Faults of the Bohemian Massif

Source of Analytical Data on Main Faults and Faulted Areas with Seismic Potential

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dbz:diendorf-boskovice_fault

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dbz:diendorf-boskovice_fault [2020/05/29 15:56]
petrs 		dbz:diendorf-boskovice_fault [2021/04/29 15:45]
petrs [Quaternary]
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   ***//Geodesy//**   ***//Geodesy//**
     * Results of some long-term geodetic measurements in the region were interpreted as tectonic deformation in several studies (Pospíšil et al. 2009, 2010, 2012; Roštínský et al. 2013). However, editor of this text is skeptical to interpretation of these observations as due to tectonic strain because:      * Results of some long-term geodetic measurements in the region were interpreted as tectonic deformation in several studies (Pospíšil et al. 2009, 2010, 2012; Roštínský et al. 2013). However, editor of this text is skeptical to interpretation of these observations as due to tectonic strain because: 
-      - the reported strain rates are too large (10<sup>-1</sup> mm/y, i.e. same order as in the eastern Alps) to be in agreement, on a regional scale, with concept of active Alpine orogen vs. stable foreland;+      - the reported strain rates and slip rates are too large (slip rate of 10<sup>-1</sup> mm/y, i.e. same order as the faults in the eastern Alps) to be in agreement, on a regional scale, with concept of active Alpine orogen vs. stable foreland;
       - the locally observed geological records rule out that such deformation at BF could have been operative on a long-term scale during Quaternary;       - the locally observed geological records rule out that such deformation at BF could have been operative on a long-term scale during Quaternary;
-      - the non-tectonic deformation (climatic/hydrogeological, slope instability, problematic long-term stabilization of measurement points) of the surface is expected to compete with such strain rates or exceed them (see the extensive literature on the effects of water management and climatic change on surface deformation and Vyskočil 1996). For example, repeated levelling on a profile crossing the BF near Tetčice (section BF_6; Roštínský et al. 2013) and suggesting vertical strain of 10<sup>-1</sup> mm/y order, was partly carried out in the area of former pond (see maps of the 1<sup>st</sup> military mapping from 1764-1768) and therefore likely affected by ongoing sediment compaction.+      - the non-tectonic deformation (climatic/hydrogeological, slope instability, problematic long-term stabilization of measurement points) of the surface is expected to compete with such strain rates or exceed them (see the extensive literature on the effects of water management and climatic change on surface deformation and Vyskočil 1996). For example, repeated levelling on a profile crossing the BF near Tetčice (section BF_6; Roštínský et al. 2013) and suggesting vertical slip rate of 10<sup>-1</sup> mm/y order, was partly carried out in the area of former pond (see maps of the 1<sup>st</sup> military mapping from 1764-1768) and therefore likely affected by ongoing sediment compaction.
  
