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.

SUMMARY S receiver functions obtained from seismograms of teleseismic events recorded at 78 European permanent broad‐band stations are used to estimate the thickness of the European lithosphere. Our results provide new, independent information about the lithospheric thickness beneath the Precambrian platform of Eastern Europe and the Phanerozoic platform of central Europe. Detailed high‐resolution images of the lithosphere–asthenosphere boundary (LAB) reveal indications for a typical continental lithosphere of about 100 km thickness beneath a majority of stations within Central Europe, whereas in the vicinity of the Trans‐European Suture Zone (TESZ), the lithosphere thickens to about 130 km. A relatively thin lithosphere of 80 km was found beneath the Upper Rhine Graben region suggesting that the Cenozoic extension affects the whole lithosphere. No clear signal from the LAB was detected beneath the Alps and Carpathians. The LAB Sp phase might be disturbed by complicated structure due to ongoing collision/subduction in these regions, or the data are not yet sufficiently dense. A relatively thicker lithosphere of about 120 km was found beneath the SW part of the Bohemian Massif that was formed during the Variscan orogeny. We found an LAB depth of about 190 km near a single station located in the Vrancea area/Eastern Carpathians, which is characterized by the occurrence of intermediate deep earthquakes. Beneath the stations located in the Precambrian platform of Eastern Europe, the LAB deepens to approximately >200 km, even though the converted phase from the LAB is not as sharp as found beneath other stations located in Central Europe or even is missing.

S U M M A R YIn the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15• E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82• N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits of the inner shelf yield P-wave velocities of 2 km s −1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30• E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent-ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

[1] An eleven-month deployment of 25 ocean bottom seismometers provides an unprecedented opportunity to study low-magnitude local earthquakes in the complex transpressive plate boundary setting of the Gulf of Cadiz, known for the 1755 Lisbon earthquake and tsunami. 36 relocated earthquakes (ML 2.2 to 4.8) concentrate at 40-60 km depth, near the base of the seismogenic layer in ∼140 Ma old oceanic mantle lithosphere, and roughly align along two perpendicular, NNE-SSW and WNW-ESE striking structures. First motion focal mechanisms indicate compressive stress for the cluster close to the northern Horseshoe fault termination which trends perpendicular to plate convergence. Focal mechanisms for the second cluster near the southern termination of the Horseshoe fault indicate a strike-slip regime, providing evidence for present-day activity of a dextral shear zone proposed to represent the Eurasia-Africa plate contact. We hypothesize that regional tectonics is characterized by slip partitioning. Citation: Geissler, W. H., et al. (2010), Focal mechanisms for sub-crustal earthquakes in the Gulf of Cadiz from a dense OBS deployment, Geophys.

[1] The SUDETES 2003 seismic experiment investigated the lithospheric structure of the eastern part of the Variscan belt of central Europe. The key profile of this experiment (S01) was 630 km long and extended southwestward from the margin of the East European craton, across the Trans-European suture zone (TESZ) and Sudetes, and across the Bohemian Massif that contains the active Eger (Ohře) rift, which is an element of the European Cenozoic rift system. Good quality first arrivals and later phases of refracted/reflected P and S waves were interpreted using 2-D ray-tracing techniques. The derived seismic model shows large variations in the internal structure of the crust, while the depth to the Moho varies in the relatively narrow depth interval of 28-35 km. Except for the Polish basin on the northeast end of the profile, the sedimentary cover is thin. The crystalline upper and middle crust with velocities of 5.9-6.4 km s À1 is about 20 km thick, and the 7-10 km thick lower crust can be divided into three regions based on P wave velocities: a low-velocity region (6.5-6.6 km s À1 beneath Eger rift and Sudetes) that is bounded on the southwest and northeast by regions of significantly higher velocity (6.8-7.1 km s À1 beneath the Saxothuringian and Moldanubian in the southwest and Fore-Sudetic Monocline and Polish Basin in the northeast). High-velocity bodies (Vp > 6.5 km s À1 ) were delineated in the upper crust of the Eger rift region. The seismic structure along the S01 profile images a Variscan orogenic wedge resting on the down warped margin of the plate margin containing the TESZ. This situation implies the northerly directed subduction of the Rheic Ocean that existed between the southern margin of the Old Red Continent and the Armorican terranes presently accreted into the Variscan belt. Closure of this ocean produced the Rheic suture between low-velocity crust of the Variscan orogenic wedge and higher-velocity crust of the TESZ.