Paleogene – Early Miocene deformations of Bukulja – Ven ~ ac crystalline ( Vardar Zone , Serbia )

Low-grade metamorphic rocks of the crystalline of Mts. Bukulja and Ven~ac, which are integral parts of the Vardar Zone, are of Late Cretaceous age. From the Middle Paleogene to the beginning of the Miocene, they were subjected to three phases of intensive deformations. In the first phase, during the Middle Paleogene, these rocks were subjected to intense shortening (approximately in the E–W direction), regional metamorphism and deformations in the ductile and brittle domains, when first-generation folds with NNE–SSW striking fold hinges were formed. In the second phase, during the Late Oligocene and up to the Early Miocene, extensional unroofing and exhumation of the crystalline occurred, which was followed by intrusion of the granitoid of Bukulja and refolding of the previously formed folds in a simple brachial form of Bukulja and Ven~ac with an ESE–WNW striking B-axis. The third phase was expressed in the Early lowermost Miocene (before the Ottnanghian), under conditions of NE–SW compression and NW–SE tension. It was characterized by wrench-tectonic activity, particularly by dextral movements along NNW–SSE striking faults.


Introduction
Crystalline of Bukulja and Ven~ac, with its non-metamorphosed Mesozoic-Cenozoic cover and Oligocene--Miocene granitoid, spatially belongs to the Vardar Zone (Fig. 1).These are terrains with complex geological compositions which have been discussed many times, often with controversial explanations.
There are dilemmas about the age of the crystalline in the first place, which directly influenced different explanations of the tectonics of these terrains.The crystalline has most often been considered to be of Paleozoic age (SIMI] 1938; FILIPOVI] 1973; FILIPOVI] & RO-DIN 1980; \OKOVI] et al 1995; TRIVI] 1998).Such an opinion is mostly based on the fact that these are rocks of different metamorphic grade, while there are no reliable paleontological proofs or even paleontological proofs of any kind.However, according to findings of globotruncana and other fauna and on the base of palynologic data from low-metamorphic rocks of Ven~ac, BRKOVI] et al. (1980) and MAROVI] et al. (2005), respectively, concluded that the Bukulja-Ven~ac crystalline is of Late Cretaceous age.
According to its age, folding of the area has also been explained in different ways.\OKOVI] & MAROVI] (1985,1986) separated three generations of folds in these terrains.These authors related the first fold generation to Hercynian deformation, which is marked by NE-SW striking fold axes.In the second phase, during older Alpine tectogenetic events, the Hercynian fold structures were refolded into E-W striking folds.The third generation of folds is the consequence of a pluton intrusion and further refolding of all the existing folds into large domes and brachial synclines.TRIVI] (1998) separated three (? four) generations of folds.According to this author, axes of the oldest, Hercynian structures are oriented in the WNW-ESE direction.These structures were refolded into folds with NNW-SSE striking axes during the first phase of Alpine deformation in the Mesozoic.Later, during the later Alpine phases, the geometry of such folds became more complex due to a pluton intrusion and strike-slip movements along E-W striking faults.MAROVI] et al. (2005) considered the metamorphic rocks of Bukulja and Ven~ac to be of Late Cretaceous age and the authors are of the opinion that there are only Alpine and no Hercynian folds in these rocks.
The relationship between the crystalline and the nonmetamorphosed Cretaceous (prevailingly Late Cretaceous) deposits, including tectonically incorporated slices of serpentinite, is unclear and has been explained in different ways.Sedimentary deposits are widespread on the surface, mostly north, east and southeast of the Bukulja-Ven~ac crystalline, and they were also drilled out under Neogene deposits of the Aran|elovac and Belanovica Basin.There are also isolated and disconnected portions of Cretaceous sediments on the southern rim of the crystalline.All this points to the possibility that the crystalline was completely covered by Cretaceous sediments.The majority of authors is of the opinion that the Cretaceous sediments transgressively overlie the crystalline.According to TRIVI] (1998), metamorphic rocks were thrust over Cretaceous sediments in certain parts of the terrain in the South.BRKOVI] et al. (1980) and \OKOVI] & MAROVI] (1986) mentioned sections where metamorphic rocks gradually transit into nonmetamorphosed Upper Cretaceous deposits.
