Tektonika południowego Bałtyku

Abstract

Tektonika zachodniej części polskiego sektora Bałtyku jest wynikiem kilkufazowych deformacji. Front fałdowań kaledońskich, na lądzie polskim związany ze strefą Teisseyre'a-Tornquista, na skraju akwenu oddziela się od niej, biegnąc ku WNW. Strefa T-T zmierza ku NW, będąc na tym odcinku odmłodzonym prekambryjskim rozłamem wewnątrzplatformowym. Deformacje kaledońskie mogły być rezultatem skośnej kolizji małych bloków skorupowych (terranów?) pochodzenia bałtyckiego lub gondwańskiego z kratonem wschodnioeuropejskim. Podział obszaru na zuskokowane bloki dokonał się na przełomie karbonu i permu, zapewne również w warunkach transpresyjnych. Po wczesno mezozoicznym okresie tensji i transtensji, związanej z otwieraniem północnego Atlantyku, niektóre uskoki zostały odmłodzone także w środowisku transpresyjnym. Udział składowej przesuwczej z biegiem czasu wyraźnie malał.  Tectonics of the southern BalticMultiphase deformational history of the western part of the Polish Baltic sector was much more complicated than that of its eastern part. The latter is situated on a stable East European craton (EEP) while the former belongs to the younger and more mobile Central European Platform. Main tectonic features of the discussed western Polish sector are presented in Fig. 5. Figs. 1-3 are compiled in successively larger scales after the earlier works, and -show the area on a broader, regional background. Caledonian deformation front (CDF) which in the Polish onshore areas coincides with the Teisseyre-Tornquist Tectonic Zone (TTZ), separates from the latter just at the Baltic coastline, and runs towards WNW north of Rugen and south on the Ringkobing - Fyn High (RFH). This bifurcation has been confirmed both by the results of two offshore boreholes, and by the occurrence of the O sesmic reflector (representing probably the top of the Precambrian basement) southwest of the TTZ. TTZ extends nortwestwards as an intracratonic fracture rejuvenated during the Mesozoic. It has been recently proposed (Eugeno-S ... , 1988) to call this segment Sorgenfrei-Tornquist Zone (STZ) because of its different crustal character in comparison to the TTZ. Strongly folded Early Palaeozoic rocks may have been even thrusted over the edge of the EEP. These deformations were most probably a result of oblique collisions of minor crustal blocks (terranes?), of either Baltic or Gondwanian provenance, with the EEP craton although the evidence for this interpretation is so far rather poor. Devonian-Carboniferous platform sediments reached presumably far to the north covering the whole southern part of the Baltic Shield. Their present post-erosional boundary, as well as the subdivision of the area into many tectonic blocks resulted from the latest Carboniferous-earliest Permian movements in the northern forefield of the European Hercynian foldbelt which were preceded by the smaller scale deformations of the similar style at the end of Devonian and Early Carboniferous. The Late Hercynian tectonism manifests itself in the significant vertical displacements along faults, sometimes exceeding 1000 m (Fig. 6). It has been assumed (F. Arthaud, P. Matte, 1977) that these movements operated in the field of regional, right-lateral shear and produced a system of conjugate faults: right-lateral trending NW-SE and left-lateral trending NE-SW. Early Mesozoic was a period of minor tensional (transtensional?) stresses which caused in our area the initial salt flowage into salt anticlines at its southern rim, as well as the formation of narrow, asymmetric, V -shaped, synsedimentary grabens, probably rooted in single sub-Zechstein faults. Present tectonic picture of the area was ultimately formed during the latest Cretaceous inversion of Mid-Polish Trough. The northwesternmost branches of the then uplifted Mid-Polish Swell extend into the offshore area as two anticlines with the Jurassic strata in their cores. These are the Kołobrzeg and Kamień Pomorski Anticlines built, respectively, over the Kołobrzeg and Gryfice Blocks. Their uplift was mainly connected with the rejuvenation of the earlier, syn-Hercynian faults of Kamień Pomorski, Trzebiatów and Koszalin. These faults converge northwestwards in the vicinities of the southern end of Ronne Graben. Crestal parts of both anticlines are bounded from the east by subordinate faults forming some sort of slightly marked horsts. Mainly in these crestal parts the clusters of smaller, strongly faulted brachyanticlines have been recorded in the Zechstein and Triassic seismic reflectors. Influences of the earlier tectonic structures are recognizable in some Mesozoic features as, for example, in a scissor-like pattern of the Kamień Pomorski and Adler Faults, and in the disturbances of courses of the Mesozoic grabens. The inversion movements, as elsewhere in Central Europe, are claimed to be a response to the orogeny in the Alpine domain. Transpressional stress field has been invoked (P.A. Ziegler, 1987) although the strike-slip component must have been much weaker than during the Hercynian epoch. Full text of this work will appear in English in the series Geology of Poland, Polish Geological Survey Bulletin, vol. IX.