The Kock Fault Zone as an indicator of tectonic stress regime changes at the margin of the East European Craton (Poland)

Abstract

Integrated tectonic interpretation of seismic data and core samples from boreholes in the vicinity of the Kock Fault Zone (KFZ) allowed us to identify several tectonic deformation events that were responsible for creating its complex structure. The KFZ is an example of a mechanically weak regional-scale tectonic structure that accumulated deformation over hundreds of millions of years and therefore is a good indicator of stress regime changes in a broader area. The KFZ is here regarded as a combination and superposition of two genetically and temporally different faults: the older Kock Fault, which is an inverted normal fault, and the younger, low-angle Kock Thrust. The first, Silurian stage of KFZ evolution occurred in a tensional stress regime that gave rise to the activation of a deeply rooted normal-slip precursor to the Kock Fault. Subsequently, this fault underwent inversion during the Late Famennian compressive/transpressive event. In the Early Carboniferous, the tectonic stress regime changed into tension/transtension, leading to extrusion of basalt magma and abundant mineralisation in the vicinity of the inverted Kock Fault, followed by tectonically relaxed sedimentation of Carboniferous strata. The deposition was terminated by a compressional event at the end of the Westphalian. Contraction resulted in the formation of the low-angle Kock Thrust decoupled in Silurian shale that cut across the upper part of the Kock Fault and displaced it towards the NE, over the East European Craton foreland.