Rozwój tektoniczny bajkaIski oraz kaledońsko-waryscyjski przedpola wschodnioeuropejskiej platformy w Polsce

Abstract

Południowo-zachodnia granica starej platformy w Polsce ma charakter rozłamu ryftowego, który powstał w epoce tektonicznej postgotyjskiej a prebajkalskiej. Na przedpolu utworzyła się geosynklina bajkalska z odchodzącym od niej ku NE aulakogenem wołyńsko-orszańskim. Na obszarze konsolidacji bajkalskiej powstał w kambrze nowy system ryftowy z siecią aulakogenów. Aulakogen krakowski i świętokrzyski trwały do końca epoki waryscyjskiej i uległy sfałdowaniu.THE TECTONIC DEVELOPMENT OF THE POLISH PART OF EAST-EUROPEAN PLATFORM FOREFIELD IN THE BAIKALIAN AND CALDONO-V ARISCAN EPOCHSThe magnetic and geological data show that the structures of the East-European Platform basement reach the boundary of magnetic provinces at different angles and they do not extend south-west of it but are rather cut by it (Figs. 1-2). The boundary follows a broken course and well agrees with the model of break-up and divergence of the continental crust along rifts. This is how the East-European subcontinent originated. The western section of . the subcontinent brokke off and its relics are at present represented .by several microcontinents incorporated into younger orogenic belts. The rift break-up is dated at the post-Gothian, Dalslanidian or Early Baikalian epoch.The Early Baikalian-Małopolska geosyncline and Volhynian-Orsha aulacogen representing an abandoned arm of triple junction, developed in the southern Poland (Fig. 3). The orientation of the aulacogen in relation to geosynclinal margin well matclJ.es the relevant models (K. Burke and J. F. Dewey, 1973; P. Hoffman, J. F. Dewey. K. Burke, 1974; P. B. King, 1975). The subsequent evolutionary stages of the geosyncline were connected with compression and folding which resulted in the origin of the Early Baikalian-Małopolska orogen. The orogen formed southern margin of the exogeosyncline which became the site of Cambrian sedimentation.The subsequent, Late Ba1kalian and Caledono-Variscan stage of development of the southern Poland was connected with the activity of geosynclines and aulacogens the location of which is shown in Fig. 4. The Holy Crass Mts aulacogen represented one arm of a triple junction, the Cracow aulacogen – the second. The third arm of the triple junction ran westward, through the Sudety area, wherefrom is known evidence for the existence of oceanic floor in the Baikalian and Caledonian epochs. The axes of the Holy Cross Mts and Cracow aulacogens converge nearby Wieluń, marking the location of the triple junction in the Early Paleozoic. The subsequent stages in the development of that region .comprised several !phases of compression and regional metamorphism.A fourth arm of the triple junction originated in the Devonian, resulting in spreading responsible for separation of the western · Sudety Mts and Upper Silesian massif. This was the site of deposition of geosynclinal rocks of a part of the Moravo-Silesian zone running arcuately from Opole to Poznań and later turning west and converging with the Rheno-Hercynian zone in the northern forefield of the. Harz Mts.During the Carboniferous took place subduction of oceanic plate plunging northward beneath central-European microcontinents locally welded by Paleozoic fold zones. Southward dip of Benioff zone was recorded by the origin of a long belt of Late Variscan granitoid intrusions. The belt stretches from Zulova and Strzelin on the east, through the Middle Odra river metamorphic area, to the boundary zone of the Saxo-Thuringicum and Rhenoherzynicum on the west. The Variscan orogen formed by the above discussed processes had no strongly developed foredeep in Poland. Up to the present, there has not been recorded any Upper Carboniferous profile of a greater thickness and there is not much place left for it in central and northern Poland. The older molasse of a large thickness is found much further to the south, in the Upper Silesian massif in the forefield of the Moravo-Silesides. Teleorogenic molasse (W. J. Chain, 1974) originated in Pomerania· and Lublin area, at large distance from the front of the Variscan externides. In turn, the younger, Early Permian molasse with volcanic rocks is found in a wilde belt parallel to the front of the Variscan externides.The other triple junction from the Caledono-Variscan tectonic epoch was situated in the western Baltic at the margin of the East-European Platform (Fig. 4). It gave rise to at least three arms: WNW-ESE oriented, Koszalin-Chojnice, NW-SE oriented, Piła, and a western one. Treating these arms as aulacogens it is possible to suggest the existence of an aulacogen system entering the area of the East-European epi-Dalslandian Platform from the prate-Atlantic side, whereas the existence of the circum-Fennoscandian branch of the Caledonides (J. Znosko, 1962; R. Dadlez, 1974) seems less probable in this area.