous domain,

Mokianubin zone

l/;iicjiil Varlscan granitoides

Brunovistulian basement: . ..**.. ..t.*..* i

f . l l . l p

* . . . a * .

++t+

I

-i-*+-t

I

El

Moravian nappes

Bmo granite

Brunov&tulian basement affected by Variscan orogeny

Permian graben

through the staurolite zone developed in the upper part of the para-autochthonous domain and the lower Mora- vian unit, to kyanite and sillimanite zones in the upper Moravian unit [25]. The peak temperature of metamorphism in pelites directly overlying basement granitoids was estimated by Hack et al. [23] and by Stipski and Schulmann [25] at 550-580 “C using garnet-biotite geothermometry [26]. Pressure estimates based on garnet-plagioclase-biotite-muscovite barometry [27] range between 5 and 7 kbar (figure 3).

However Hack [6] demonstrated that the NE-SW trending mineral isogrades cut across the tectonic boundaries at the southern margin of the Dyje dome. Similar geometrical relationships between metamorphic isogrades and lithotectonic boundaries have been suggested by Stipska and Schulmann [25] at the northern termination of the Dyje window (figure 3). The obliquity of metamorphic isograds with respect to tectonic boundaries has been explained recently by a model of large scale buckling of nappe sequences in association

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M. Lobkowicz, K. Schulmann, R. Melka

I A

cl B

c, ‘ak)- ZNOJMO

Figure 2. A. Simplified geological map of the Czech part of the Dyje massif. B. Idealized profile of deformation structures within the Dyje massif. (1) Devonian sediments, Cadomian basement; (2) slightly deformed granodiorites; (3) deformed granites\granodiorites; (4) strongly deformed granites\granodiorites: (a) at the contact with the Moravian units, (b) in shear zones within the massif; (5) quartz-diorite lenses: (6) tonalites containing metasedimentary relics as xenoliths.

Figure 2. A. Carte du massif de Dyje. B. Profil interpretatif des deformations dans le massif de Dyje. (1) Sediments devoniens, Socle cadomien ; (2) granodiorites peu deformdes ; (3) granodiorites dtformtes ; (4) granodiorites trts deformees : (a) au contact avec des unites moraves, (b) zones de cisaillement dans le massif m&me ; (5) lentilles quartzodioritiques ; (6) tonalites a reliques mtta- stdimentaires et xenolithes.

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01234SB

Figure 3. Mineral isograde map of the Dyje tectonic window. Modified after Hock et al. [23]. PT and compositional data from deformed granites and surrounding metapelites. A. The PT diagram with estimated P and T conditions calculated using garnet-biotite thermometry and garnet-plagioclase barometry for the westernmost part of the Dyje Massif. B. Chemical composition of recrystallized plagioclase (western mylonites). C. Composition of white micas in recrystallized domains (western mylonites).

Figure 3. Carte des isogrades dans la fenetre tectonique de Dyje. D’aprbs Hock et al. [23]. Estimations PT et donnees chimiques du granite deform6 et des ptlites (zone mylonitique, ouest). A. Diagramme PT base sur le thermometre biotite-grenat et barometre grenat-plagioclase. B. Composition chimique des plagioclases recristallises. C. Composition des micas blancs dans les domaines recristallises.

with inverted metamorphic zonation [28]. In this model the Moravian nappes are considered as a multilayer- crustal system which is passively buckled during the final stage of uplift.

NE trending mineral zones are present also in the dominantly granitic autochthonous domain. In the western part of the Dyje massif, oligoclase is stable in recrystallized metagranitoids whereas the albite/oligoclase transition line truncates the massif in its central part trending roughly parallel to NE-SW mineral isogrades in the overlying nappes [23]. This obliquity of mineral isograds cannot be related to ductile folding.

3. Deformation structures

The wide range of deformation structures developed in the Dyje massif suggests a long-lasting and polyphase tectonic history. The first Variscan deformation phase

D,, is superimposed on pre-Variscan structures and occurred under upper greenschist facies conditions. The second deformation phase D, is connected with the development of localized discontinuous shear zones and with the formation of arrays of slickenside surfaces.

3.1. Pre-Variscan structures

Pre-Variscan structures are represented by magmatic flow fabrics in igneous rocks, by metamorphic low- grade foliation in the metasedimentary host rocks or by alternation of leucosome and mesosome in relics of migmatites [ 191.

In the Dyje granitoids, pre-Variscan fabrics are marked mainly by schlieren of mafic magmas in granodiorites, by flattening and elongation of mafic inclusions, and by compositional banding formed by the mixing of basic and acid magma. Mafic rocks show well

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q B

Figure 4. Sketch maps showing the regional distribution of mylonitic foliations (A) and stretching and mineral lineations (B) in the Dyje massif: (1) Mylonite belt at the contact with the Moravian units, (2) foliations, (3) internal shear zones within the body, (4) lineations. Stereoplots show the orientations of the foliation and hneations in cross-sections perpendicular to the structural trend. Lower hemisphere projection, contoured at multiples of uniform distribution, about 50 measurements per diagram.

Figure 4. Schema structural du massif de Dyje: (I) zone mylonitique au contact avec les unites moraves, (2) foliations, (3) zones de cisaillement 51 I’interieur du massif, (4) lineations. Diagrammes montrant les orientations des foliations et des lintations. Projection sur hemisphere inferieur, contours en multiples de la distribution altatoire.

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DEFORMATION FABRICS IN THE DYJE BATHOLITH

I Ar

- t@ P

LS fabric

o 1 2 3 4 5km

Figure 5. Schematic diagram showing ductile structures typical for individual domains within the Dyje massif. A. Fabric pattern in the mylonitic belt indicating mostly L fabric. B. Low-strain domains in the transitional region from mylonites to undeformed granite. C. SC pattern in localized shear zones in the interior of the granitoid massif.

