Seismic structure and location of a CO₂ source in the upper mantle of the western Eger (Ohře) Rift, central Europe

Abstract

P‐SV conversions provide new insights into the lithosphere of the western Eger (Ohře) Rift, a presently active CO2 emanation area, Quaternary volcanic field, and earthquake swarm region in central Europe. Gas and isotope (He and C) mapping of free gas phases in mineral springs and mofettes proved the origin of CO2‐ dominated gases from a subcrustal magmatic fluid reservoir. Analyzing teleseismic data from several seismic networks in the western Bohemian Massif, the source region of these gases was investigated. Moho Ps conversions have 3 to 4.5 s delay. Crustal thicknesses vary between 27 and 38 km; vp/vs ratios vary between 1.63 and 1.81. Beneath the western Eger Rift an approximately 40 km wide Moho updoming up to 27 km exists. Locally observed weak conversions indicate a complex Moho transition zone in this area. A local “6 s phase” possibly originates at a discontinuity in approximately 50 to 60 km depth or may represent multiples from velocity inversions at the base of the upper crust. Moho updoming and the distribution of the “6 s phase” coincide with the CO2 degassing fields and the positions of Quaternary volcanoes at the surface. We hypothesize the release of CO2‐dominated fluid/magma from isolated melt reservoirs in the depth range of 60 to 30 km, separation of CO2 from the melt at 29 to 21 km depths, and CO2 transport through the crust. The geophysical indications may point to presently active magmatic underplating beneath the study area, supporting the results of gas geochemical and isotope investigations. This is the first attempt that combines seismic and gas geochemical data for a tectonic model. Our model may be transferable to other continental rift areas worldwide.

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

“…comm., 2005). In accord with the thinning of the crust (Geissler et al, 2005), both profiles show also a lithosphere thinning beneath the NK area to about 80 km (Babuška and Plomerová, 2000) and possibly up to 65 km (Heuer et al, 2006).…”

“…However, recent investigations of fluids of mantle origin (e.g., Bräuer et al, 2005;Geissler et al, 2005) showed a possible link between the upper mantle and the earthquake swarm activity. Tertiary to Quaternary volcanism and the present-day escape of fluids of mantle origin around the major upper crustal fault zones may suggest that these faults can be linked with the upper mantle.…”

“…They also speculate that episodic seismicity and associated hydrofracturing drive volatile transfer from the mantle to the crust. Geissler et al (2005) hypothesized the release of CO 2 dominated fluid/magma in the western BM from isolated melt reservoirs fed from the asthenosphere and resting in the depth range of 60 to 30 km. We propose that mantle fluids may escape directly from the asthenosphere and channel into the crust along the block boundaries of the mantle lithosphere (Fig.…”

“…1a,b). The Tertiary Eger (Ohře) Rift (ER), a 300 km long ENE-WSW striking structure characterized by high heat flow and Cenozoic volcanism is part of the European Cenozoic Rift System (ECRIS; Prodehl et al, 1995) and its formation is thought to be related to Alpine collision (Ziegler, 1992; see also review of the ER development in Geissler et al, 2005). Active tectonics is primarily manifested by frequent weak to moderate earthquake swarms (Horálek et al, 2000), emanations of CO 2 dominated gases of mantle origin (Weinlich et al 1999;Bräuer et al, 2003), and by neotectonic crustal movements (Bankwitz et al, 2003).…”

Crustal and upper mantle seismic structure of the Svalbard Archipelago from the receiver function analysis

“…To calculate the receiver function (RF) few steps are usu− ally done (e.g. Owens et al 1984;Kind et al 1995;Geissler et al 2005): (1) pre− paring a list of teleseismic events occurred during the analysed time with the de− sired magnitude and epicentral distance; (2) selection of three−component seismo− grams with high signal−to−noise ratio; (3) restitution filtering of seismograms and/or low−pass or band−pass filtering of seismograms; (4) picking the onset of di− rect wave or calculating the onset based on one−dimensional global model; (5) cut− ting the seismograms in time window, usually tens of seconds before and tens of seconds after the onset of direct wave; (6) rotation from Z, N, E components into Z, R, T components or into a ray coordinate system (L, Q, T) based on theoretical back−azimuth and incident angles of rays or angles calculated from polarization analysis of seismograms; (7) deconvolution of a vertical component from horizon− tal components in frequency domain or in time domain; (8) normalization of com− ponents of receiver function to preserve the absolute amplitude; (9) manual quality check of the calculated receiver function. Later, receiver functions can be move− −out corrected and stacked in distance or back−azimuth bins to improve sig− nal−to−noise ratio.…”