Rola bloków litosfery i ruchy przesuwcze w przedmolasowym rozwoju waryscydów na brzegach Masywu Czeskiego; uwagi i implikacje

Zbigniew Cymerman



Oberc (48) has recently presented the new model of the Variscan evolution in Central Europeon the Bohemian Massif margins. In this model a dominant role play lithosphere blocks and strike-slip movements. The purpose of this paper is discussion the Oberc's version of Variscan belts evolution on the Bohemian Massif margins. The most discrepancies (in my opinion) concern Oberc's assumptions about geotectonic models, used by him structural basis, the age and a number of deformation stages, and regional implications.
Oberc explained the development and structure of the central European Variscides on the basis of the blocks model in connection with the theory of expanding Earth. As a result of increasing of Earth wavelength there are "keyboard uplift blocks". This pushing out of blocks is due to increase of dip angles of a upper surface of astenosphere. Increasing of the dip angles are due to a gravitational displacement of blocks on sloping surfaces of astenosphere. According to Oberc this almost horizontal displacements of "gravitational spreading" type caused folding of the content of geosynclines, which are between blocks of the Cadomian orogenesis.
The most discrepancies to the Oberc's geotectonic model are as follows: l. To reach "gravitational compression" there must be a great angle of dip on the upper surface of astenosphere. However, it is difficult to believe that with uplifting of litosphere together with astenosphere blocks take place a significant putting out of these blocks. A more likely alternative is that adjustment of lithosphere to a decrease in curvature of the Earth, like in Don's model (21). 2. In order to overcome internal resistances in Cadomian blocks a "gravitational flow" has to be relatively fast. Such fast gravitational flow caused only one generation of tectonic structures. With sequence of complex and several stages of Variscan deformation there must be assumed that there were also several phases of gravitational flow. I consider it unlikely.
Oberc has rejected subduction and thus a classical version of plate tectonics model of the European Variscides as unproven. In Variscan belts are several zones subduction, however. Segments of oceanic crust are present also in Sudetes Mountains (SW Poland), perhaps in form of series of oceanic basins. The fragments of ocean crust could undergo obduction stage, also.
Reservations has also related with Oberc's criterion of fold axes orientation as principal one for correlation in structural analyses. The use of the criterion of fold axes orientation is generally considered unreliable for correlation purposes. The best criterion is orientation of extensional lineations (oriented parallel to the X-axis of the finite strain ellipsoid, X> Y≥ Z). For that reason the Oberc's suggestion in terms of changing structural plans of the Variscan belts during the seven phases of deformation cannot be accepted on the basis of orientation of fold exes.
Did really there were seven phases of Variscan deformation. It would have been strange that every deformation phase had been marking of different attitude of tectonic displacements. That problems can not examined without timing of particular deformation phases. Controversy has been begun with attemption of definition of significance of the Cadomian orogenesis in the European Variscides. Oberc has been suggested that the Cadomian orogenesis deformed and metamorphosed the only mezozonally metamorphosed geological units (for example in Sudetes). In the Sudetic Mts. are similarity in style and deformation sequences between Variscan geological units (for example epimetamorphic Kaczawa Mts.) and units (terranes) mezozonally metamorphosed (for example the metamorphic complex of Śnieżnik Mts). Additionally a large number of data (among another radiometrical dating) indicate gradual transitians from "Caledonian" to Variscan metamorphic events. Without geochronological age dating, differentiating of these orogenic phases is exceedingly difficult if not impossible.
Results of geophysical study in Western Europe show a subhorizontally dipping to south thrust sheets or planes of nappes. According to this data it is difficult to accept Oberc's model of blocks, which are bounded by vertical, deep faults downing to astenosphere. Any scenario for the tectonic history of European Variscan is necessarily speculative.

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