Direct field evidence for polyphase active faulting and associated coseismic landslides in the accretionary wedge of the Outer Western Carpathians

<p>The Outer Western Carpathians represent an accretionary wedge of the Alpine-Himalayan orogenic zone in central Europe, where the Mesozoic and Cenozoic sedimentary (flysch) rocks were deformed and thrust over the European foreland during the Paleogene and Neogene. The thrusting processes terminated in the Upper Miocene, and the contemporary instrumental earthquake distribution records suggest that OWC are already stabilized and belong to the European Plate. This hilly to mountainous region has been intensively affected by various types of shallow slides, debris flows as well as deep-seated slope failures of different magnitudes. Due to the lack of direct evidence of intense seismicity, the permafrost thawing on a turn of the Pleistocene and intense rainfalls in the Holocene have been considered as their principal triggers. However, our current research revealed landforms attributed to the coseismic Holocene polyphase strike-slip faults&#180; surface ruptures. The associated coseismic sedimentary structures include injected sand and flame structures of fluviolacustrine sediments, large angular boulders in riverbed fluvial sediments, etc. Alongside with those structures and landforms, we documented also evidence for coseismic slope failures in the close surrounding of the faults near Lide&#269;ko Village and in the summit area of the Javorn&#237;ky Mts. in the E part of the Czech Republic and north-western Slovakia, respectively. Several distinct faulting phases with offsets up to a few meters were dated from ca. 10.000 ¹⁴C ka up to 1240 ¹⁴C ka BP. Morphological analysis of LiDAR digital elevation models with a 1 m resolution revealed clustered populations of prehistoric presumably coseismic landslides that accompany the fault traces at both sites for a distance up to a couple of kilometres. Field inspections allowed description of their source zones, which were usually several meters up to first tens of meters deep and up to 500 m long. The landslide masses travelled for a distance up to 1-2 km, while being often transformed to debris flows and rock avalanches. They are characteristic with their distinctly depleted source zones at generally gentle slopes ranging from 8 to 20&#176;. The contribution presents the faulting styles, particular events and associated coseismic landslide characteristics in detail and broader regional context, providing the first comprehensive evidence for possible coseismic origin of the deep-seated slope failures in the Outer Western Carpathians.</p><p>The research was supported by the international bi-lateral project &#8220;Earthquake-triggered landslides in recently active and stabilized accretionary wedges&#8221; of the Czech Science Foundation (GA&#268;R 22-24206J) and the Taiwanese Ministry of Science and Technology (MOST 111-2923-M-008-006-MY3).</p>