Figure 5. Bloc diagramme illustrant la repartition de differentes structures ductiles caracttristiques dans le massif du Dyje. A. Structures ductiles dans la zone mylonitique indiquant la predominance des fabriques L. B. Regions de deformation faible dans la zone de transition du granite non deform6 et de la zone mylonitique. C. Fabrique S-C dans les zones de cisaillement discontinues de la partie centrale du massif granitique.

developed preferred orientations of biotite or amphibole parallel to the elongation of enclaves. The magmatic foliation is subvertical and N-S to NNE-SSW trending. The flat geometry of aplitic dykes suggests sub-vertical extension during the late stages of magma ascent.

Metasedimentary host rocks show a well developed schistosity that dips westwards in a shallow manner. Syntectonic garnets up to 1 cm in size exhibit a complex curvature of inclusion trails indicating tectonometamor- phic pre-Variscan history [2]. Granitic and granodioritic dykes cross-cut the metamorphic foliation or define sills parallel to the schistosity.

3.2. D, ductile deformation

detached metasedimentary cover towards the core of the Dyje massif to the east. Its western margin is homogeneously deformed and granitoids are converted into foliated, equigranular or augen orthogneiss, forming an irregular, l-2 km thick mylonitic belt. The mylonitic fabric has a well developed foliation and stretching lineation (figure 4). Foliation planes defined by recrystallized quartz ribbons, preferred orientation of micas and planar alignment of feldspar phenocrysts, strike NNE- SSW and dip 50-80” to the west. At the northernmost termination of the Dyje Massif, where quartz diorites predominate, the foliation strikes almost N-S and the dip becomes progressively shallower reaching maximum values of 30”. Here the zone of high strain intensity is wider than in the south, reaching 3 km of homogeneously mylonitized rocks.

The intensity of ductile deformation of the Dyje gra- Mineral stretching lineations are horizontal or sub- nitoids decreases from the western contact with the horizontal and strike NNE-SSW forming an acute angle

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with the tectonic boundary between para-autochthonous meta-sediments and the autochthon. They are marked by elongate quartz aggregates, streaking and boudinage of fractured feldspar clasts, and the oriented growth of quartz and micas between pulled-apart fragments of feldspars. In several places, elongate mafic inclusions are oriented in the regional stretching direction. Even within the highly sheared mylonite belt, the intensity of deformation is heterogenous. The mylonites display alternations of less intense LS fabric with zones of strong L fabric on an outcrop scale. Although good strain markers are lacking, the prolate shapes of mafic inclusions and the rodding of feldspar aggregates suggest constrictive strain viguve 5).

Strain intensity decreases gradually towards the core of the Dyje massif where the penetrative foliation is less pronounced and strain is concentrated within steep shear zones that anastomose around low-strain domains (figure 5). These domains consist of isotropic granite and are elongated in a NE-SW direction, parallel to the regional stretching direction. Farther east, NNE-SSW striking subvertical shear zones or flat thrusts truncate isotropic granodiorites and quartz diorites which other- wise display little evidence of ductile deformation. In some places, the shear zones reactivate the early pre- Variscan fabric, especially in the central quartz dioritic belt. These shear zones are sub-vertical and trend N-S. Stretching lineations contained within the internal foliation of NE-SW trending shear zones are generally sub- horizontal and strike NNE-SSW, parallel to the stretching lineation elsewhere in the massif. Outside shear zones, in the central and eastern parts of the massif, the metamorphic foliation consists of discrete non-penetrative, sub-parallel planes trending NNE-SSW and dipping 50-70” to the west. These planes are assumed to be the less pronounced equivalent of the foliation in the orthogneiss belt.

Sense of shear indicators in the homogeneously foliated orthogneiss belt, such as small-scale zones of very intense ductile shearing surrounded by moderately mylonitized granite, S-C type fabrics, and asymmetrical feldspar porphyroclast-tail systems, indicate a top to the NNE sense of shear (figure 5). The NE-SW trending shear zones acted as dextral strike slip zones and show consistent kinematics with the marginal mylonite belt. The N-S trending shear zones or reactivated pre-Varis- can foliations exhibit a sinistral sense of shear.

3.3. D, deformation

The D, deformation pattern is locally overprinted by the D, deformation responsible for the formation of discontinuous shear zones. These zones, ranging from a few centimetres to some metres in width, affect granitoids with variable D, deformation. Deformation in the shear zones is marked by development of ultramylonites and phyllonites. The boundaries between D, shear zones


and the surrounding undeformed granitoids or ortho- gneisses are often very sharp

Most of the D, shear zones show a variable dip towards the NW and the poles to foliations within the shear zones form a broad girdle, reflecting a fan-like disposition around a SW-NE axis (figure 6). Mineral and stretching lineations defined by recrystallized quartz aggregates and the preferred orientation of chlorite and muscovite, strike SSW-NNE, generally parallel to the strike of the foliation planes f$gure 6). A dextral sense of movement dominates in moderately and steeply west dipping shear zones, whereas flat-lying shear zones have acted as reverse faults.

Figure 6. Orientation diagrams showing the orientation of fabric elements of D2 shear zones. (L) Stretching lineation, (N) poles to shear zone planes.

Figure 6. Diagrammes dCcrivant la disposition des zones de cisaillement D2. (L) LinCation d’ktirement, (N) pbles des plans de cisaillement.

Slickenside surfaces cutting across undeformed rocks occur in the vicinity of D2 shear zones figure 7). The sense of movement along these brittle faults is indicated by the local offset of aplite dykes or by the oriented growth of minerals on slickenside surfaces. Most faults are typified by a mean strike of N 65” E, approximately vertical dip, and a dextral sense of shear. A second set of steeply dipping faults is oriented NlS-25”E, with NW plunging slickensides showing a sinistral or normal sense of movement. A third set of fractures, striking approximately NW-SE and dipping 45-60” SW or NE, show normal displacements.