Finally, in accordance with different interpretations of the geologic composition, the geotectonic position of the Bukulja-Ven~ac crystalline unit is also controversial.Its metamorphic content resembles the Drina-Ivanjica crystalline (\OKOVI] et al. 1995).The Bukulja--Ven~ac crystalline is located on the eastward extension of the Jadar Block, which is made of Paleozoic rocks.This fact led FILIPOVI] (1995), FILIPOVI] & JO-VANOVI] (1998) and FILIPOVI] ( 2005) to include at least a part of it (western part of Bukulja) into the Jadar entity.There is also the opinion that Bukulja-Ven~ac crystalline is completely different from the metamorphic rocks of both the Drina-Ivanjica and Jadar developments and that it is made of metamorphosed Cretaceous deposits belonging to the Vardar Zone (BRKOVI] et al. 1980;MAROVI] et al. 2005).
The above-cited problems concerning the geologic composition of the Bukulja-Ven~ac crystalline are a challenge for further investigations directed toward new and better documented solutions.The results of one of these studies, which represent a contribution to a better understanding of the Paleogene-Early Miocene tectonics of these regions, are presented in this paper.
(1) The Bukulja-Ven~ac crystalline is made of rocks of different degrees of metamorphism, mostly low-grade metamorphics and, to a smaller extent, medium-to-highgrade metamorphics.These are mostly sedimentary rocks which were subjected to regional metamorphism and also to contact metamorphism in the vicinity of the granitoid.The lowest structural position is occupied by gneisses (and also leptynolites in places), which are followed by: micaschists, sericite schists, meta-quartz conglomerates, phyllites and sericite schists, marbles, calcschists, metacalcarenites and metasiltstones.Also epidote-actinolite-and chlorite schists occur subordinately in the low-grade metamorphic complex.Rocks with a higher grade of metamorphism are found in the vicin- ity of the granitoid, while going away from it -towards the Ven~ac, low-grade metamorphics predominate.The Bukulja-Ven~ac crystalline is of Late Cretaceous or maybe partly even of Paleogene age.The second author (I.\.) is of the opinion that Ven~ac domain of the crystalline is of Late Cretaceous age, while the rest of it is Paleozoic and resembles the Drina-Ivanjica Paleozoic.During these investigations, rich microfloral association, which indicates Late Cretaceous age, was found in the calcschists and metacalcarenites of Ven~ac.This is in full agreement with the results on the crystalline age based on globotruncanas (BRKOVI] et al. 1980).However, this age most probably does not refer to the whole crystalline.Based on a lithostratigraphic correlation, FILIPOVI] ( 2005) is of the opinion that the metamorphic rocks west of Bukulja are similar to the Jadar Paleozoic, thus that they are Devonian and Carboniferous in age.
(2) Cretaceous sequence of non-metamorphosed deposits and serpentinite are exposed on the northern, eastern and southern slopes of the Bukulja-Ven~ac morphostructure.The Cretaceous sediments are represented by reefal and stratified limestones, rarely also by Early Cretaceous clastites and, for the largest part, by various types of carbonates, clastites and Late Cretaceous flysch (BRKOVI] et al. 1980).Smaller tectonic slices of serpentinite of Jurassic age occur locally near the Cretaceous sediments.
(4) A Neogene-Quaternary cover is represented by loosely bound coarse-grained, gravely-sandy, clayey-sandy and clayey deposits.These are mostly fresh-water equivalents of the Ottnangian-Karpatian and, to a lesser extent, also marine deposits of the Badenian and Sarmatian.The highest stratigraphic level is represented by different types of Quaternary deposits.

