Collapse wedges in periglacial eolian sands evidence Late Pleistocene paleoseismic activity of the Vienna Basin Transfer Fault (western Slovakia)

Abstract

Studies of seismically triggered deformations in the depositional record contribute significantly to our knowledge of earthquake recurrence over geological time. This paper describes soft sediment deformation structures preserved in a periglacial Upper Pleistocene succession of eolian sand in the eastern Vienna Basin, Central Europe. The strata were formed dominantly by wind ripples migrating across a sand sheet, less frequently by deposition of fine-grained sand and silt from suspension during decreased wind speed and by small-scale eolian dunes. Most prominent deformations observed are collapse wedges, which are interpreted as being formed along dilatational fractures. Further detected deformations include chaotically disturbed strata, folded strata, and a pop-up structure caused by sliding. The tensile fractures are oriented systematically in a prevailing N–S direction, perpendicular to the W–E extension suggested for the transtensional Vienna Basin Transfer Fault, which underlies the study area. Repeated seismic shock is identified as the trigger of the deformations. The mechanism of deformation implies some degree of cohesion within the deformed strata, and this may be attributed to seasonal frost and the presence of vadose water within the upper levels of the sediment. These deformations appear periodically, in 25 distinct horizons of strata internally deformed by collapse wedges, and in 40 horizons if deformation both by collapse wedges and/or chaotically disturbed beds is considered. The vertical distribution of deformations allowed the calculation of the recurrence periods of earthquakes with the use of a Bayesian age-depth model; this, in turn, was based on seven OSL ages. The calculation yielded mean recurrence periods of ca. 100 years, though this figure is biased by a relatively high degree of uncertainty in the dating and in the age-depth model. Even with this caveat, the present study reveals the underexplored potential of periglacial eolian deposits to preserve paleoseismological signals.