Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the late Early Cretaceous nappe-stacking

. Fault-related folds and hanging-wall structures reflect the geometry of the main thrusts in fold-thrust belts. The results of the structural analysis of the Supragetic Unit metamorphic basement in eastern Serbia at map-, outcrop- and thin-section scale, and its importance for the late Early Cretaceous nappe-stack-ing are presented in this paper. The Supragetic Unit metamorphic basement includes various volcano-sedimentary rocks of Ordovician-Si-lurian protolith age. They were metamorphosed to the low greenschist facies with temperatures reaching 300–350°C and pressure reaching 0.3–0.5 GPa. The microscale studies show that quartz and albite demonstrate dominantly bulging and locally subgrain rotation recrystallisation, while chlorite, sericite and muscovite define spaced to continuous foliation recognised both at the outcrop- and the thin-section-scale. The statistical analysis based on the available map data shows low- to high-angle west-dipping foliation which is interpreted as an indicator of flat-ramp geometry of the Supragetic thrust, rather than east-vergent tight to isoclinal folding. At the thin-section scale ductile to semi-ductile C’-S structures indicate top to ESE thrusting. Subsequent kinking, recognised both at the outcrop- and the thin-section-scale, deform the older foliation. Those kink bands are the result of WNW–ESE to NW–SE compression and could represent the later stage of a continuous deformation event during which C’-S structures were formed. The youngest, brittle deformation is represented by subvertical joints with no offset recognised in thin-sections. The structural the Unit low-grade metamorphic basement studied areas, events could imply a possible initi-ation of the late Early Cretaceous nappe-stacking in the ductile to semi-ductile/semi-brittle domain.

The late Early Cretaceous nappe-stacking resulting in ductile deformations was researched in Western Bulgaria (KOUNOV et al., 2010). The same tectonic event was also recognised in tectonothermal studies conducted in other parts of the orogen (BOJAR et al., 1998;GRÖGER et al., 2013;ANTIĆ et al., 2016).
The aim of this study is to constrain the internal structure of the Supragetic Unit basement of Serbian Carpathians (Caras group of IANCU et al., 2005b;Caras terrane of BALINTONI et al., 2009 andBALINTONI et al., 2014) and to estimate its significance for the late Early Cretaceous nappe-stacking in terms of the geometry of the regional thrusts and crustal levels in which the thrusting occurred.

Geological settings
The most prominent tectonic unit of the Eastern and Southern Carpathians, the Dacia Mega-Unit (sensu SCHMID et al., 2008) was built of various nappes, of which the Supragetic-Getic system (KRÄUTNER & KR-STIĆ, 2002;SCHMID et al., 2008) was formed during the late Early Cretaceous nappe-stacking. Those two nappe-sequences are in contact along the complex, high-offset thrust, along which metamorphic basement of the Supragetic Unit is thrust on the top of Getic sedimentary cover (KALENIĆ & HADŽI-VUKO- VIĆ, 1973;SCHMID et al., 2008).
In the Serbian Carpathians segment of the Carpatho--Balkanides Mts., the Supragetic nappe is mostly covered with Miocene to Quaternary sediments of the Velika Morava Corridor (i.e. the southern prolongation of the Pannonian Basin), but it crops out in three regions ( Fig. 1) -between the towns of Golubac, Petrovac and Kučevo, north of Despotovac, and southwest of Paraćin. This study is focused on the northernmost of the three regions, on the areas of Golubac (Fig. 2) and Kučevo (Fig. 3). These areas were chosen for this research due to the great number of the available outcrops and evident and not overprinted structural contact between the Supragetic and the Getic Units.