Methodology of research
Geologic mapping of the Bukulja-Ven~ac crystalline (including the granitoid) and its non-metamorphosed cover of Late Cretaceous age provided information relevant for solving the tectonic setting of the area.These were data on bedding, foliation, folds of different scale and faults.They were analyzed within different scale ranges and homogeneous domains and the obtained data were incorporated in a tectonic synthesis, together with knowledge on the lithostratigraphic units.
Particular attention was paid to the determination of the orientation of fault planes and associated slip direc-tion, which was used for the reconstruction of paleostress and deformation phases manifested from the middle Paleogene to the beginning of the Miocene.
Reconstruction of faulting succession and displacement was based on the criteria given by PETIT (1987) and GAMOND (1983GAMOND ( , 1987)).Reduced deviatoric paleostress tensors were computed for a cogenetic fault population which was separated from polyphase sets, based on field observations and kinematic compatibility.The method of numerical and graphical inversion proposed by ANGELIER & MECHLER (1977), ANGELIER (1979ANGELIER ( , 1989) ) and method of numerical dynamic analysis (NDA) by SPERNER et al. (1993) were used.Computation of the data for paleostress analysis was performed using Tectonic FP software (ORTNER et al. 2002).

Structural features
In a structural sense, three large homogeneous domains can be distinguished within the research area: (1) Bukulja-Ven~ac crystalline, (2) the thrust-fold sequence of non-metamorphosed Cretaceous deposits with tectonically incorporated slices of serpentinite and (3) Neogene basins.The first two structural domains are discussed in this paper, because they resulted from Paleogene-Early Miocene deformations, which were the subject of the research.
The structural setting of the Bukulja-Ven~ac crystalline is very complex with a polyphase-deformation history and at least two phases of folding.The area is dominated by a large (Dkm) brachial-antiform structure, the hinge of which plunges toward ESE.The bestdeveloped fabric element is foliation, which actually makes this antiform (Fig. 2A).Foliation is unevenly developed: it is best-developed in gneisses and micaschists, less present in phyllites, sericite schists and calcschists, while it is poorly developed in metacalcarenite, metasiltstone and "massive" marble.
The foliation is probably the result of flattening perpendicular to the foliation planes.Isoclinal intrafolial folds of cm and dm scale are indicators of shearing along foliation.They are particularly well-visible in the metacalcarenites of Ven~ac, and locally, also in quartzsericite schists (Fig. 3).The Folds are mostly rootless and represent thickened hinge zones, while their limbs are strongly flattened and sheared.These folds are west-northwest-vergent with fold axes plunging toward NNE and SSW (Fig. 2B).Crenulations of foliation are noticed locally.The crenulation axes plunge toward south-southeast to, southeast and northwest and they are genetically related to the formation of the brachial antiform (Fig. 2C).Foliation and intrafolial rootless folds could have been formed in an almost horizontal position.All this indicates refolding in the Bukulja--Ven~ac crystalline.
Foliation is developed in the granitoid as well.It has a periclinal distribution (Fig. 2D) compatible with foli- The thrust-fold stack of non-metamorphosed Cretaceous sediments with tectonically incorporated slices of serpentinite also have a very complex structure as well.Today, this unit is preserved within several small, more or less homogeneous structural regions on the northern, eastern and southern rims of the Bukulja-Ven~ac antiform.The structure is dominated by bedding and faults.The Bedding planes are well-exposed and penetrative.
Terrains on the northern slopes of Mt.Ven~ac are composed of non-metamorphosed deposits of Cretaceous age.Despite the fact that a large part of the area is covered with deluvium, a lot of information was acquired for fault analyses.
On the diagram F (Fig. 2), poles to bedding are mostly concentrated in the NW quadrant, marking a monoclinal dip toward southeast.However, field investigations showed that the folds in this area are not simple but that it is a folded unit with normal and overturned limbs of NNW (NW) vergent folds, similar to the folds of the first generation in the underlying Bukulja-Ven~ac crystalline, only less developed with less strain.Cretaceous deposits north of Bukulja are identically deformed (Fig. 2E).
East of Ven~ac, there is an intensely tectonized zone in the Cretaceous deposits and serpentinite.Unfortunately, this area is mostly covered, with no outcrops of Cretaceous deposits, thus a comprehensive measuremant the of bedding attitude could not be performed.According to the data from the wider surroundings (BR-KOVI] et al. 1980), the area is characterized by a thrustfold pattern marked by West-vergent recumbent folds and reverse faults, developed under dextral transpressio.
Terrains made of non-metamorphosed Cretaceous deposits on the southern and southwestern slopes of Ven-~ac are mostly covered with deluvium and are unfavorable for structural investigations.The scattering of the bedding data, presented on diagram G (Fig. 2) is probably a consequence of the rotation of faulted blocks, but also of the small number of measurements which are statistically not representative.Field observations showed that the Cretaceous deposits here are also intensely folded, with the occurrence of overturned west-northwest-vergent folds.