Quantitative fault analysis shows heterogeneous and probably also polyphase development of the stress field. Paleostress estimates were carried out using a modi- fication of the Carey and Brunier [29] technique (figure 7). The mean stress tensor was calculated first using measured orientations of fractures and striations. The orientation of measured striations was then com- pared with the orientation of calculated ones in order to select the best fit planes. The selected planes and striations were then used to calculate the best fit orientation of stress axes. The calculated q-axis of the stress ellipsoid either shallowly plunges 10-20” toward the SSW or at a generally higher angle 15-60” toward the SW. The

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56 128

Figure 7. Schematic diagram showing brittle structures in the Dyje massif (central part of the Massif near Znojmo). First column shows sets of microfaults from three different localities, second column sets of microfaults chosen using the Carey and Brunier technique 1291. Third column shows idealized block-diagram of main slickenside-surfaces active in the Dyje massif and a stereoplot of the maximum stress axes from several sites: o,-maximum principal stress, a,--intermediate axis, cr-maximum principal stress.

Figure 7. Diagrammes dtcrivant la disposition des structures fragiles dans le Massif de Dyje (partie centrale du Massif de Dyje autour de Znojmo). Premiere colonne - ensemble de microfractures mesurees dans 3 localitts differentes; deuxieme colonne - ensemble des microfractures choisies a I’aide de la mtthode de Carey et Brunier [29]. Troisitme colonne - bloc-diagramme des systtmes de failles principales activees dans le Massif du Dyje et sttreogramme des axes de la contrainte maximum calcules dans plusieurs affleurements; 0, - contrainte principale maximum, 0 2 - axe intermediaire, (5 a - contrainte principale minimum.

mean q-axis of the stress ellipsoid plunges mostly to the E or SE, at angles up to 20”.

Fault slip analysis provides information about late increments of brittle deformation in the Dyje Massif. Analysis from the whole massif shows that large volumes of rocks have been extended in a WSW-ENE direction. The cluster of calculated O, directions may be interpreted, therefore, as the n-axis of the latest extensional deformation [30]. The cluster of or directions is oriented in a SW direction and indicates a late compres- sive axis of deformation. The average direction of CF, is similar to that of maximum of poles of the D, shear zones tjigures 6 and 7). The geometrical consistency between kinematics deduced from slickenside surfaces and from ultramylonitic shear zones suggest their con-

temporaneous origin. The analysis of slickenside surfaces has demonstrated that the D, extensional x-axis is shifted about 30-40“ westward from the NNE plunging x-direction of the D, deformation, represented by regional stretching lineations.

4. Microstructural evolution

4.1. D, microstructures in granodiorites

in the western mylonitic zone

Weakly deformed granite and granodiorite show dynamic recrystallization of plagioclase and brittle- ductile deformation of K-feldspar. Fractured and partly

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M. Lobkowicz. K Schulmann, R. Melka

recrystallized K-feldspars and extensive development of myrmekites (figure 8), occurring at the sides of the feldspar clasts parallel to the foliation, characterize the less recrystallized samples. The extensive growth of myrmekite suggests that diffusion of alkalies has been associated with deformation [31]. K-feldspars show dynamic recrystallization along exposed sides and along small micro-shear zones. Plagioclase exhibits a higher degree of dynamic recrystallization and is partly converted into a fine-grained matrix of albite, oligoclase, quartz, and white mica (figure 3h). Deformation of large magmatic quartz grains initially leads to the development of typical mantle structures (figure 9) marked by large subgrains defined by prismatic boundaries in the grain core and small rounded subgrains and new grains at its margin [32]. Recrystallization of biotite occurred along crystal margins and kinked boundaries without significant chemical changes. The composition of recrystallized white mica shows enrichment of the phen- gitic component up to Si 3.25 (figure 3c), which indicates pressures of about 5-6 kbar for temperatures of around 500 “C [33]. The mineral assemblages and com- positions of sheared granites are consistent with upper greenschist-lower amphibolite facies conditions of deformation [34, 351.

Intense deformation of granites and granodiorites is marked by the dynamic recrystallization of plagioclase and K-feldspar with significant grain-size reduction (figure IO). The matrix consists essentially of small, slightly elongate oligoclase and albite grains, 50 mm in size, quartz and white mica. In strongly deformed granodiorite, quartz occurs as elongate aggregates up to

Figure 8. Micrograph of progressive replacement of K-feldspar by plagioclase and quartz myrmekitic integrowth in X2 sections. Granodiorite.

Figure 8. Plagioclase et quartz rempla$ant progressivement les feldspath potassiques donnant naissance B des myrmekites et aux facits soumis a une contrainte maximum qui se propa- gent vers I’interieur des porphyroclastes. Granodiorite.

Figure 9. Micrograph of core and mantle effect developed in large quartz grain. Granodiorite.

Figure 9. Effet << mantle and core >> dtveloppe dans les grains de grande taille du quartz. Granodiorite.

1.5 mm thick and several mm in length, made up of new subsequant or elongate grains 0.2-0.25 mm in size with serrated boundaries (figure II) indicating grain boundary migration [36]. The size of recrystallized quartz grains does not vary with respect to ribbon boundaries. The assymmetric shape of relict K-feldspar grains [37] generally indicates a top-to-NE sense of shear (figures 12a and j). Biotite porphyroclasts often show mica-fish geometries [38] with a sense of shear consistent with other kinematic indicators (figure 124.

