TRIASSIC-JURASSIC EVOLUTION OF THE POMERANIAN SEGMENT OF THE MID-POLISH TROUGH - BASEMENT TECTONICS AND SUBSIDENCE PATTERNS

Abstract

Based on reflection seismic data, a regional tectonic model was constructed for the sub-Zechstein basement of the Pomeranian (NW) segment of the Mid-Polish Trough (MPT). This model is based on the concept that the thick Zechstein salts acted on a basin-wide scale as a mechanical decoupling layer during the Mesozoic evolution ofthe MPT. Due to this regional decoupling effect, Mesozoic extensional faulting was mostly restricted to the sub-Zechstein salt basement whilst normal faulting played a subordinate role in the Mesozoic syn-extensional sedimentary series characterized by gradual lateral thickness changes. Locally, normal faulting affecting Mesozoic series triggered the development ofsalt diapirs. Mechanical decoupling ofMesozoic series from their pre-Zechstein substratum played also an important role during the Late Cretaceous-Paleogene inversion of the Mid-Polish Trough. Taking into account: 1 - the location of Mesozoic thickness gradients, 2 - the structural configuration ofthe sub-Zechstein basement, and 3 - the location of salt structures, a tectonic map was constructed showing the inferred sub-salt fault zones that were active during the subsidence and inversion of the Pomeranian part ofthe MPT. A high degree of correlation was achieved between the seismically mapped regional sub-salt structural patterns and magnetic and gravity features, as well as the main inversion structures. Moreover, a very good correlation was established between the inferred basement fault zones and the gross thickness patterns ofthe Triassic-Jurassic successions. The NE boundary of the MPT was generally controlled by the SW margin ofthe East European Craton, whilst its SW boundary coincides with a system of fault zones most probably inherited from earlier tectonic phases. Contrary to previous hypotheses, there is no evidence for important strike-slip faulting transverse to the main axis of the Pomeranian segment of the MPT.