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| Titel |
Structure and Tectonics of the Cheb Basin (NW-Bohemia) from a shallow reflection seismic survey |
| VerfasserIn |
Felix Halpaap, Marco Paschke, Florian Bleibinhaus |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104710
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| Publikation (Nr.) |
 EGU/EGU2015-5619.pdf |
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| Zusammenfassung |
| In the seismically active region of Northwest Bohemia, we imaged structural characteristics
of the Cenozoic Cheb Basin with a shallow 3.5 km reflection seismic survey to find proof of
faulting along the PoÄatky-Plesná shear zone (PPZ). Previously, the shear zone’s existence
has been inferred from earthquakes that occur in swarms and concentrate in the focal
zone of Nový Kostel, below the Cheb Basin, along a plane striking at 170°. The
difference in strike between the planar focal zone and the 145° oriented, crustal-scale
eastern border fault of the Cheb Basin, which forms the northern termination of the
geomorphologically dominant Mariánské LaznÄ fault, was interpreted to hint to the
existence of a second major crustal fault zone. With additional interpretations of river
drainage patterns, a distinct 25 m terrain escarpment and the distribution of Quaternary
sediments around the Plesná river, the surface outcrop of the PPZ was thought to be
found.
A P-velocity model which we obtained from tomographic inversion of the first arrivals
revealed an uppermost layer of very slow seismic velocities (about 1 km/s) that varies
strongly in thickness. We interpret this layer as unconsolidated Quaternary sediments,
which impacted the quality of our recorded shot gathers negatively with increasing
thickness of the layer. The result of our standard reflection seismic processing,
challenged by strong ground roll, is an image of the eastern Cheb Basin’s layers
and several tectonic features along a cross-strike profile with varying resolution.
Our seismic image shows undisturbed younger sediments of the upper neogene
Vildštejn and Cypris Formation, overlying the early miocene Main Coal Seam
Formation and a structured basement. The imaged maximum basin depth of 300 m and
unconformities below and above the Vildštejn Formation correspond well with
litostratigraphic borehole data and previous sedimentological and tectonic models. We
observe reverse faults in the lower part of the Main Coal Seam Formation on top of the
basement, which we interpret as signs of a pop-up structure related to the N-S oriented
PoÄatky-Plesná shear zone. Additional draped folds and signs of normal faults
inside the Main Coal Seam Formation illustrate the early Miocene east-west directed
extensional regime in the area. Sediment layers thicken towards the east, which
indicates that synsedimentary extension of the Cheb Basin was accommodated at the
basin-bounding Mariánské LaznÄ fault. We do not observe any vertical fault offsets in the
younger sedimentary layers, which suggests that any normal or reverse faulting
must be older than 20 Ma. Our explanation for the formation of the escarpment at
the eastern bank of the Plesná valley, which has previously been interpreted as
outcrop of the PPZ, relies solely on incision of the Plesná river into Quaternary
sediments. |
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