Results of paleostress analysis
Paleostress analysis in the area of the Bukulja-Ven-~ac crystalline, non-metamorphosed Cretaceous deposits and the granitoid show three kinematic stages, the first probably being of Middle Paleogene, the second of Oligocene to Oligocene-Miocene and the third of Early Miocene (Pre-Ottnangian to Karpatian) age.The relative chronology of these events is deduced from crosscutting map-scale faults in key outcrops.

Deformational event (D 1 ) -E-W compression
This paleostress tensor group comprises a conjugated pair of NW-trending sinistral and NE-trending dextral strike-slip faults (Fig. 4).These faults are overprinted by mainly extensional structures on numerous outcrops.
Folds of the first generation with a NNE (NE)-SSW (SW) striking axes probably originated in such a stress field.Today, they are exposed as intrafolial folds in the Bukulja-Ven~ac crystalline, as well as in WNW (NW) vergent folds in non-metamorphosed Cretaceous deposits.

Deformational event (D 2 ) -N-S-to-NE-SW extension
The second paleostress tensor group comprises WNW to NW and NE-trending normal faults (Fig. 5).These faults are probably related to an Oligocene unroofing of the Bukulja-Ven~ac crystalline and the granitoid intrusion.In this case, WNW to NW trending normal faults often form conjugate sets: synthetic, gently sloping northwards and antithetic, with steeper dips toward the south.They were formed above the brittle-ductile detachment zone along which the extensional unroofing occurred.

Deformational event (D 3 ) -wrench tectonic regime, NE-SW compression and NW-SE tension
The third paleostress tensor group comprises NNW to NW trending dextral and WNW trending sinistral strike-slip faults (Fig. 6).Fault systems with these kinematic characteristics, which originated in the stress field with NE-SW compression and NW-SE tension, can be

