Zagadnienie analizy strukturalnej w Sudetach, jej cele i metodyka

Abstract

Structural analysis in the Sudetes Mts. A complete structural analysis of the area of the Sudetes Mts should comprise studies of macrostructures and mesostructures, supplemented by microstructural investigations of selected areas. A detailed mapping of macrostructures in the Sudetes Mts. is progressing rapidly. Less work has been done on mesostructures, while microstructures have been investigated only in few points.' Studies of mesostructures carried out in all metamorphic regions of the Sudetes Mts. were often spotty and non-methodical. Such studies require meticulous investigations of the character and form mesostructures in relation to the competency of rocks subjected to these deformations and to metamorphic crystallization. The determination of the origin of these discontinuities is of primary importance. The age of mesostructures can not be determined in metamorphic series of unprecised stratigraphic position. Even in such cases when the stratigraphic position of the deformed rocks is exactly known, the determination of the age of deformations is not easy, as the age of the rock does not define the age of deformations affecting this rock. The discontinuities present in rodks can be much younger than the rock itself. In the opinion of the present author all attempts to date the age of mesostructures in old crystalline complexes of the Sudetes Mts. are premature and unjustified, as no supporting argumentation is presented. Both linear structures and planar structures are present among mesostructures in the Sudetes Mts. The linear structures may be divided into folds (F) and lineations (L). They are regarded in the Sudetes Mts. as corresponding to the kinetic B-axis. Two or three system's of linear mesostructures are present often in the metamorphic complexes of the Sudetes Mts. Usually they are regarded as differing in age. More than three intercrossing systems of mesostructures can be observed locally, chiefly in rocks of low competency, in which even very small stresses can be recorded. Planar mesostructures studied in the Sudetes Mts comprise bedding, foliation and secondary cleavage. As a rule, foliation is parallel to sedimentary surfaces. The present writer proposes the symbol SQ for this foliation. The secondary cleavage, usually related with the principal lineation, is of the fracture cleavage type, or of the strain-slip cleavage type. Axial plane cleavage is also present. Joints, also studied in the Sudetes Mts. form another type of planar mesostructures. DIFFICULTIES IN DETERMINING THE SEQUENCE OF LINEAR STRUCTURES OF DIFFERENT AGE The age and sequence of linear structures can be determined approximately in Early Palaeozoic formations of the Sudetes Mts. Instead, the old crystalline complexes regarded generally as Pre-Cambrian, even the sequence of two intercrossing sets of linear structures remains often obscure. An example is provided by linear mesostructures in shists and gneisses of the Mt. Śnieżnik massif, studied by the present writer. Three systemes of minor folds and lineations, differing in age in the opinion of the present author, occur in this area. Locally they are accompanied by fracture cleavage showing th re e different directions (Fig, I, Fig. 2 and Fig. 3). The principal system of minor folds and lineations is parallel to the axes of macrostructures, and, generally speaking, has a north-south direction. The present author designated formerly this system by the symbol Bu regarding it as the oldest set of deformations. The line ar structures of this system are traversed by minor folds and lineations striking NW-SE, which were formerly designated by the symbol B2. Small sharp folds of the third system, present locally, are striking W-E. These folds have generally the character of rigid deformations of the kinkband type. The succession of the three sets of mesostructures outlined above proved doubtful in the light of more detailed studies. These studies revealed details of the structures which suggest that the N-S system is probably younger than the mesostructures of the NW-SE system. In th e majority of cases studied (47 out of 50) the N-S lineation yields curved lines after the development of folds of the NW-SE system. According to the generally known rules this indicates that the lineation is younger than the traversing folds. The N-S lineation is syngenetic with a partial granitization of the schist series. One of the phases of this granitization is recorded by the appearance of albite blasts. The albite poikilofolasts of same paragneisses display well preserved relicts of wrinkling belonging to the NW-SE lineation (Fig. 5). Also this detail speaks against the conception set forth previously by the author. However, the folds of the NW-SE system are often very complicated. Joints, concordant with axial planes appear, and are accompanied by quartz veins, and occasionally by irregular quartz nests (Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, and Fig. 11). Some folds of the NW-SE system pass into structures of kink-band type, which are characteristic for a late stage of deformations (Fig. 6). This evidence is tentatively explained in the following way: the NW-SE fold system was formed during the first phase deformations. The second deformation was associated with granitization of the schists and produced the linear mesostructures of the N-S system. During a subsequent, third deformation phase a rigid overprint of the folds of the NW-SE system 'took place (joints, quartz veins and nests, formation of kinknband structures). Such an interpretation is also supported by the observation of disruptions of the N-S lineation on the joints, and by the presence of strong deformations along the quartz veins associated with folds of the NW-SE system. Assuming th a t all phases of formation of the NW-SE folds precede the N-iS lineation and granitization, it should be concluded that the fractures disrupting the NW-SE folds and the associated quartz veins and nests survived the granitization processes; however this conclusion seems improbable. Further studies are required for a definite solution of the problems outlined here. In order to avoid numeral symbols determining the sequence of deformations, the present author proposes the symbol Ls for the lineation of the N-S system and the symbol Fs for the folds of this system (mesostructures typical for the Mt. Śnieznik massif). The folds and lineations of the NW-SE system are denoted Fk and Lk, as these mesostructures are characteristic for the Krowiarki range. The youngest structures of the kink-band type (striking E-W) and the associated lineation are denoted Fw and Lw, the subscript „w” for the Wilczka stream, where these mesostructures were discovered first. REMARKS ON JOINTS IN THE SUDETES MTS. Studies of joints are currently carried out in the Sudetes Mts. The author measured c. 25 000 joints in the Mt. Śnieznik massif and in the eastern part of the Kaczawa Mts. The principal results are outlined below. 1. In the eastern part of the Kaczawa Mts. the most frequent joint directions are concordant with large faults or with minor local dislocations (Fig. 12, Fig. 13, and Fig. 14). 2. The joints are often predisposed by small fissures perpendicular to the lineation (a—c fissures), secondary cleavage, and sometimes by poorly marked foliation. 3. The maxima of joint direction often form belts on cumulative diagrams; this can be explained by rotation of stress during the deformation. 4. The cumulative diagrams of joints, even those constructed for the same rock types and for neighbouring outcrops may differ greatly in details. 5. Striae on fault-slip planes are most often sub-horizontal or slightly inclined. Steeply inclined striae are rare (Fig. 15). 6. Joints and small faults striking N-S or NNE-SSW are especially frequent in the eastern part of the southern branch of the Kaczawa Mts. These directions are corresponding with the directions of Q joints determined by H. Cloos ((1925) in the eastern part of the Karkonosze granite massif. 7. Significantly, 70 per cent of faults mapped by the author in the eastern part of the southern branch of the Kaczawa Mts. lie between the azimuths 350°—20°. It seems that th e dislocations and joints having these directions form the major disjunctive deformations in the Western Sudetes Mts.