Farther east the degree of plagioclase and biotite destabilization is more important. A typical feature is the growth of epidote inside weakly deformed plagioclase clasts. In more deformed samples, epidote and

Figure 10. Micrograph of groundmass formed by small recrystallized and elongated albite, subordinate K-feldspar grains and small muscovite in granodiorite.

Figure 10. Matrice form&e de petits grains de feldspath recris- tallis& (plagioclase pour la majoritt, K-feldspath subor- don&). Granodiorite.

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Figure 11. Micrograph of polycrystalline quartz ribbon em- bedded in the completely recrystallized feldspar matrix. In- dividual quartz grains show signs of internal deformation and serrated grain boundaries. Notice the difference between grain-size of feldspar and quartz.

Figure 11. Ruban de quartz polycristallin dans une matrice de feldspath recristallists. Les grains de quartz montrent des signes dune deformation inteme et des joints lobes. Noter la difference de taille entre les grains de feldspath et ceux de quartz. Toutes les photos ont CtC prises dans la section XZ.

new garnets are aligned along trails parallel to the foliation. New recrystallized plagioclase is represented only by albite. Biotite is smeared out and often transformed to chlorite. A large amount of fine grained white mica replaces partly fractured K-feldspar. These mineral transformations are consistent with greenschist facies conditions of deformation [34, 3.51. Quartz forms elongated aggregates composed of irregular grains with migrated boundaries often forming an internal oblique grain shape foliation.

4.2. D, microstructures in quartz-diorites

Deformation microstructures developed in quartz diorites in the northern part of the massif are quite different from those seen in granodiorites. Recrystalliza- tion of feldspar is lacking or minor and the deformation is concentrated into narrow zones filled by new white mica, clinozoisite and quartz (figure 13). These micro- shear zones cut across plagioclase porphyroclasts irre- spective of the lattice orientation. The top-to-the-NE sense of shear suggested by these shear bands is consistent with that indicated by other sense-of-shear indicators. Individual shear bands are several microns thick and displacement along them attains several milli- metres. K-feldpars are generally fractured and rotated, their longest planes facing parallel to the XY plane of finite strain. Neck zones between segmented crystals are filled with quartz and phyllosilicates.The geometry of


these fractured grains indicates the sense of shear in

tonalitic rocks cfigures 12b, c and e). Quartz is often present in the form of strongly elon-

gated large grains, 1.5 mm thick and up to 10 mm long, parallel or subparallel to the mesoscopic foliation. These large grains are often surrounded by small elongate recrystallized grains of similar orientation aligned oblique to the mesoscopic foliation figure 14). New grains also develop from small sub-grains along kinked boudaries indicating a sub-grain rotation recrystallization mechanism. The recrystallized grain size is strongly variable ranging from 35 to 90 urn, according to ribbon position with respect to rigid K-feldspars.

The microstructural zones described above, generally showing a N-S trend are schematically represented in figure 17.

4.3. Microstructures within D, shear zones

Deformation within these very high strain zones is marked by intense grain-size reduction of all minerals (figure 15). The rock is almost fully converted into an ultrafine-grained groundmass, with grain size reaching 5-10 urn, made of albite, quartz, sericite and other break-down products of feldspar and mica. Quartz forms relict clasts consisting of large, globular grains with prismatic subgrain boundaries perpendicular to the foliation or of single-crystal ribbons elongate parallel to the foliation (figure 15). The groundmass is affected by shear bands indicating top-to-the-NE displacement.

5. Quartz c-axis fabrics

5.1. Granodiorites

In granites and granodiorites of the western mylonite belt, the quartz c-axis fabrics comprise a number of point maxima forming a rather scattered pattern (figure 16). In some cases, the c-axis fabric tends towards a Lister type II crossed girdle distribution [39] with c-axis maxima situated close to the periphery of the diagram near the XZ plane of the finite strain ellipsoid, or near the Y-axis of finite strain. The synoptic plot for granodiorites yields an assymmetric pattern, consistent with top-to-NNE shearing. The opening angle 35% 40’ of a small circle centred around the Z-axis of the c- axis pattern is commonly interpreted as the result of the combined activity of basal ‘a’ slip and prism ‘c’ systems [40]. This type of fabric indicates higher temperatures of quartz deformation consistent with the observed microstructures [41]. These quartz c-axis patterns are associated with deformation microstructures typical of the highest temperatures in the area studied (figure 16).

The quartz c-axis pattern in the eastern part of the weakly deformed granodiorites is marked by the development of a typical type I cross girdle pattern

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Yf Btt,Chl, Msk

(a)

?tt,Chl, Msk

NNE

imm

NNE

NNE Kf

a Kf imm

Figure 12. Kinematic indicators represented by sigma-type porphyroclasts (a, f), pull-apart feldspars and plagioclase book-shelves (b, c, e) and biotite mica-fish (d). All of sense of shear indicators are consistent with the northeastward sense of shear.

Figure 12. Criteres cintmatiques representes par les porphyroclastes de feldspath (a, f), structures de type book she[fdans des feldspaths (b, c, e) et dam des biotites (d), tous coherents avec un cisaillement vers le nord-est.

(figure 1.5). Stronger maxima are developed close to the pole of the foliation plane and more often near the Y-axis of finite strain, indicating a combination of prism ‘a’ with subordinate basal ‘a’ slip activity. This type of quartz c-axis fabric is consistent with greenschist facies conditions of deformation.The sense of the c-axis fabric asymmetry with respect to the macroscopic foliation is in accord with the northward sense of shear deduced from other kinematic indicators.

5.2. Quartz diorites

In the quartz diorite, the c-axis orientations form single girdles rotated with respect to the normal to the foliation plane in the same sense as the imposed shear (figure 15). Remnants of a conjugate crossed-girdle developed in stress protected regions marked by large grain size, reaching 90 urn. This type of microfabric is

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Figure 13. Micrograph of mica rich micro-shear zones truncat- ing the plagioclase clasts and extensional gashes filled by quartz and sericite. Quartz-diorite.