 :!: **Basing on this local evidence the fault activity is currently evaluated in a following way:** :!: **Basing on this local evidence the fault activity is currently evaluated in a following way:**
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 <WRAP GROUP> <WRAP GROUP>
 <WRAP column 60%>  <WRAP column 60%> 
-  *Pronounced scarp of WNW-ESE direction dividing the hilly terrain with narrow valley of Danube to the NE from comparatively flat area with wide Danube valley to the SW. Interrupted and multiple morpholineament continues in ESE direction towards Hafnerbach, partly reflecting the structure of the crystalline basement. This scarp, very likely a fault (not identified in geological maps), possibly a minor dip-slip fault once terminating now eroded Tertiary Molasse sediments or thick regolith, seems to lack any horizontal offset by Diendorf fault (sections DF_1, 1a and 9), therefore providing evidence against significant strike slip of the latter after the formation of the former. +  *Pronounced scarp of WNW-ESE direction dividing the hilly terrain with narrow valley of Danube to the NE from comparatively flat area with wide Danube valley to the SW. Interrupted and multiple topolineament continues in ESE direction towards Hafnerbach, partly reflecting the structure of the crystalline basement. This scarp, very likely a fault (not identified in geological maps), possibly a minor dip-slip fault once terminating now eroded Tertiary Molasse sediments or thick regolith, seems to lack any horizontal offset by Diendorf fault (sections DF_1, 1a and 9), therefore providing evidence against significant strike slip of the latter after the formation of the former. 
-  *This structure is likely a part of a system of WNW- to NW- striking faults with small slip amplitude documented by multiple observations and indicated by penetrative presence of morpholineaments in an extensive area hosting the whole DF and southern BF. The age of these faults is largely unknown but some of them were observed to displace Quaternary strata. In Dunkelsteiner Wald and Wachau this fault system may host sources of weak seismicity. +  *This structure is likely a part of a system of WNW- to NW- striking faults with small slip amplitude documented by multiple observations and indicated by penetrative presence of topolineaments in an extensive area hosting the whole DF and southern BF. The age of these faults is largely unknown but some of them were observed to displace Quaternary strata. In Dunkelsteiner Wald and Wachau this fault system may host sources of weak seismicity. 
-  *Also note the N-S oriented morpholineaments near Aggasbach Dorf north of here which seem to coincide with deflected valley of Danube river (out of extent of the map shown here).\\ +  *Also note the N-S oriented topolineaments near Aggasbach Dorf north of here which seem to coincide with deflected valley of Danube river (out of extent of the map shown here).\\ 
 </WRAP> </WRAP>
 <WRAP column 30%> <WRAP column 30%>
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   *The 7 m thick sediment succession includes (from bottom to top; see photo an scheme below left): sandy eluvium, coarse gravels (partly cemented), silty clays with angular detritus, layer of colluvium, fine loams with paleosol, and  loess loams to loess with interlayer of laminated or cross bedded fine sands. While the layers no. 6 and 7 (see figure) give OSL ages within the span of 36-63 ka, dating of the deeper strata remains problematic. Ages largely exceeding 100 ka are very likely and Tertiary age is not ruled out (Prachař 2017).   *The 7 m thick sediment succession includes (from bottom to top; see photo an scheme below left): sandy eluvium, coarse gravels (partly cemented), silty clays with angular detritus, layer of colluvium, fine loams with paleosol, and  loess loams to loess with interlayer of laminated or cross bedded fine sands. While the layers no. 6 and 7 (see figure) give OSL ages within the span of 36-63 ka, dating of the deeper strata remains problematic. Ages largely exceeding 100 ka are very likely and Tertiary age is not ruled out (Prachař 2017).
   *Clay-filled fault within the shear zone contains slickenside (dipping 66° to NW) with 2 sets of slickenlines, the younger showing normal dip-slip. The fault does not continue to the covering sediment (Prachař 2017).   *Clay-filled fault within the shear zone contains slickenside (dipping 66° to NW) with 2 sets of slickenlines, the younger showing normal dip-slip. The fault does not continue to the covering sediment (Prachař 2017).
-  *Thin blind sand dike cuts through the lower part of the sedimentary sequence above the fault. The dike connetcs directly to the fault, but no systematic offset of hosting strata was observed, only local disturbation of single bed of coarse sand adjacent to the dike (Prachař 2017).+  *Thin blind sand dike cuts through the lower part of the sedimentary sequence above the fault. The dike connects directly to the fault, but no systematic offset of hosting strata was observed, only local disturbation of single bed of coarse sand adjacent to the dike (Prachař 2017).
   *This clastic dike is interpreted as an effect of local liquefaction generated by earthquake-induced shaking. Its location above the observed fault is likely an effect of stress concentration within loose and water-saturated deposit above the breakpoint in bedrock topography and lithology (structurally controlled failure of gravitationally unstable sediment)  as the coseismic slip at the fault (dynamic control) is ruled out by the absence of strata offset (Špaček et. al. 2018).   *This clastic dike is interpreted as an effect of local liquefaction generated by earthquake-induced shaking. Its location above the observed fault is likely an effect of stress concentration within loose and water-saturated deposit above the breakpoint in bedrock topography and lithology (structurally controlled failure of gravitationally unstable sediment)  as the coseismic slip at the fault (dynamic control) is ruled out by the absence of strata offset (Špaček et. al. 2018).
   *Although the inferred hypothetical earthquake is not necessarily related to Diendorf fault, it is recommended to be assumed related for purpose of SHA (Špaček et. al. 2018).   *Although the inferred hypothetical earthquake is not necessarily related to Diendorf fault, it is recommended to be assumed related for purpose of SHA (Špaček et. al. 2018).
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 {{:dbz:stosikovice.jpg|Structures in Stosikovice sand pit.}} {{:dbz:stosikovice.jpg|Structures in Stosikovice sand pit.}}
  
