Tektonika piętra waryscyjskiego rejonu dębnickiego w świetle badań drobnostrukturalnych

Józef Krokowski


Variscan tectonics of the Dębnik region (south Poland) in the light of mesoscopic studies

Palaeozoic rocks which appear in the Dębnik area near Cracow are the oldest exposed fragments of the epi-Caledonian platform between the Sudety and Holy Cross Mts (Znosko 1970, Bogacz 1977). They crop out in the north-eastern periphery of the Upper Silesia Coal Basin. The platform basement consists of Lower Palaeozoic rocks. Devonian and Carboniferous which appear as a series of dolomites and limestones form the platform cover. Palaeozoic rocks of this area belong to an elevated tectonic unit — the so called Meta-Carpathian arch. The Meta-Carpathian arch disappears to the south in the Carpathian foredeep. There is a marked disagreement in opinions concerning the geological structure of the Palaeozoic rocks of the Dębnik area. Until recently there was generally accepted fold tectonics as dominant in this region. Fault structures has been treated as playing only a secondary role and as not connected with formation of folds which were supposed to occur much earlier. Only; recently the investigation of Bogacz (1977) have changed these opinions. Bogacz stresses the importance of dislocation, flexures and deformations occurring in the cap rock of magmatic intrusions of laccolith type. According to a third group of opinions, significant thrusts of the Palaeozoic rock masses connected with intensive folding occured in the area of Dębnik. Since there are such divergent opinions on the geological structure of this area, an attempt to solve this problem on the basis of the structural mesoscopic studies has been undertaken.

Mesoscopic studies were carried on simultaneously with cartographic works of Bogacz (1977 see Fig. 1). The measurements were plotted as diagrams in eqiual-area projection. To reckon the means of particular sets of folds axes, bends and axes II, the vector method of Fisher — Watson was employed (Fisher 1953, Watson 1966, Cruden, Charlesworth 1972). The investigated area was divided into some structurally homogenic units (Fig. 1). They are in close correspondence with tectonic structures existing in the Palaeozoic of Dębnik. The mesostructural data underwent two-stage, statistical generalization. The first stage was concerned with the analysis within a given unit in the next stage relations between units were examined.

Attitude of strata
Distribution of the attitude of strata (Fig. 2) of the Palaeozoic rocks points to a heterogeneous character of the geological structure of the examined area. Three systems of folds were distinguished: F1, F2 and F3. The trend of the deformations F1 and F3 is approximately NWW—SEE while the trend of F2 is NNE—SSW. Deformations F1 and F2 differ in dip angles which are low in the F1 and steep in the F3 system.

Lineation В is represented by: axes of minor folds and bends found as [beta]-axes on the diagrams on measured in the field; axes of the great circles of the attitude of strata (axes П); intersection lines of the bedding planes with longitudinal joints and cleavage. The majority of B1 lineation measurements is accumulated around NWW—SEE direction with a tendency to plunge to the NWW (Fig. 6). Few other directions can be explained by later deformations, mainly in the F2 period (Fig. 7). Reorientation of В1 lineation at the stage F2 was rather small. Only locally (unit D) this reorientation is significant. B2 lineation (Fig. 8) is represented mainly by the translational structures appearing in the Unit D. Probably these deformations developed along one of the sets of the J(II)—J(IV) jointing system. Measurements of B2 lineation accumulate around the N—S direction with a tendency to plunge to the south.
Cleavage occurring in the Palaeozoic rocks of the Dębnik area is rather poor developed. Planes with trends approaching the NWW—SEE direction are dominant (Fig. 9). In a way it is a logitudinal F1 fold cleavage. In the narrow zones in the vicinity of faults there appears often cleavage having a general W—E direction. This cleavage is better developed than the former F1.