Figure 13. Micro-zones de cisaillement fragmentant les porphyroclastes de plagioclase. Elles sont remplies de mica blanc et de sericite. Quartz-diorite.


6. Grain size analysis and paleopiezometry

Laboratory experiments have demonstrated that the sub-grain and grain size generated during steady state deformation depend primarily on the magnitude of the differential stresses [43, 441. According to experimental studies temperature only has a minor influence on these three parameters. Numerous paleopiezomeric studies exist showing grain size evolution and flow stress variations in a single shear zone [45]. However, regional studies showing the variation of flow stress in crustal scale shear zones in quarz bearing tectonites are lacking. In this work we present a grain size paleopiezometric study from a mylonitic domain with a natural strain and temperature gradient.

Detailed grain size analysis of recrystallized quartz grains was performed in deformed granitic rocks along three profiles perpendicular to the regional foliation trend. Quartz grains were measured in ribbons exhibit- ing steady state recrystallized grain size [46] i.e. with no relics of original undeformed quartz grains. About 150-200 grains were measured per thin section using the line transect method. Grain sizes were plotted as histograms (figure 17) and were used for paleopiezometric calculations.

Figure 14. Monocrystalline quartz ribbons recrystallized margins. Note crystallization of small grains at the low- angle boundaries and grain size variations in the vicinity of feldspar clasts.

In the western part of the massif, recrystallized grains originated by migration recrystallization mechanisms and show the largest average grain size, reaching 280 pm. The maximum grain size varies from 480 to 560 pm. Moving to the east, the grain size decreases reaching an average value of 170 pm and a maximum value of 280 urn. In the northern and eastern parts of the Dyje massif, a rotation recrystallization mechanism pre- dominates within quartz and consequently the steady state recrystallized grain size is significantly smaller, varying between 35 urn and 100 urn (figure I#). These variations in grain size can be observed on a thin section scale and are dependent on the amount of rigid feldspar clasts in the sample [13].

Figure 14. Rubans monocristallins de quartz recristallists. Noter la cristallisation de petits grains aux joints a faible angle entre les rubans.

the most typically developed and could be interpreted as the result of dominant activity of basal ‘a’ and subordinate activity of prism ‘a’ slip systems. The maximum concentration of c-axes is located close to the y-axis in highly sheared aggregates close to rigid feldspar clasts in which rotation recrystallization leads to the development of exceptionally small grains up to 35 urn in size (figure 14). A unique maximum close to the y-axis is commonly interpreted as result of dominantly prism ‘a’ slip in non-coaxial flow [42] and is probably related to increasing shear stress [ 131.

The size of grains that have recrystallized by grain boundary migration shows strongly assymmetrical distribution with dominance of large grains. On the con- trary, the size of new grains originated by sub-grain rotation recrystallization show a very homogeneous grain size distribution within the polycrystalline aggregates. Therefore, the median value of grain size for migration crystallization mechanisms and the average value for rotation recrystallization mechanisms was used to estimate the flow stress in deformed quartz aggregates. For sub-grain dynamic rotation recrystallization the experimental calibration of Christie and Ord [47] has been used with constants A = 4090, n = 1.11. For grain boundary migration recrystallization the empi- rical paleopiezometer of Werling [48] was applied with the constants A = 2302, n = 0.59. The results of paleopiezometry calculation for given stress exponents and material constants are shown in figure 17 (left upper

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sw L NE

HumQw Of nndom disWiMlon

Figure 15. Quartz c-axis fabrics in the graniteslgranodiorites and the quartz diorites. Fs = quartz foliation. Lower hemisphere equal area projection, contours at multiples of uniform distribution. Lower diagrams are the synoptic plot of maxima with density below the stereoplots. Shaded regions indicate microstructural zones. First column: quartz microfabrics related to amphibolite facies recrystallization; second column: quartz microfabrics related to greenschist facies recrystallization; third column: quartz microfabrics related to greenschist facies recrystallization in quartz diorites. 1: mylonitic belt; 2: weakly deformed granodiorite; 3: D3 shear zones.

Figure 15. Fabrique des axes c du quartz dans les granites/granodiorites dune part et dans les quartz diorites d’autre part. S = plans S, F, = foliation inteme du quartz. Projection sur l’hemisphere inferieur, contours en multiples de la distribution aleatoire. Les zones ombrees indiquent la zonation des microstructures. Premiere colonne : recristallisation en facies amphibolite ; deuxieme colonne : recristallisation du facits schistes verts ; troisitme coionne : recristallisation du facits schistes verts dans les granodiorites. 1 : ceinture mylonitique ; 2 : granodiorite peu deformee ; 3 : zone de cisaillement D2.

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Figure 16. Globular quartz clasts and shear bands in the fine- grained ultramylonite. XZ section of finite strain.

Figure 16. Clastes de quartz globulaires dans une matrice ultra-mylonitique. Photo prise dans la section XZ.

inset). Stress estimates for grain boundary migration mechanisms vary between 100 and 60 MPa (figure 17) with lower flow stress values concentrated at the westernmost highly deformed mylonitic part of the Dyje massif. Stress values increase towards the east and the north reaching values of up to 100 MPa. In the northernmost and eastern regions, where rotation recrystallization of quartz dominates. Stress values range between 35 and 95 MPa.

These stress intensities are characteristic of most crustal shear zones [43, 441. The lowest flow stress values calculated using grain boundary migration paleopiezometry were obtained in the western mylonitic belt which exhibits the highest strain intensities and amphibolite facies conditions. Stress values increase steadily to the east towards the regions of weaker greenschist facies deformation and drop again in the easternmost and northern quartz-diorites where rotation recrystallization is dominant.