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Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the ...   The Upper and Lower Getic units, the parts of the Getic nappe system, are located structurally below the Supragetic Unit (Fig. 5). They are represented by the high-grade to low-grade metamorphic basement and the Permian to Late Cretaceous sedimentary cover (i.e. KALENIĆ & HADŽI-VUKOVIĆ, 1973;KRÄUTNER & KRSTIĆ, 2003;IANCU et al., 2005a;IANCU et al., 2005b). The Permian sediments of the Getic nappe sequence do not outcrop in the studied areas and their position in the cross sections (Fig. 5) is interpreted based on their surface exposure south of the studied areas (ATONIJEVIĆ et al., 1963;VESELINOVIĆ et al., 1964;KALENIĆ & HADŽI-VUKOVIĆ, 1973).
The Early Miocene to Quaternary sedimentary cover is represented by the lacustrine infill of Kučevo and Rakova Bara basins and smaller intramontane depressions, as well as by lacustrine to marine sediments of the Velika Morava Corridor that cover most of the Supragetic Unit west of the studied areas (Fig. 1B;KALENIĆ et al., 1980).

Tectonic framework
The Dacia mega-unit of the Eastern, Southern and Serbian Carpathians was initially part of the active continental margin of Europe, which was partially separated from the continent during the Jurassic opening of the Ceahlau-Severin branch of the Alpine Tethys (SCHMID et al., 2008). The obduction of the East Vardar ophiolites on top of the Dacia mega-unit occurred during the latest Jurassic (CSONTOS & VÖRÖS, 2004;SCHMID et al., 2008). Although the true nature of the East Vardar ophiolites is highly debated, the thermal and metamorphic events of Late Jurassic age were recognised in the Dacia mega-unit (REISER et al., 2017, and references therein).
The late Early Cretaceous nappe-stacking, the 'Austrian' phase, in the Dacia mega-unit, followed the closure of the Ceahlau-Severin Ocean and its collision with the Moesian platform (CSONTOS & VÖRÖS, 2004;SCHMID et al., 2008). This event was recognised in various parts of the Dacia mega-unit (i.e. GRÖGER et al., 2013, and references therein, in the Eastern Carpathians;BOJAR et al., 1998, IANCU et al., 2005a  The late Early to early Late Cretaceous, post 'Austrian', cooling event which was reported in Serbo-Macedonian Massif (ANTIĆ et al., 2016) was interpreted as post-collisional extension. The Late Cretaceous Banatitic magmatism in the Apuseni-Banat-Timok-Srednogorie magmatic arc is associated with that extension (VON QUADT et al., 2005;GALLHOFER et al., 2015).
The 'Laramian' phase overthrusting of the whole Dacia nappe-stack over the Ceahlau-Severin and the Danubian Units took place after the cessation of the Late Cretaceous Banatitic magmatism, during the latest Cretaceous-Early Paleocene (BOJAR et al., 1998;IANCU et al., 2005a;SCHMID et al., 2008;ANTIĆ et al., 2016, and references therein).
The interplay of extension in the Pannonian basin and Paleogene-Neogene clockwise rotation and oroclinal bending of the Dacia mega-unit around Moesian promontory had a significant influence on tectonothermal and structural evolution of the orogen (LINZER et al., 1998;SCHMID et al., 1998;MATENCO & SCHMID, 1999;USTASZEWSKI et al., 2008, BALÁZS et al., 2016. This was associated with orogen-parallel extension and significant strain partitioning (FÜGENSCHUH & SCHMID, 2005;KRÉSZEK et al., 2013). The subsequent collision and locking in the Eastern Carpathians and the inversion in the Pannonian basin in the Miocene-Pliocene times represent the last stage in the tectonic evolution of the Dacia mega-unit ; MATENCO Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the ...

