Tektonika wału pomorskiego

Abstract

 Omówiono i zilustrowano podstawowe cechy strukturalne wału pomorskiego: morfologię powierzchni

strukturalnych, rozkład uskoków oraz antyklin solnych i niesolnych. Rola tektoniki synsedymentacyjnej, w tym tektoniki solnej, jest większa niż poprzednio przypuszczano. Przeprowadzono podział na bloki tektoniczne, zgrupowane w dwa zespoły. Różnice w ich stylu strukturalnym zależą od charakteru podłoża podcechsztyńskiego. Geneza wału środkowopolskiego,  jak i bruzdy sedymentacyjnej, z której się on wyłonił, odniesione są do ruchliwości strefy Teisseyre'a-Tornquista i przypisane głównie ruchom pionowym, z niewielkim udziałem naprężeń kompresyjnych i składowej przesuwczej. Mogą być one uważane za wynik reakcji sztywniejszej skorupy kontynentalnej na przemieszczenia odbywające się w sąsiednich basenach oceanicznych. Istnieje ścisły związek między etapami ewolucyjnymi prowincji Tetydy i Północnego Atlantyku a blokiem kontynentalnym zachodniej i środkowej Europy.

TECTONICS OF THE POMERANIAN SWELL (NW POLAND)

Review of the tectonics of the Pomeranian Swell (northwestern part of Mid-Polish Swell) is based upon the seismic reflection data and the results of about 50 boreholes piercing the Zechstein-Mesozoic structural complex. Pomeranian Swell is an uplifted, elongated tectonic unit of this complex, about 14000 sq. km in area. Jurassic (mainly Lower Jurassic) strata subcropping there on the pre-Cainozoic surface, contrast with the Upper Cretaceous rocks infilling the surrounding deep troughs.

Various structural features of this unit (configuration of the Zechstein bottom and the pattern of faults – see Fig. 1; alignment of the salt and non-salt anticlines - see Figs. 3 and 4) allow to distinguish within it the two assemblages of tectonic blocks (Fig. 5). The first one of them (Wolin, Gryfice and Kołobrzeg Blocks) is characterized by multidirectional faults, occurrence of Late Triassic-Early Jurassic synsedimentary grabens and half-grabens, the lack of regional salt tectonics, and reduced sequences of the Zechstein and Mesozoic strata. The second assemblage (Czaplinek, Więcbork,

and Piła Blocks) is marked by well developed non-piercement salt domes of WNW-ESE to NW-SE trends. They are separated by the extensive zones out of which the Zechstein salts have been completely or almost completely squeezed. The intensity of salt tectonics increases towards SE, where isolated salt pillows merge into elongated salt swells in which the thickness of tectonically accumulated salts exceeds 2000 m. The differences in the structural style of both assemblages depend on their basement. Whereas the northwestern assemblage is underlain by the block-faulted Devonian-Carboniferous rocks, partly covered by a thin and disrupted veneer of the Rotliegendes clastics, the southeastern one is developed above a labile trough filled in with Rotliegendes shales and siltstones of remarkable thickness. Mid-Polish Swell (and the Pometanian Swell as a part of it) formed at the place of a sedimentary furrow along which the subsidence during the prevailing part of the Mesozoic had been greater than in the adjacent units. In its Pomeranian segment, the total thickness of the Zechstein and Mesozoic sediments attained 4500 m in the northwest, and more than 7000 m in the southeast (see Fig. 7 for the Zechstein through Lower Cretaceous thickness). This mean thickness had been locally modified by the salt flowage, initiated during the Late Triassic and having manifested itself throughout the rest of Mesozoic. As a result, in the early Late Cretaceous a furrow with well developed salt domes existed there in the central and southwestern part. At that time (Coniacian – Santonian?) a general inversion of the unit began, followed by erosion, initially intermittent and subaqueous. Culmination of uplift and the beginning of subaerial erosion fell on the earliest Tertiary (Early Palaeocene? Palaeocene ?). Vertical uplift of the Pomeranian Swell can be estimated at about 1500 m in its northwestern part and more than 3000 m in the southeastern part.

The origin of both the Mid-Polish Furrow and Mid-Polish Swell is clearly connected with the well-known Teisseyre-Tornquist tectonic zone (T – T Zone). Being an old and major lineament, it constituted during the discussed period a direct structural link between two geotectonic provinces: the North Atlantic and the Western Tethys (Fig. 6). At the times of the Pangea break-up, a roughly triangular crustal block has been separated by this zone from the stable East European Precambrian Platform. It was bounded from the northwest and south by the above geotectonic provinces. Geological events, such as rifting, ocean-floor spreading, ocean closure and mountain building, as well as the state of tectonic stresses in both these domains must have influenced the geological evolution of the area lying between them. Actually, the Late Triassic – Early Jurassic tectonic activity were there a weak response to the then incipient rifting in both domains. Later periods of activity (earliest Middle Jurassic, latest Jurassic –earliest Cretaceous) broadly coincided with further spreading phases in the the state North Atlantic and the Tethys. Towards the end of Mesozoic a substantial change to the compressional regime took place in the latter province, having caused a reorientation of stresses and inversion movements within the basins of its northern forefield (Fig. 8). These movements began almost synchroneously with the first (pre-Coniacian) major folding in the Eastern Alps and Western Carpathians, while their peak in the earliest Tertiary did not coincide with any marked orogenic phase in the Western Tethys.

Inversion processes were due mainly to the vertical uplifts. There is no evidence for a regional compressional crumpling (holomorphic folds). Reverse faults which occur particularly in some more southern areas, are local features which may have originated at the boundaries of the basement blocks as a result of their differential vertical displacements with minor compressional or strike-slip components.

Inversion was then probably an effect of subcrustal isostatic flow coupled with small-scale transcurrent movements within the continental lithosphere, as the latter tended to adjust itself to the displacements in adjoining oceanic basins. Transcurrent movements advanced partly along the lines inherited from the earlier Late Hercynian dextral shears. They may have resulted both from the transmission of compressive stresses from the Tethys and from the differences in spreading velocity between the various segments of the North Atlantic.

Mid-Polish Furrow was the most persistent and most strongly subsided unit among the similar Middle European basins. Mid-Polish Swell in turn is the most extensive, most prominently uplifted and deeply eroded element among the inverted highs which emerged from within these basins (Fig. 8). Situation of both these units in the narrowest place between the T – T Zone and the from of Hercynian foldbelt seems to be a decisive tectonic agent in this respect.