7. Discussion

7.1. Microstructural zonation-result of intracontinental underthrusting

Microstructural zonation described above can be interpreted in terms of eastward decrease of temperature. In the west dynamic recrystallization of feldspars and grain boundary migration recrystallization mechanisms of quartz dominate. Farther to the east, the deformation of K-feldspar is more brittle, plagioclase is


unstable and quartz exhibits signs of grain boundary migration recrystallization. In the easterly located quartz diorites, plagioclase is converted totally to a mica rich matrix, K-feldspar is fractured whereas quartz is deformed by dislocation glide accompanied by subgrain rotation recrystallization.

The microstructural boundaries represent metamorphic isogrades and are sub-parallel to metamorphic zones defined by Hock et al. [23]. This is in agreement with data of Gapais [ 1 I] who proposed that the switch between grain boundary migration and a sub-grain rotation recrystallization mechanisms in quartz occurs at 500 “C. We suggest that the microstructural zonation found in the Dyje Massif represents a paleothermal gradient developed during underthrusting of the Brunovis- tulian basement below the Moldanubian domain.

The consistent grain size in individual quartz ribbons of deformed granodiorite can be explained by a persis- tence of a load-bearing framework structure [49] marked by a low viscosity contrast between matrix and quartz ribbons and uniform stress distribution [ 131. The boundary between low- and high-flow stress domains trends north-south parallel to microstructural zones (figure 17). The decrease of quartz grain size to the east, associated with flow stress increase, can be interpreted in terms of temperature increase towards the west. Increasing strain intensity in conjunction with an increase in steady state recrystallized grain size along the western HT margin resulted from the decrease in yield strength of quartz-feldspar tectonites with increasing temperature. This assumption is valid only for a constant strain rate [48] of Variscan deformation along the whole E-W section, indicating that the flow stress boundaries reflect metamorphic isograds.

In the quartz diorite, deformation is concentrated in the weakest fraction, represented by quartz, and exhibits an interconnected weak layer (IWL) structure after very low strains [ 131. Grain size variations in the tonalitic mylonite are due to large viscosity contrasts between the rigid feldspar and quartz and to varying spacing factors between stronger feldspar clasts. In regions with low clast spacing, the recrystallized grain size is about 70 pm, corresponding to approximately 55 MPa, and probably reflecting more realistic real flow stress. The quartz diorite tectonite was more homogeneously deformed at lower temperatures than the westerly located granodiorite, which exhibits more localized deformation in narrow shear zones but microstructures typical for higher temperatures. Due to the presence of an interconnected quartz layer structure in quartz-diorite this rock is weaker and shows higher strain intensity, lower flow stress and lower yield strength than the westerly granodiorites. The difference in yield strengths between granodiorite and quartz diorite at similar temperatures is responsible for more homogeneous deformation of the weaker rocks.

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GBM

•1 $1 - 70’MPa

B 71 -80MPa

@j 81-90MPa

91 -102MPa

Figure 17. Histograms showing grains size distribution in steady state recrystallized quartz grains in tonalite and granodiorite. Note the decrease of grain size in grain boundary migration domain from the west to the east and the eastward transition to sub-grain recrystallization domain. Left-upper inset: Diagram showing calculations of flow stress using steady state recrystallized grain size of quartz in deformed granite. Shading indicates microstructural zones: west-amphibolite facies recrystallization, centre-greenschist facies recrystallization, east-greenschist facies microstructures in quartz diorites.

Figure 17. Histogrammes decrivant la repartition de la taille des grains de quartz recristallists dans la quartz-diorite et la granodiorite. Noter une decroissance de la taille des grains dans le domaine de migration des joints d’ouest en est et la transition vers le mtka- nisme de rotation, a I’est. Le diagramme en haut, a gauche sur la figure, montre le calcul des contraintes differentielles base sur la taille des grains recristallises. Les zones ombrtes indiquent la zonation des microstructures : ouest - recristallisation en facies amphibolitique, centre - recristallisation de basse temperature et a I’est recristallisation de basse temperature dans les quartz diorites.

7.2. Obliquity of microstructural isograds with respect to thrust front-result of oblique exhumation

Microstructural zonation reflects a stage of homogeneous underthrusting of the Brunovistulian domain under the Moldanubian terrane (figure 18a). More deeply subducted parts of the Brunovistulian zone were later detached and transported upwards as Moravian nappes and the inverted metamorphic zonation within the Moravian nappe pile originated [25]. However, thrusting of nappes alone cannot be responsible for the

obliquity of isogrades with respect to thrust boundaries and a model involving folding of nappes and metamorphic isogrades has been suggested by Stipska et al. [28].

Isogrades in the autochthonous domain are neither inverted nor refolded and the metamorphic-microstructural and stress zonations are only exhumed. We suggest that D2 extensional faulting is responsible for exhumation of the early paleothermal gradient in a WSW direction as indicated by quantitative fault analysis (figure 7) so that the Moravian nappes have slipped down during late WSW directed extensional movements (figure 18b) as indicated by the maximum a, directions. The direction of the x-axis of this late extension is inclined at

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Variscan deformation, microstructural zonation and extensional exhumati‘on

Figure 18. Schematic block diagrams showing the tectonic evolution of the Dyje massif. A. Underthrusting of Brunovistulian domain below Moravian nappe pile. Note that microstructural/metamorphic isogrades are subparallel with thrust-wrench boundary of Moravian nappes. B. Westward sliding of buckled Moravian nappe pile in the direction of brittle-ductile D, extension. Isostatic rebound resulted in uplift of underthrusted granitoids of Dyje Massif.