Methods
In order to constrain the internal structure of the Supragetic Unit basement, research at map-, outcropand thin-section-scales was conducted.
The regional-scale structures and the data collected during the geological mapping in the studied areas (KALENIĆ & HADŽI-VUKOVIĆ, 1973) were analysed at the map-scale. In this way, the statistical mean orientation of foliation in the three stratigraphically lowermost metamorphic sequences of the Supragetic Unit basement was obtained. Those metamorphic sequences were chosen for the analysis because they were expected to record deformations at the deepest levels. This analysis was performed separately for Golubac and Kučevo areas (Fig. 2, 3) in order to constrain possible changes in the geometry of the hectometre-scale structures along the strike. On the basis of the map and field data, two regional cross sections were constructed (Fig. 5) for both studied areas and were analysed independently afterwards.
The field studies included observations and measurements (studied sites in Fig. 2 and Fig. 3) of the outcrop-scale structures, collecting samples for microstructure analysis and the determination of the grade of metamorphism. The kink bands in the foliated rocks can be used as paleostress indicators (i.e. DE-WEY, 1965;GAY & WEISS, 1974;SRIVASTAVA et al., 1999). The kink bands can be conjugate, either symmetric or asymmetric, or monoclines. The geometry of a single kink band includes the external and internal foliation separated by two kink planes or boundaries (see Fig. 1 of GAY & WEISS, 1974). In order to perform paleostress analysis of kink bands, the field orientation of the internal foliation and the kink plane was measured. In a kink band, the slip direction is parallel to the kink plane and perpendicular to the intersection of the kink plane and the internal foliation (SRIVASTAVA et al., 1999). The sense of movement was determined based on the geometry of the kink band. The paleostress analysis was carried out using the NDA method in TectonicsFP software (ORNTER et al., 2002). A 60° θ angle between σ 1 and the kink plane was used as an optimal angle for the kink bands, taking into account that this deformation occurs at deeper levels than the brittle deformation (SRIVASTAVA et al., 1999).
For micro-scale studies, six oriented samples were collected, four of them in the metavolcanics near Golubac (sites GO1, GO7, GO8 and GO9 in Fig. 2) and two in the chlorite schists in Kučevo area (site KU1 in Fig. 3). The optical microscopy technique was used for the thin section analysis. Based on mineral assemblages of the two metamorphic sequences and recrystallisation types of dominant minerals the metamorphic grade was determined. The microstructures were analysed for determining a deformation sequence and the tectonic transport at the thin-section scale.

Results
The observations were carried out at three scales and the corresponding results are presented in this section.

Thin section-scale deformations
In four samples from Golubac area (sites GO1, GO7, GO8 and GO9, Fig. 2) the main rock-forming minerals are quartz, albite, sericite, chlorite, muscovite and metallic minerals, while zircon and apatite are accessory ( Fig. 6A-C, 7A, B). The protolith of this rock is interpreted to be acidic to intermediate tuff.
Based on the grain morphology, quartz and albite demonstrate dominant bulging recrystallisation and locally sub-grain rotation recrystallisation (Fig. 6A-C). Undulose extinction is a common feature in quartz and albite. The observed mineral assemblage as well as the quartz and albite grain features indicates lower greenschist facies metamorphism with temperatures reaching 300°C and pressure of 0.3 GPa (p/T estimates based on BEST, 2003).
In slightly deformed metavolcanics, the most prominent structure recognised in thin sections of this metamorphic sequence is a spaced foliation defined by narrow bands of chlorite, sericite and muscovite, separated by wider bands of coarser quartz and albite. The second set of structures, cutting the older ones at the acute angle and forming C'-S structures that demonstrate top to ESE to top to SE reverse tectonic transport is less evident (Fig. 7A, B).
Two thin sections from the lowermost metamorphic sequence of the Supragetic Unit basement (Kučevo area, site KU1 in Fig. 3) show that, compared to samples from Golubac area (Fig. 6D-F, 7C-F), this rock is built of finer mineral grains (generally less than 0.1 mm in diameter, except for albite). The main rockforming minerals here are quartz, sericite, epidote, chlorite, calcite and metallic minerals and they define protolith as Ca-rich quartz keratophyre. As in the metamorphic sequence analysed above, in this rock quartz grains demonstrate dominant bulging recrystallisation and undulose extinction (Fig. 6D-F). The analysis of the mineral assemblage and dynamic recrystallisation of quartz implies that this sequence was metamorphosed under lower greenschist facies with temperatures reaching 350°C and pressure of 0.5 GPa (p/T estimates based on BEST, 2003).
The penetrative spaced foliation observed in thin sections is the most dominant structure of this lower-Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the ... most metamorphic rock sequence. It is defined by alternating narrow bands of chlorite and sericite and wider bands of dominantly quartz, calcite and albite. The second foliation cuts the previous foliation at the acute angle, forming C'-S structures. C'-S structures together with chlorite and sericite growth in strain shadows of albite porphyroblasts clearly define top to ESE reverse shearing (Fig. 7C, F). Both sets of the previously described foliations (S and C') are deformed by thin section-scale kink bands that are antithetic to sub-perpendicular to S planes and demonstrate a reverse sense of shearing (Fig. 7E). NEMANJA KRSTEKANIĆ et al.

