The current state of the hidden magmatic process beneath the western Eger Rift after the 2011 earthquake swarm traced by isotope studies
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The western Eger Rift area is part of the European Cenozoic Rift System (ECRIS), renowned for the occurrence of CO2-rich degassing and for recurring swarm earthquake activity in the eastern part of the Cheb Basin (CB). The CO2 degassing in the western Eger Rift area has been comprehensively investigated during active and quiet seismic phases for 25 years. The latest data of the regional distribution pattern of gas compositions and isotopic compositions of helium and CO2 from 26 gas exhalations were recorded between 2014 and 2016. Mantle-derived CO2 as the major component as well as mantle-derived helium occur at all degassing sites. The carbon isotope ratios of CO2 cover a small range (δ13C = -1.7 to -5.1‰) and the 3He/4He ratios range from 1.9 to 5.9 Ra (Ra= 3He/4He of air). The highest 3He/4He ratios were found at locations along the Počatky-Plesná Fault Zone (PPZ), followed by a degassing site in a clay pit on the Nová Ves Fault (NVF) and by the locations on the Mariánské Lázně Fault (MLF) at the eastern boundary of the CB. The detailed characterization of degassing in the clay pit confirms the existence of the E-W striking NVF, underscores the importance of fault-related degassing, and evidences that the degassing site in the clay pit is supplied by the same magmatic reservoir that supplies the locations along the PPZ. Despite very high gas flow and CO2 concentrations in all degassing centers, the fractions of mantle-derived helium are different, with presently up to 94% (with respect to the subcontinental lithospheric mantle (SCLM) 3He/4He of 6.32 Ra) in the CB, up to 73% in the Mariánské Lázně and ~35% in the Karlovy Vary degassing centers. At the locations in the eastern part of the CB, a clear progressive increase of the mantlederived helium fractions was observed since the first sampling campaigns about 25 years ago, while they remained essentially the same at other degassing sites. Superposed to this progressive increase of the 3He/4He ratios in the eastern part of the CB, short-time increases of the 3He/4He ratios (up to 6.3 Ra) were recorded at locations along the PPZ several months before the 2000 and 2008 earthquake swarms and are interpreted as indication for an ongoing magmatic process beneath that area. The CB is located close to the Nový Kostel focal zone, where four strong periods of seismicity (with magnitudes >3) occurred since the beginning of our investigations. The latest gas data confirm our earlier findings from detailed time-series studies, showing that in relation to seismic events, decreased 3He/4He ratios are repeatedly observed due to admixed seismically released crustal helium. In relation to the seismically active period in 2014, such decreased 3He/4He ratios were observed at locations along the PPZ, at the boundary of the CB on the MLF and on the NFV (clay pit). According to the long-term data (1994 to 2016), an ongoing hidden magmatic process is indicated beneath this area only. Essentially, the sampling frequency determines whether and when such seismically triggered anomalies or anomalies due to ascending magma are recorded. In future, more frequent sampling than possible in the past, or even better continuous monitoring, could increase our knowledge about the geodynamic development in the area. In the ECRIS, there are only few locations where free gases with plain SCLM signature escape at the surface. Gases from the SCLM can only retain their signature to a large extent when they are transported on a highly permeable migration path reaching down to the SCLM. In the western Eger Rift area, this is only fulfilled for the mofettes along the PPZ. Presently, the eastern part of the CB is the most active non-volcanic region in the ECRIS.
Publications
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2017. Active magmatic underplating in western Eger Rift, Central Europe. Tectonics, 36
Hrubcová, P., Geissler,W.H., Bräuer, K., Vavryčuk, V., Tomek, C., Kämpf, H.
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2018. Monitoring of helium and carbon isotopes in the western Eger Rift area (Czech Republic): Relationships with the 2014 seismic activity and indications for recent (2000 – 2016) magmatic unrest. Chemical Geology, 482,131-145
Bräuer, K., Kämpf, H., Niedermann, S., Strauch, G.