Joints belong to two systems of factures: J(I)—J(III) and J(II)—J(IV) (Fig. 12). It has been found that the joints express fairly good cathetal relation to the bedding planes (Hancodk 1964, 1968). J(I)—J(III) system is orthogonally interlocked. The sets J(I) and J(III) take the position almost parallel and normal to the direction of B1 lineation. Locally the Jm set forms an interlocked system of conjugate shears of a small dihedral angle (Muehl'berger 1961). The fractures of the J(II)—J(IV) system exhibit shear features. They often form en échelon pattern. In relation to the present attitude of strata the sets of joints show different density and morphological features of the surface (Fig. 12). It eliminates the possibility of explaining jointing as result of only one tectonic stage connected with the radio-concentric stress field, which could be a result of an arched lifting of the Palaeozoic rocks caused by the magmatic masses (compare: Dżułyński 1953). So jointing indicates the polyphase development of the geological structure of this area. Strata rotation to the horizontal position round axes of the local structures was carried out in order to reconstruct the primary position of joints and their structural systems (Fig. 12). The rotation was carried out twice, taking into account two phases (F1 and F2) of rotation. After the rotation, joints show a fairly good special arrangement (Fig. 12). Means of sets concentrate on the periphery of diagrams. It indicates the existence of a network of master joints. The best developed sets are those which at present take the position almost parallel to the strike and dip of strata. They also show the strongest calcite mineralization. Most probably joints originated in the period before the rotation of the Fi system. Their formation was facilitated by accumulation of the elastic energy and existence of weakened surfaces in the place of future discontinuities (Price, 1956, 1966; Secor, 1965). There is also a possibility that jointing origined in a stress field of „gravital — normal” type which preceeded the F1 folding. A significant evolution of the stress field occured during the development of the F1 and F2 deformations. As a result of this evolution joints carry some features synkinematic to those deformations. The opening and mineralization of jointing could happen during the folding or later; most probably after the F2 stage.

Shears and minor faults (Fig. 14 and 15) have generally W—E strike and form a system of normal dip-slip configuration of kinematic features. Most probably they can be related with the F1 tectonic stage. The connection of fault tectonics with the N—W structural direction is very well expressed. This is represented by sets of strike-slip and oblique slip faults having the NNE—SSW direction (Fig. 1, 14, 15 and 18). These faults form en échelon pattern of the NW—SE or NWW—SSE axis and dextral turn of dislocations (Fig. 18). Probably they have steep surfaces. Such arrangement of dynamic features of faults comes from observations of slip structures, slicolites and feather cleavage accompanying dislocations. Cartographic data corroborate the mesoscopic observation (Fig. 1 and 18). Shears and minor reverse faults of meridional trends occur locally. They are correlated with horizontal compression in the W—E direction. Probably they represent F2 deformations developed after the rotation of strata.

Slickenside structures are presented together with slicolites. Considerably wide dispersion of planes on which slickenside structures and slicolites appear is characteristic. Tectonic transport in the W—E direction is dominant, especially among slickenside structures which occur on the bedding surfaces (Fig. 16). Most probably it represents lineation of F2 deformations (mainly unit С and D). Meridional tectonic transport marked by the subhorizontal scratches on the steep planes having meridional strikes is also of significant importance (Fig. 18). It is probably connected with NNE—SSW strike-slip faults.

Stylolites form a population consisting of two groups. Stylolites of the first group occur more often and they appear on the interbedding planes. Their rods are normal to the bedding (Fig. 17). This group of stylolites originated in the period before the rotation of strata. Stylolites of the second group arranged parallel to the bedding are less frequent (Fig. 17). Their origin can be related with the F1 folding as a result of compensation accompanying fold deformations (Choukroune, 1969).

Tectogenesis of the Palaeozoic rocks of the Dębnik area is a multistage process (Bogacz 1977). Normal dip-slip field stress preceding the folding. The origin of stylolites of the first group can be related to the period from before the rotation of strata during F1 folding. The stylolites originated in the stress field of ,,gravital-normal’’ type ([delta]1 — vertical) by dissolving under the pressure of the cap-rock. Most probably lots of small discontinuities of normal dip-slip nature can be associated with that tectonic period. In some places these discontinuities are synsedimentary.