Outcrop-scale deformations
The most dominant structure at the outcrop-scale in both studied areas is a penetrative foliation which was also observed in thin sections. This foliation generally has west to northwest dipping at variable (from low to high) angles. Low-to moderate-angle eastward-dipping foliation was observed only at GO2 site (Fig. 2, 5).
Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the ... A different orientation of kink bands deforming the main foliation (Sf) was observed in Golubac area (Fig.  8A-F). One set of conjugate kink bands striking NE-SW (Fig. 8B, D, F) and the second set of kink bands, striking NW-SE (Fig. 9C) were recognised. As for the single monocline kink bands, the most com-mon orientation of the kink planes (Sk) is antithetic in respect to external foliation (i.e. Fig. 8A). This is the case for both the eastward-and westward-dipping external foliation. However, in respect to external foliation, synthetic kink bands were also recognised (Fig.  8E). Paleostress analysis of all the kink bands recog-NEMANJA KRSTEKANIĆ et al. nised in Golubac area resulted in quite uniform stress axes, with σ 1 302/03, σ 2 032/01 and σ 3 141/86 (Fig.  9C), indicating NW-SE compression.
Stretching lineation statistical analysis, based on the standard Gaussian counting model in SpheriStat with the following statistics parameters: mean E=0.26, standard deviation S=0.357 and kappa k=100, resulted in maximum of 304/40 (Fig. 9D).
The other structures recognised in outcrops of the Supragetic Unit basement are changes of foliation dip angle at the decimetre scale (Fig. 8C). No older tight or isoclinal folds or any evidence of transposition was observed in the field.

Map-scale structures
The total of 85 measurements in the Supragetic Unit basement (Sco, Sse and Sab units in Fig. 2, 3) were used for the purpose of the statistical analysis of foliation in Golubac area. The standard Gaussian counting model in SpheriStat was used with the following statistics parameters: mean E=0.85, standard deviation S=0.645 and kappa k=100. The results show the stretched maximum in 270/31, and the less pronounced submaximum in 267/85 (Fig. 9A).
In Kučevo area, 64 foliation measurements were statistically analysed using the standard Gaussian Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the ... counting model (E=0.64, S=0.56, k=100). The results show similar west-dipping foliation with a more pronounced variation in dip angles than in Golubac area. The statistical maximum is 270/49, and the two submaxima are 246/28 and 274/82 (Fig. 9B).

