A Signal of Ice Loading in Late Pleistocene Activity of the Sudetic Marginal Fault (Central Europe)

Thursday, 18 December 2014
Petra Stepancikova1, Thomas K Rockwell2, Daniel Nývlt3, Filip Hartvich1,4, Jakub Stemberk1,4, Dylan H Rood5,6, Jozef Hók7, Maria Ortuňo8, Madeline Myers9, Karen M Luttrell9 and Neta Wechsler10, (1)Academy of Sciences of the Czech Republic, Institute of rock structure and mechanics, Prague, Czech Republic, (2)San Diego State University, San Diego, CA, United States, (3)Faculty of Science, Masaryk University, Department of Geography,, Brno, Czech Republic, (4)Charles University, Prague, Czech Republic, (5)University of California, Santa Barabara, Earth Research Institute, Santa Barbara, CA, United States, (6)Scottish Universities Environmental Research Center at the University of Glasgow, East Kilbride, United Kingdom, (7)Faculty of Science, Comenius University, Department of Geology and Paleontology, Bratislava, Slovakia, (8)Faculty of Geology, University of Barcelona, Department of Geodynamics and Physics, Barcelona, Spain, (9)Lousiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, (10)Tel Aviv University, Department of Geophysics, Atmospheric and Planetary Sciences, Tel-Aviv, Israel
We combine paleoseismic trench and cosmogenic dating results to study the late Pleistocene-Holocene history of morphologically pronounced NW-SE trending Sudetic Marginal Fault (SMF) situated at the northeastern limit of the Bohemian Massif in central Europe. Eighteen trenches were excavated at the Bila Voda site to study 3D distribution of a beheaded alluvial fan on the NE block of the fault and to find the offset “feeder channel” that sourced the deposits. We interpret a small drainage of about 40-60 m to the SE of the fan apex as the feeder channel. A 2.5 m depth profile was collected for cosmogenic exposure dating from a well-preserved part of the fan. Using a simple model that accounts for pre-depositional exposure (inheritance) and assuming no surface erosion, 10Be concentrations are well-fit with an apparent exposure age of ~12 ka. However, this is a minimum limiting age if the surface was eroded by gelifluction in the late Pleistocene. Assuming a ~25 ka OSL age for the base of the fan apex it gives a left-lateral slip rate of ~2 mm/yr. As the Holocene deposits do not show significant displacement, most of the recorded slip took place during Late Pleistocene with corresponding slip rate of 2.8 to 3.5 mm/yr. Bila Voda site lies ~150 km south from the Late Pleistocene Weichselian maximum (~20 ka) ice-sheet front. Thus, we hypothesize that the slip rate acceleration was due to ice-loading and subsequent unloading during deglaciation. To test this, we calculated the stress induced in the lithosphere from ablation of the Weichselian ice sheet modeled as a flexing elastic plate. Preliminary modeling results indicate that complete deglaciation alters the stress field such that it would inhibit left lateral failure on the SMF, consistent with observations suggesting no slip occurred during the Holocene. Although the SMF is ~150 km from the Weichselian ice sheet front, it is well within the flexural rebound area of the ice sheet, causing normal stress on the SMF to vary by ~1 MPa from deglaciation. Our model is consistent with loading mechanisms reported from other regions due to deglaciation, but is the first documented evidence in central Europe, having a great implication for seismic hazard assessment. Research was supported by Czech Science Foundation (No. P210/12/0573) and Czech Ministry of Education, Youth and Sports (No. LH12078).