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1 Acta Geologica Polonica

1 2008

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Cenozoic tectonic evolution of the Poznań-Oleśnica Fault Zone, central-western Poland

Widera M., Ćwikliński W., Karman R.

Acta Geologica Polonica

2008

Vol. 58, no. 4

455-471

The Poznań-Oleśnica Fault Zone (P-OFZ) of central-west Poland is an over 150 km long, up to 10 km wide, and up to 200 m deep graben that developed during Early Oligocene to Pliocene times on the flank of the NE-dipping Fore-Sudetic Homocline. Fault systems of this extensional structure appear to reach pre-Zechstein basement in the area ofthe Fore-Sudetic Homocline that forms an integral part of the Bohemian Massif. The P-OFZ was affected by several stages of subsidence, separated by periods of inversion and/or tectonic quiescence between the Triassic and Cenozoic. Structurally, this dislocation zone can be linked with the Variscan faults, reactivated at that time. During the second half of the last century, the northern parts of the P-OFZ graben were intensively explored by cored boreholes because of their lignite content. Conventional back-stripping methods could not be applied to the tectonic analysis of the POFZ due to the limited thickness of the graben fill, its poor dating and the presence of relatively thick lignite seams. Therefore, phases of accelerated subsidence were determined by comparing the thickness of individual lithostratigraphic units within the graben and on its flanks. The total subsidence of the different graben segments was quantified by determining the aggradation coefficient (AC) and by taking the consolidation coefficient (CC) of lignite seams into consideration. Subsidence analyses indicate that the initial Early Oligocene extensional phase of the P-OFZ was followed by an Early.Middle Miocene extensional subsidence phase and a probably latest Miocene.Pliocene final extensional phase.