Stage F1
F1 tectonic stage had probably various stress fields. F1 folds originated under compressive regime ([delta]2 almost horizontal, oriented NWW—SEE; [delta]1 — horizontal, oriented NNE—SSW). Small dip angles are characteristic for these folds which represent flexural-slip type of folding. Stylolites of the second group resulted from the compensation during the F1 folding. Some slips on the bedding planes mark a movement of layers one towards the other during the F1 folding. Jointing and a part of cleavage originated also in this stage.

Stage F2
During the next tectonic stage there were formed flexure-fault structures F2 of meridional trend. They are represented by:
— a system of strike-slip and oblique faults
— deformations of the flexure type
Vertical movements associated with subsurface migration of magmatic masses played an important role in the formation of structures of that system (Kozłowski 1955, Zajączkowski 1964, Bogacz 1977). They caused a concentric rotation of strata and of structures existing earlier (compare: Dżułyński 1955). The magmatic intrusions are responsible e.g. for formation of the Dębnik brachyanticline and for the present joint pattern. But the activity of intruding magma was controlled by already existing geological structure which had originated in the Fî and partly F2 periods. It leads to the reinforcement of the effects of the F2 stage during which the magmatic intrusions took place. The faults of NNE—SSW set originated while the [delta]2 were in vertical position, probably in a compressive mechanical regime. The pair of forces of the NW—SE or NNW—SEE trends and dextral turn participated in this process. These faults can be interpreted as a set of low-angular, homothetic shears R (Riedel) or as the set T. Cleavage in the form of high-angular shears R (conjugate Riedel) occurs in the vicinity of faults. This cleavage could use the already existing discontinuities of fold cleavage Fj. Locally (units С and D) there existed a stress field of „thrust” type having a subhorizontal principal stress alt oriented W—E. Stress [delta]2 was in the same direction as lineation B1. The reverse attitude of strata and compressive translational structures in the unit D1, as well as the conjugate shears and minor reverse faults of meridional trends are results of the activity of that field. The Czatkowice flexure (unit D) probably originated as a deformation connected with a fault (Bogacz 1977). Later on this deformation was modified by ,,thrust” stress field exerted by intruding magma. Fold deformations of the F2 system lead to the reorientation of b1 lineation. But the reorientation is limited to the areas where F2 deformations were very intensive.

Stafe F3
F3 system has the same direction as the F1 folds. It is represented by the flexural bend of strata in the narrow zone close to the Czernka dislocation (unit E). This bend can be interpreted according to Trevisan’s model (fide de Sitter 1959) and Mattauer (1973) as a deformation derived from the dextral oblique slip dislocations belonging to the NNE—SSW set (Fig. 19). This bend occupies the position of compressive structures in the experiment of Wilcox et al. (1973). Still, this interpretation it not reliable because of rather small range of strike-slip NNE—SSW faults in relation to the size of the total thrust of the Czernka dislocation, and to the size of the flexural bend discussed above (Bogacz 1977). Vertical forces caused by the deep-seated movements of magmatic masses were of great importance during the development of these deformations.

The studied area belongs to the Variscan zone which separates two pre-Caledonian massifs: Małopolska Massif and Upper Silesia Massif (Pożaryski 1974). Natural movements of these massifs were most probably the main tectogenetic factors responsible for the deformations discussed above (Bogacz 1977). Old, deep fractures of the Earth crust having almost NW—SE direction (Dworak, Paproth 1969; Pożaryski 1971; Bogacz 1977) had strong influence on the development of structural phenomena of the Variscan cycle in the studied area. Variscan deformations carry the features characteristic for the structures developed in the cap-rocks of big strike-slip or oblique-slip dislocation zones (Moody, Hill 1959, 1964; Burtman et al. 1963; Pavoni 1961, 1969 Harding 1973; Tchalenko, Ambraseys 1970, Jaroszewski 1972 and others). Considering the problem from the point of view of mesoscopic studies it seems that the Variscan structures of the Dębnik area were developing in the platform regime (Stille 1964, Chain 1974).

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