The Late Alpine structural development of the Kleszczów Graben (Central Poland) as a result of reactivation of the pre-existing, regional dislocations

Abstract

The Kleszczw Graben forms the easternmost part of the Tertiary graben system, which occurs on the North European Paleozoic platform, in the foreland of the Polish Carpathians. It is concluded that structural features of the graben, and its development during the Tertiary-Quaternary, were strongly influenced by a pre-existing fault pattern within the Paleozoic-Mesozoic basement. Tectonic analysis of the drill hole data revealed the occurrence of two main fault sets reactivated or established during the Kimmeride and Laramide phases. One of the fault sets trends NE-SW to ENE-WSW, and the second trends NW-SE to WNW-ESE. Most of the faults were formed as branches of two distinctive dislocations of regional extent: the Kluczbork-Bechatw Fault (KBF) and the Holy Cross Fault (HCF). Our reconstruction of the Late Alpine evolution of the Kleszczw Graben was based upon two data sets: large number of boreholes, and detailed field investigations in the open-pit brown coal mine "Bechatw". The Kleszczw Graben developed in three stages. Stage I -The opening of the graben took place in the Late Oligocene-Early Miocene as a result of the reactivation of the WSW-ENE and NW-SE trending, preexisting faults in transtensional regime (the maximum stress axis parallel to WNW-ESE). Subsequently, a succession of pull-apart basins, arranged along the WSW-ENE trend, was formed. This stage ended in the Late Sarmatian-Early Pannonian with a vertical displacement of the Cenozoic series. This was the effect of the movements along the NE-SW trending faults in the extensional regime (maximum stress axis plunging at a high angle). Stage II -The structural rearrangement of the graben occurred during the Late-Middle Pleistocene and was caused by strike-slip movements on the NW-SE reverse faults in transpressional regime (the maximum stress axis along NW-SE). Stage III -The recent reactivation of some dislocations in the Kleszczw Graben area, induced in part by mining works and evidenced by numerous earthquakes, of up to 4.6 in magnitude. Most of these seis-Correspondence to: R. Gotowaa (romgot