Fault slip versus slope deformations: Experience from paleoseismic trenches in the region with low slip-rate faults and strong Pleistocene periglacial mass wasting (Bohemian Massif)

Abstract

Successions of colluvia and loesses covering the faults in the eastern and central parts of the Bohemian Massif were exposed in six trenches (two trenches at each fault, up to 6 m deep) and studied in detail in terms of stratigraphy and deformation. Based on OSL and radiocarbon ages these sediments were dated as Weichselian Early Glacial to Younger Dryas/Holocene (∼120 ka to 11 ka). On the Kosíř fault (NW-SE strike), the offset strata bring evidence for Late Pleistocene oblique slip (normal and strike-slip components) on the main fault plane with minimum slip rate in order of 0.1 mm/a. Although the undisrupted topsoil suggests the absence of significant slip in Holocene and no clear evidence of paleoseismic events was found, this fault structure should be included in seismic hazard assessment. Conversely, on the Hluboká and Diendorf-Boskovice faults (NW-SE and NNE-SSW strikes, respectively), the tectonic slip is contradicted for the last 15–23 ka based on dating of undeformed strata sealing the fault planes. Multiple independent evidence suggests that these two faults were not active in Late Pleistocene at least and their prominent scarps is probably largely due to exhumation by differential denudation. Examples given document the mechanisms of slope-related deformations which may lead to destruction of the records of older faulting and occasionally produce deformation structures resembling the tectonic ones.