Discussion and Conclusions
Investigations in the area of the Bukulja-Ven~ac crystalline showed the following: • The Bukulja-Ven~ac crystalline is of Late Cretaceous age, maybe even partly Early Paleogene.It was intruded by an Early Miocene granitoid.
• The crystalline is overlain mostly by Late Cretaceous non-metamorphosed clastic-carbonate rocks and flysch.
• Metamorphic grade in the crystalline decreases from the granitoid to the periphery and toward the upper structural levels, where there is a gradual transition into non-metamorphosed members of the Late Cretaceous.
• There is a similar manner of folding (fold shape, vergences) in both sequences of Cretaceous deposits: the metamorphosed and the non-metamorphosed ones, but deformations in the crystalline is more intense and occurred in the ductile domain.Two phases of folding are noticed.
• Reconstruction of paleostress fields points to three major phases of brittle formation : in the middle of the Paleogene, in the Oligocene-Early Miocene and in the Early Miocene.
The above presented facts point to a unique tectonic-sedimentary environment in this area during the Late Cretaceous (maybe also in the ?Early Paleogene), which was inverted in the middle of the Paleogene.Such an environment is consistent with the model elaborated by PAMI} (1993), PAMI} et al. (2000, 2002), according to which the northern part of the Vardar Zone (Vardar--Sava) is the result of obliteration in the Upper Cretaceous-Paleogene active continental margin of Southern Europe, with well-defined island arc and back-arc basins.This sedimentation area was inverted and included into the Dinaridic orogene by collisional processes in the Eocene.According to PAMI} et al. (2000PAMI} et al. ( , 2002)), this phase was followed by intense deformation of the Jurassic ophiolitic mélange, metamorphism and magmatism.
The Bukulja-Ven~ac sedimentation and deformation area (Fig. 7) was probably generated in a similar tectonic setting.In the middle of the Paleogene, the Bukulja-Ven~ac area was subjected to shortening in the approximate E-W direction, when a thick WNW vergent thrust-fold sequence was formed.The second author (I.\.) is of the opinion that these structures were formed only in the Ven~ac domain of the crystalline, while, in its other parts, the Hercynian structures were refolded by a Mesozoic-Cenozoic tectonic event.The lower parts of the sequence reached the zone of ductile deformations and underwent regional low-to medium-grade metamorphism.The whole process was followed by the formation of tight and isoclinal folds with hinges striking NNE (NE)-SSW (SW) with strong axial plane cleavage, and subsequent transposition of bedding along the cleavage, the formation of foliation.Rem-nants of these folds are preserved today as intrafolial folds.
In the brittle-ductile and brittle domain, above the metamorphites, this phase of tectogenesis resulted in the formation of distinctly WNW (NW) vergent overturned, sometimes also recumbent, folds with axes striking NNE (NE)-SSW (SW) and the formation of conjugated NW trending sinistral and NE trending dextral strike slip faults.
Extension, probably ductile, followed by intrusion of granitoid, volcanism and exhumation of the Late Cretaceous metamorphics (metamorphic core complex) is characteristic for the second phase, which that was expressed in the Late Oligocene and up into the Early Miocene.
The process of exhumation metamorphism and emplacement of the granitoid was marked by refolding of the foliation and the previously formed folds, when the distinct brachial-antiform of the Mts.Bukulja and Ven-~ac (with an ESE plunging axis) was formed.There are certain indications that a shallow synform), rim synform, which is presently mostly burried with Neogene-Quaternary deposits, was formed northeast of the antiform.
Unfortunately, the detachment zone along which the ductile extension occurred has not been defined, which certainly does not mean that it does not exist.Further detailed investigations are necessary for its determination.
In the brittle domain in the area of extensional allochthon, WNW to NW and NE trending normal faults were activated, often as pairs of synthetic and antithetic sets.
After the exhumation of the metamorphic core complex of Bukulja and Ven~ac, tectonic shortening affected the area.It is expressed through dextral transpression with NE-SW compression and NW-SE tension.Activation of the NNW-NW trending dextral and WNW trending sinistral strike-slip faults is characteristic for this phase.In the domain of the first system, small NE-trending normal faults (probably "pinnate" faults) were activated.Under transpressional conditions, west-vergent folds and thrusts were formed, particularly on the eastern periphery of Ven~ac.This transpressional event affected the Vardar Zone, the Serbian-Macedonian Unit and the Carpatho-Balkanides, all the way to the Moesian Plate (wrench corridor, MAROVI] et al. 2001).
The process of destruction of the previously formed structures, related to the shaping of the Pannonian Basin and its periphery, commenced after the transpressional events, already from the Ottnangian.
The performed investigations stress the problem which demands more detailed research and application of new methods in order to obtain more reliable and precise solutions.This refers, in the first place, to the necessity of performing detailed structural investigations and registering kinematic indicators of extensional processes and stress fields in general.Particular attention should also, be paid to an explanation of the manner of extensional unroofing, transpressional tectonics and related phenomena.

Fig. 1 .
Fig. 1.Geologic sketch of the wider area of Bukulja and Ven~ac.

Fig. 2 .
Fig. 2. Equal -area Lower hemisphere stereograms of: A, foliations in the crystalline; B, B-axis intrafolial folds; C, crenulation lineation; D, foliations in the granitoid; E, bedding in Cretaceous deposits north of Bukulja; F, bedding in Cretaceous deposits northwest of Ven~ac; G, bedding in Cretaceous deposits south of Ven~ac.Spheristat software was used for the analysis.

Fig. 4 .
Fig. 4. Distribution of stress states related to a E-W compressional event.Stereographic projection of the measured outcrop -scale faults and calculated stress axes.The circle, rectangle and triangle represent the orientation of the maximum, intermediate and minimum principal stress axes, respectively.

Fig. 5 .
Fig. 5. Distribution of stress states related to an extensional event with N-S to NE-SW trending V 3 .Explanation the same as for Fig. 4.

Fig. 6 .
Fig. 6.Distribution of stress states related to a dextral strike-slip regime with V 1 , trending NE-SW.Explanation the same as for Figs. 4. and 5.