-//Structures in Stošíkovice sand pit. System of small-ampitude NW-striking faults is cut by few younger, E-W striking faults with partly incorporated sandy gravels of assumed Quaternary terrace (Špaček, unpublished). //+//Structures in Stošíkovice sand pit. System of small-amplitude NW-striking faults is cut by few younger, E-W striking faults with partly incorporated sandy gravels of assumed Quaternary terrace (Špaček, unpublished). //
 </WRAP> </WRAP>
 </WRAP> </WRAP>
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   *Trench HOS-1 (25 m long and 4 m deep; WGS84: 48.9476°N, 16.2726°E) with fault in Lower Miocene sediments sealed by Late Pleistocene sands and loess loam. Špaček et al. (2017, 2018)\\   *Trench HOS-1 (25 m long and 4 m deep; WGS84: 48.9476°N, 16.2726°E) with fault in Lower Miocene sediments sealed by Late Pleistocene sands and loess loam. Špaček et al. (2017, 2018)\\
-  *Strong imbrication, large density of deformation bands and content of tectonic clasts of kaolinized crystalline rocks at the contact of clays and sands indicate large slip on the fault after deposition of Miocene sediments (assumed age Eggenburgian-Ottnangian based on lithostatigraphic correlation). Horizontal slip component was inferred on small faults which offset the thin clayey beds in sands, but the sense of slip was not resolved.+  *Strong imbrication, large density of deformation bands and content of tectonic clasts of kaolinized crystalline rocks at the contact of clays and sands indicate large slip on the fault after deposition of Miocene sediments (assumed age Eggenburgian-Ottnangian based on lithostratigraphic correlation). Horizontal slip component was inferred on small faults which offset the thin clayey beds in sands, but the sense of slip was not resolved.
   *Deformed Neogene sediments are unconformably overlain by Pleistocene strata. The irregular, up to 30 cm thick basal layer of sands and gravelly sands with mostly angular clasts of local provenance has a sharp base and the luminescence dating of a single sample gives the age of 16-18 ka. Locally, at the base of Quaternary a thin sub-horizontal layer of parallel-laminated sand is developed and partly incorporated in coherently bended or apparently undeformed parts of Miocene sand. Luminescence age of a single sample from this layer is 20-24 ka.   *Deformed Neogene sediments are unconformably overlain by Pleistocene strata. The irregular, up to 30 cm thick basal layer of sands and gravelly sands with mostly angular clasts of local provenance has a sharp base and the luminescence dating of a single sample gives the age of 16-18 ka. Locally, at the base of Quaternary a thin sub-horizontal layer of parallel-laminated sand is developed and partly incorporated in coherently bended or apparently undeformed parts of Miocene sand. Luminescence age of a single sample from this layer is 20-24 ka.
   *The above described sediments are overlain by 1-1.8 m thick layer of loess loam which is mixed with sands at the base. Its luminescence age is 26-29 ka. This age inversion (if real) suggests the emplacement of the loess loam without resetting of the luminescence signal (e.g. in a form of mudflows derived from older loess). Apparent age inversion due to dating errors can not be ruled out, however.   *The above described sediments are overlain by 1-1.8 m thick layer of loess loam which is mixed with sands at the base. Its luminescence age is 26-29 ka. This age inversion (if real) suggests the emplacement of the loess loam without resetting of the luminescence signal (e.g. in a form of mudflows derived from older loess). Apparent age inversion due to dating errors can not be ruled out, however.
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 **editor:** [[spacek@ipe.muni.cz|Petr Špaček]] **editor:** [[spacek@ipe.muni.cz|Petr Špaček]]
  
-  *Refraction and reflection seismic profile crossing the fault line (Alexa 2017; diploma thesis supervised by J. Valenta) showing a >50 m vertical offset at the base of low-velocity strata (assumed Lower Miocene sediment and/or strongly weathered crystalline) and a >40 m thick, apparently vertically undisplaced, top layer with even lower velocity, covering this offset and a burried pediment in crystalline rocks. The upper sedimentary body is of assumed Eggenburgian/Ottnangian age (~17-21 Ma) and it fills up adjacent paleovalley. The structure documents the erosional destruction of older fault scarp near (or in) the paleovalley and the absence of vertical slip component on the fault after Early Miocene (which is the minimum age of the paleovalley). Note the qualitative similarity of this observation with that at [[[[diendorf-boskovice_fault#Limberg: Secondary faults. Timing of dip-slip.|Limberg site]]. Small younger horizontal slip is not ruled out here.\\ +  *Refraction and reflection seismic profile crossing the fault line (Alexa 2017; diploma thesis supervised by J. Valenta) showing a >50 m vertical offset at the base of low-velocity strata (assumed Lower Miocene sediment and/or strongly weathered crystalline) and a >40 m thick, apparently vertically undisplaced, top layer with even lower velocity, covering this offset and a buried pediment in crystalline rocks. The upper sedimentary body is of assumed Eggenburgian/Ottnangian age (~17-21 Ma) and it fills up adjacent paleovalley. The structure documents the erosional destruction of older fault scarp near (or in) the paleovalley and the absence of vertical slip component on the fault after Early Miocene (which is the minimum age of the paleovalley). Note the qualitative similarity of this observation with that at [[[[diendorf-boskovice_fault#Limberg: Secondary faults. Timing of dip-slip.|Limberg site]]. Small younger horizontal slip is not ruled out here.\\ 
  
 <WRAP GROUP> <WRAP GROUP>
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 ===== Other notes===== ===== Other notes=====
  
-**Ongoing reserach and possible future work to be done:**+**Ongoing research and possible future work to be done:**
   *EM conductivity mapping of fault and covering sediments south of Hostěradice (Fojt and Špaček, ongoing), near Ivančice, Lesonice and Bořitov (planned)   *EM conductivity mapping of fault and covering sediments south of Hostěradice (Fojt and Špaček, ongoing), near Ivančice, Lesonice and Bořitov (planned)
   *studies on cross structures   *studies on cross structures
dbz/diendorf-boskovice_fault.txt · Last modified: 2024/05/29 16:39 by petrs

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