Model ekspansyjnego rozwoju Sudetów Zachodnich w paleozoiku (propozycja)

Jerzy Don



Studies on evolution of the Sudety Mts show the lack of space and time correlation of geological processes affecting units nowadays separated by the Intra-Sudetic Fault (15, 16). The units differ on both history of facies-structural development and tectonics. In areas situated SW of the fault, major foldings and accompanying metamorphism have taken place in the Caledonian orogeny (D1), that is in times when sedimentation under geosynclinal conditions was still continuing in areas situated NE of the fault. In the latter areas rock series were folded and partly affected by epizonal metamorphism not before the Sudetic phase, before the Late Carboniferous. It follows that the fault divides the Western Sudety Mts into Caledonian Kłodzko - Izera - Lusatia and Hercynian Bardo - Kaczawa - Zgorzelec segments. In the former geological units are elongate and longitudinally (with some deviation to SW - NE) oriented, and in the latter they mainly show SE "Sudetic" direction (Fig. 1).
The question how the two nowadays juxtaposed segments could develop in different ways during so long time interval (i.e. from the Eocambrian to Permian, inclusively), is highly intriguing. This is because of the fact that a possibility of large horizontal off sets along the above mentioned Intra-Sudetic Fault (15, 16) is precluded as the available data speak against its extension into an area of transversally folded Eastern Sudety Mts. The paper presents an attempt to find a model which would give the most plausible explanation of so large differences in evolution of the nowadays juxtaposed segments. A part of that model has been recently proposed by Brause (6) in his reconstruction of development of geosyncline and tectogene of the Zgorzelec Slaty Mts (Gorlitzer Schiefergebirge).
The model (Fig. 2) also well (or even better) explains evolution of the Kaczawa geosyncline. It shows break-up of Proterozoic basement into blocks (microcontinents) gradually shifting apart from one another. The rift-type geosynclinal basin originating between the blocks became the site of sedimentation of younger and younger Paleozoic series. An opposite process started by the end of the Devonian and intense foldings accompanied by epizonal metamorphism of the strata took place in the Middle Visean. The model well explains a jump in metamorphism at the Intra-Sudetic Fault as well as presence of diabase-spillite series related to Cambrian initial volcanism in the Kaczawa geosyncline, and the lack of granitoids in the geosyncline, despite of their intense development in the neighboring Caledonian segment at that time (Fig; 3). However, the former segment did not behave passively but it was at the same time subjected to an advanced rebuilding which was taking place from the Cambrian to Permian in relatively stable stress field (different from that of the rift area).
This is shown by latitudinal elongation of structural units of that segment, differing from one another in age (Fig. 1): from Cambrian age of Doberlug Syncline to Lower Permian age of series in southern Karkonosze Mts and Intra-Sudetic Trough. The contrast in tectonics of the two segments becomes especially well visible when we compare latitudinal elongation of Lower Carboniferous Doberlug-Kirchhain Basin and similar elongation of Świebodzice Depression (filled up with thick molasse series) with folds and scales of coeval flysch series in the Hercynian segment, linear, elongate and NW - SE oriented.
As it was mentioned above, the stress field remained stable from the Cambrian to Permian but different in each segment. This has to be due to some global factor and such factor should at the same time be responsible for differences in evolution of the segments. This requirement should be taken into account in modifying the Brause model (Fig. 2) to explain also evolution of the Kłodzko - Izera - Lusatia segment. It is surprising but the only model matching relatively well the above features and at the same time explaining the majority of apparent discrepancies is that assuming expansion of the Earth (26 and others) in a pulsatory way (8 and others). Authors of that model suggest a mechanism of folding related to tangential compression and explainable in terms of adjustment of ancient litho, sphere to a decrease in curvature of the Earth in result of its expansion. The process, shown in two-dimensional way in the cross-section (Fig. 4A - A'), has to be projected on vast massif which was becoming subjected to radial fracturing (Fig. 4B) and a break-up in the course of its adjustment to a new curvature of the globe. The break-up proceeded from its marginal parts to the center, leading to disintegration of external parts into segments gradually shifting a part from one another (= Brause model; Fig. 2).
The increase in separation was accompanied by a rise of compression directed along longitudinal axes in a given segment and leading to its transversal folding (Fig. 4C). This was accompanied by origin o( a tension in deep- seated parts of the segment, leading to initiation of granitization processes. The latter processes were gradually spreading on shallower and shallower zones, resulting in increase in volume and, therefore, further increase in compression in inter-dome areas. Diastrophic and granitization-plutonic processes gradually developing in the folded Kłodzko - Izera - Lusatia segment (Figs. 1 and 4C) were not transmitted outside its boundaries, into the area of the Kaczawa rift, opening at that time. The above mentioned time migration of major folding phases in the two segments also appears relatively easy to explain in this model. It follows that the time of the maximum compression in the Caledonian segment corresponds to a peak in geosynclinal conditions in rift-like opening Hercynian segment. The geosynclinal stage in evolution of the latter segment became suddenly broken in fairly energetic way towards the end of Early Carboniferous, which was presumably due to cyclic shrinkage of the Earth (8, 31 ). Diastrophic phenomena affecting the Hercynian segment, labile till the Early Carboniferous, were not transmitted on any regional scale into the area of earlier consolidated Caledonian segment. It should be also noted that elongate tectonic structures of the latter segment are set transversal to direction of subsequent foldings in the Hercynian segment.

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