Discussion
In fold-thrust belts, fault-related folds can have various geometries depending on different mechanical and rheological characteristics of the deforming rocks (SHAW et al., 2004). Such folding and other structures related to the thrusting can be used to infer the geometry of the main thrust fault (JoHNSON & BERGER, 1989;SAVAGE & COOKE, 2003).
The results of the statistical analysis of the foliation measurements (Fig. 9A,B) and the field observations indicate west-dipping foliation at various dip angles. This can be interpreted in two ways. The first one is that significant variations in dip angles but not in dip directions can be the result of hectometre-scale tight east-vergent folding. The alternative interpretation is that these statistical results indicate a flat-ramp geometry of the Supragetic thrust. However, both of these interpretations, as well as the mineral assemblage and the termochronological data (BOJAR et al., 1998;KOUNOV et al., 2010;GRÖGER et al., 2013;ANTIĆ et al., 2016, REISER et al., 2017 indicate that the Supragetic Unit basement was deformed in ductile domain during the late Early Cretaceous. Furthermore, the field observations also show E-dipping foliation in the northernmost sections (site GO2 in Fig. 2) representing the western limb of a fault-related syncline in the hanging wall of the flat segment of the thrust (Golubac area cross section, Fig. 5).
The ductile to semi-ductile shearing recognised both at the outcrop-and the thin section-scale (Fig. 7, 8) demonstrate top to ESE tectonic transport. The WNW-dipping stretching lineation (Fig. 9D) is in accordance with this observation. Furthermore, kink bands deforming the penetrative foliation also imply WNW-ESE to NW-SE compression phase (Fig. 9C). Foliation parallel shearing and conjugate NE-SW striking kink bands are interpreted as the result of continuous deformation event starting in the ductile domain and continuing in the semi-ductile/semi-brittle domain when kinking occurred as hanging-wall strain accommodation (see Fig. 10; BONINI et al., 2000;SCHLISCHE et al., 2014;ROSAS et al., 2017). The dominant opposite dip-direction of kink bands and main foliation is regarded as an interesting observation. Such configuration is expected for the kink bands of the hanging wall which are antithetic to main thrust (BONINI et al., 2000;ROSAS et al., 2017). This was generally the case in the studied locations, however the field observations of the opposite dipping kink bands and foliation in the E-dipping fold limb indicated that the kink bands could have been formed along the axial planar cleavage of the pre-existing fault-related fold. The kink bands striking NW-SE are interpreted as parts of semi-ductile tear faults, transferring strain between lateral segments of the nappe during thrusting. Faults of, generally, the same orientation were also recognised at the map-scale (Fig. 2, 3, KALENIĆ & HADŽI-VUKOVIĆ, 1973).
The offset of the Supragetic thrust increases southwards, outcropping deeper sequences of the basement in the hanging wall of the thrust front (Fig. 2, 3, 5). Corresponding to this are the mineral assemblages and micro-scale deformations of different metamorphic sequences, all indicating 1.5-2 km greater exhumation in Kučevo area compared to Golubac area.
In this study, based on all the available data, the mineral assemblages, the amount of deformation at outcrop-and thin-section-scale and the lack of observable tight or isoclinal folds and inverse foliation, the internal structure of the Supragetic Unit basement is interpreted as an indicator for flat-ramp geometry of the main thrust (Fig. 10). These structures were formed during the late Early Cretaceous, starting in ductile to semiductile/semi-brittle domain. This interpretation does not completely exclude the possibility of tight to isoclinal folding of the Supragetic Unit metamorphic basement related to the Alpine nappe-stacking.

Conclusions
The structural analysis of the Supragetic Unit metamorphic basement at different scales was performed in order to constrain its internal geometry and its relations to the late Early Cretaceous nappe-stacking.
Based on microstructural observations, field evidence and map data analysis, it was concluded that the Supragetic thrust most possibly has a flat-ramp geometry, and that this geometry is reflected in the internal structure of its hanging wall represented by the Supragetic Unit metamorphics. The main foliation which was deformed during the late Early Cretaceous nappestacking suggests a fault-related folding and flat-ramp geometry that was subsequently deformed by kinking. Kinking accommodated strain in the hanging wall during transition from ductile to brittle domain. Moreover, the data presented in this study indicate an increase in the offset and the amount of exhumation along the Supragetic thrust from north to south.
These conclusions were made based on the available data and the published papers. However, further research is needed in order to constrain more precisely the timing of deformations in the Supragetic metamorphic series.