Figure 18. Bloc diagrammes schtmatisant l’tvolution tectonique du Massif de Dyje. A. Sous-charriage du domaine Brunovistullien sous la pile de nappe moravienne. Noter que les isogrades microstructurelmetamorphisme sont sub-paralleles aux dtcrochevau- chements des nappes moraviennes. B. Glissement vers l’ouest des nappes moraviennes avec evolution de la deformation depuis le ductile vers Ie fragile dans le niveau superficiel. La remontee isostatique r&rite de I’uplift des granitdides sous-charrits du Massif de Dyje.

about 30-50’ with respect to the regional L, lineation, indicating an oblique slip of the Moravian nappe pile with respect to almost N-S metamorphic zonation. The reason for exhumation of the central part of the autoch- tonous domain may be viewed as the differential uplift of the hinge zone of a crustal scale fold as suggested by StipskB et al. [28], accomodated by large scale normal limb faults. This normal faulting has been docummented by Schulmann et al. [9] in the adjacent Moldanubian region where the boundary between the Moldanubian region and the Moravian zone is traced by a large scale fault showing a combination of normal and dextral strike slip movements.

Exhumation of metamorphic isogrades in the autochthonous Dyje massif is interpreted as a result of isostatic rebound by extensional stripping over a very short

period, as indicated by Ar/Ar geochronology ranging between 328 and 325 Ma for the Moravian zone [7]. This collapse was initiated by large scale buckling of the Moravian nappes along steeply inclined surfaces and by telescopic sliding together with the Moldanubian terrane along megafold limbs towards the west (figure 18b). Almost radial extension and spreading of the Moravian zone is well documented by gravitational folds deform- ing the Moravian nappes and by numerous greenschist facies shear bands and normal shear zones affecting the whole nappe pile [9]. During isostatic rebound, almost 18 km of crust was exhumed during a period not exceed- ing 15-20 Ma, which indicates an erosion rate close to 1-2 mm yr-‘. Such a high exhumation rate is probably responsible for preservation of microstructures from later thermal annealing during uplift.

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Acknowledgements This work benefited from financial support from the

Czech National Foundation Project No. 37709. We are grateful to Mrs Andrea KolarikovB, and Mr Stano Ulrich for help with grain size measurements. We also thank Mr Vladimir Tolar and Mr Ondrej Lexa for many draw- ings. Mrs Pavla Stipska performed the PT calculations. We are also grateful1 to Mr Patrick Ledru and an anon- ymous reviewer for substantial improvements to the manuscript.

References

[ 1) Dudek A., The crystalline basement block of the Outer Car- pathians in Moravia, Bruno, Vistulikum, Rozpr., ESAV 90 (8) (1980) l-85. [2] Preclik K., Zur Analyse des Moravischen Faltenwurfes im Thayatale, Verh. Geol. B.-A. 24 (1925) 180-192. [3] Waldmann L., Zum geologischen Bau der Thayakuppel und ihre Metamorphose, Mitt. geol. Ges. Wien 21 (1930) 133- 152. [4] Suess F.E., Die moravischen Fenster und ihre Beziehung zum Grundgebirge des Hohen Gesenkes. Denkschr. Osterr. Akad. Wiss., math.-naturwiss., 88 (1912) 541-631. [!I] Suess F.E., Intrusionstektonik und Wandertektonik im variszischen Grundgebirge, Borntrager, Berlin, 1926. [6] Hock V., Mineralzonen in Metapeliten und Metapsammiten der Moravischen Zone in Niederosterreich. Mitt. Geol. Gesell. 66-67(1975)49-60. [7] Dallmeyer D., Neubauer E, Hock F., 40Ar/39Ar mineral age controls on the chronology of late-Paleozoic tectonother- ma1 activity in the southeastern Bohemian massif, Austria (Moldanubian and Moravosilesian zone), Tectono-physics 210 (1992) 135-153. (81 Fritz H., Neubauer E, Kinematics of crustal stacking and disoersion in the south-eastern Bohemian Massif, Geol. Rdsch. 82’(1993) 556-565. [9] Schulmann K., Melka R., Lobkowicz, M., Ledru P., Lardeaux J.-M., Autran A., Contrasting styles of deformation during progressive nappe stacking at the southeastern margin of the Bohemian Massif, J. Struct. Geol. 16 (1994) 355-370. [IO] Carter N.L., Tsenn M.C., Flow properties of continental lithosphere, Tectonophysics 78 (1978) 27-36. [ 111 Gapais D., Les orthogneiss: Structures, mecanismes de deformation et analyse cinematique. (Exemple du granite de Flamanville), MCmoires et documents du Centre Armoricain d’Etude Structurale des Socles, 1989. [12] Burg J.P., Laurent P., Strain analysis of a shear zone in granodiorite, Tectonophysics 47 (1978) 15-42. [13] Handy M.R., The solid-state flow of polymineralic rocks, J. Geophys. Res. 95 (1990) 8647-8661. [14] Fitzgerald J.D., Stunitz H., Deformation of granitoids at low metamorphic grade. II. Granular flow in albite rich mylonites, Tectonophysics 221 (1993) 299-324. [15] Finger F., Frasl G., Hock V., Steyrer HP, The Granitoids of the Moravian Zone of Northeast Austria: products of a Cadomian Active Continental Margin? Precamb. Res. 45 (1989) 235-245. [ 161 Preclik K., Das Nordende der Thayakuppel, Svor. SGD 6 (1926) 373-398. [17] Frasl G., Zur Geologie des Kristallins und Tertittrifs der weiteren Umgebung von Eggenburg, Excursion guide, Oster- reichische Geol. B.-A., 1983.


[IS] Preclik K., Das Nordende des Thayabatholithen, Vlst. SGir 13 (1937) 34-61. [19] Hajek T., The mantle rocks of the Dyje massif and their relation to the Moravicum and Brunovistulicum, Eas. Min. Geol. 35 (1990) 251-259. [20] Scharbert S., Batik P., The Age of the Thaya (Dyje) Pluton, Verh. Geol. Bundesanst. 3 (1980) 325-331. [21] Frasl G., Fuchs G., Matura A., Thiele 0.. Einftlhrung in die Geologie des Waldviertel Grundgebirges, in: Arbeitstagung der Geologischen Bundesanstalt (edited by Geol. Bundesan- stalt), Spec. pubis. of Austrian geological survey, Wien, 1977. [22] Frasl, G, Hock, V., Finger, F., The Moravian zone in Aus- tria-field-guide-Bohemian Massif, International Conference on Paleozoic Orogens in Central Europe, IGCP project 233, Gottingen, 1990. [23] Hock V., Marschallinger R., TOPA D., P-T evolution in metapelites of the Moravian zone of Austria, in: Conference Abstracts. International Conference on Paleozoic Orogens in Central Europe-IGCP project 233, Gottingen, 1990. [24] Dudek A., Kristallinische Schiefer und Devon ostlich von Znojmo (Znaim), Sbor.oUG 26 (1960) 101-141. [25] Stipska P., Schulmann K., Inverted metamorphic zonation in a basement-derived nappe sequence, eastern margin of the Bohemian Massif, Geological Journal 30 (1995) 385-413. [26] Hodges K.V., Spear F.S., Geothermometry, geobarometry and the Al $10, triple point at Mt. Moosilaukue, New Hamp- shire, Am. Mineral. 67 (1982) 1118-l 134. [27] Hodges K.V., Crowley P.D., Error estimation and empiri- cal geothermobarometry for pelitic systems, Am. Mineral. 70 (1985) 702-709. [28] Stipska P., Schulmann K., Hoeck V., Complex metamorphic zonation of the Thaya Dome: a result of buckling and gravitational collapse of imbricated nappe sequence, in: Cosgorove J. (Ed), Journal of Geol. Sot. of London. Special Volume, Forced Folding, (in press). [29] Carey E., Brunier B., Analyse theorique et numerique d’un modtle mecanique Clementaire applique B l’etude d’une population de failles, C. R. Acad. Sci. Paris 269 (1974) 891- 894. [30] BlCs J.L., Feuga B., La fracturation des roches, Manuels et Mtthodes, Bureau de Recherches Geologiques et Minitres, 1981. [31] Simpson C., Wintsch R.P., Evidence for deformation induced K-feldspar replacement by myrmekite, J. Metamor- phic. Geol. 7 (1989) 261-275. [32] White S., The effects of strain on the microstructures, fabrics, and deformation mechanisms in quartzites, Phil. Trans. R. Sot. Lond. 283 (1976) 69-86. [33] Massone H., Schreyer W., Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and auartz. Contrib. Mineral. Petrol. 96 (1987) 212-224, (341 le Goff E., Ballevre M., Mtthodes d’cstimation des conditions pression-temperature dans les orthogneiss: analyse des relations de phases, CR. Acad. Sci. Paris 3 11 (1990) 119-125. [35] le Goff E., Ballevre M., Geothermobarometry in albite- garnet orthogneiss: a case study from Gran Paradiso nappe (Western Alps), Lithos 25 (1990). [36] Poirier J.P., Guillope M., Deformation induced recrystallization of minerals, Bull. Sot. fr. Miner. Cristallogr. 102 (1978) 67-74. [37] Passchier C.W., Simpson C., Porphyroclast systems as kinematic indicators, J. Struct. Geol. 8 (1986) 831-843. [38] Lister G.S., Snoke A.W., S-C mylonites, J. Struct. Geol. 6 (1984) 617-639.

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[39] Lister G.S., Crossed-girdle c-axis fabrics in quartzites plastically deformed by plane strain and progressive simple shear, Tectonophysics 39 (1977) 51-54. [40] Lister G.S., Hobbs B.E., The simulation of fabric development during plastic deformation and its application to quartz- ite: the influence of deformation history, J. Struct. Geol. 2 (1980) 355-370. [41] Nicolas A., Poirier J.P., Crystalline Plasticity and Solid State Flow in Metamorphic Rocks, John Wiley and Sons, London, 1976. [42] Bouchez J.L., Plastic deformation of quartzites at low temperatures in an area of natural strain gradient, Tectono- physics 39 (1977) 25-50. [43] Kohlstedt D.L., Weathers MS., Deformation-induced microstructures, Paleopiezometers and differential stresses in deeply eroded fault zones, Jour. Geoph. Res. 85 (1980) 6269- 6285.

[44] Twiss R.J., Theory and applicability of recrystallized grain size paleopiezometer, Pure Appl. Geophys. 11 (1977) 227-244. [45] White S., Grain and sub-grain size variations accross a mylonite zone, Contrib. Mineral. Petrol. 70 (1979) 193-202. [46] Etheridge M.A., Wilkie J.C., An assesement of dynami- cally recrystallized grain size as a paleopiezometer in quartz- bearing mylonite zones, Tectonophysics 78 (1981) 475-508. [47] Christie J., Ord A, Flow stress microstructures of mylonites: example and current assesment, J. Geophys. Res. 85 (1980) 6253-6262. [48] Werling E., Tonale-, Pejo und Judiacien-Linie: Kinematik, Mikrostrukturen tmd Metamorphose von Tektoniten aus raum- lich interferienden aber verschiedenaltrigen Verwerfungszo- nen, Dissertation ETH.Nr. 9923, 1992. [49] Tharp T.M., Analogies between the high-temperature deformation of polyphase rocks and the mechanical bevior of porous powder metal, Tectonophysics 96 (1983) Tl-Tl 1.

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