Pravčice Rock Arch (Bohemian Switzerland National Park, Czech Republic) deterioration due to natural and anthropogenic weathering

The dominant factors affecting the development of weathering forms on the Prav?ice Rock Arch (PRA) (Bohemian Switzerland National Park, Czech Republic) are discussed, based on the in situ monitoring of weathering forms, as well as on laboratory studies of materials taken from this site. The in situ monitoring shows the progressive development of distinct weathering forms (delamination of case-hardened rock surfaces, cyclic efflorescence, and granular disintegration of exposed weakly cemented rock mass) in specific portions of the PRA, whose location is controlled by lithology, jointing/local hydrodynamics, and exposure to climatic factors. From the analysis of efflorescence, the dominance of sulphates is evident; particularly of gypsum and alums. However, the synergistic effects of the several salt species (that also include minor nitrates and chlorides) upon the mechanical disintegration of sandstone can be expected.     

Fabric transpositions in granite plutons — An insight from non-scaled analogue modelling

Investigations on a set of experimental models of highly viscous intrusions were carried out in order to study the internal strain pattern during vertical ascent and emplacement of granite intrusions. The strain pattern was determined by means of anisotropy of magnetic susceptibility (AMS) resulting from the orientation of magnetite particles in a liquid plaster medium. The modelled intrusions show distinct fabrics reflecting the flow of a rheologically complex, non-Newtonian material. During the vertical growth of the intrusion, constrictional vertical fabrics are transposed into flattening fabrics, and along with the development of low-intensity fabric domains are passively transported upwards. Vertical growth takes place along subvertical thrust shear zones that satisfactorily explain the discordant magmatic fabrics in granites along intrusion sides. The resulting complex fabric patterns suggest that the vertical movement of material in ascending intrusions is accommodated by various flow mechanisms operating simultaneously.     

Non-double-couple mechanisms of microearthquakes induced during the 2000 injection experiment at the KTB site, Germany: A result of tensile faulting or anisotropy of a rock?

Moment tensors of microearthquakes induced during the 2000 injection experiment at the KTB deep drilling borehole at a depth level of 5.4 km are studied. A family of 37 most reliable moment tensors contains significant non-double-couple (non-DC) components. The DC is on average 60% and the non-DC is 40%. Fault plane solutions computed from the DC part show preferred strike-slip mechanisms with small normal or reverse components. A predominant azimuth of P and T axes is in the range of N320°–340°E and of N230°–250°E, respectively. The non-DC components contain both the isotropic (ISO) and compensated linear vector dipole (CLVD) components. The mean value of ISO is 1.5%, the mean value of CLVD is − 5.7%. The predominantly negative CLVD components are inconsistent with the concept of the non-DC mechanisms as a result of tensile faulting due to fluid injection into the rock. The main origin of the non-DC components is probably anisotropy in the focal area. The other origins are errors produced by mismodelling of the medium when calculating the Green functions, and numerical errors produced by noise and limitations of input data. Adopting four alternative models of anisotropy obtained by other seismic measurements at the KTB, we have employed the non-DC components for estimating an optimum orientation of anisotropy in the focal area. The optimum orientation of the symmetry plane of anisotropy is nearly vertical with a strike of N335°–340°E. This strike coincides well with the strike of 330° typical for many major lithological units and faults and with the orientation of the transversely isotropic model inferred by other authors. After removing the anisotropy effects from the moment tensors by calculating the source tensors, the distribution of ISO is significantly narrowed. This indicates predominantly shear, but not tensile faulting.     

Interpretation of the microtremor spectra at the Zafarraya basin, southern Spain

The microtremor spectra observed at the sediment-filled basin of Zafarraya show a peak near 2·8 Hz, independent of the local basin depth. Based on geoelectric and geologic data we constructed a tentative model of the site. The seismic response for vertically incident plane waves, both SH and P-SV, was computed by the finite-difference method. The observed spectral peak was found to be due to a combined effect of the 2-D basin bottom shape and the horizontally layered basin fill. Neither of the two effects explain the spectra when treated separately. Synthetic seismo-grams indicate a transitional basin behaviour, little investigated so far, between that typical of local basin-induced surface waves and a global 2-D resonance. A practical conclusion is that the site effects cannot be simply assessed by means of the basin shape ratio (thickness to half-width ratio) and the velocity contrast at the bottom.     

Seismic model and geological interpretation of the basement beneath the Doupovské Hory Volcanic Complex (NW Czech Republic)

The Doupovské Hory Volcanic Complex (DHVC) is the best-preserved large volcanic suite of the Cenozoic intraplate volcanism in the Bohemian Massif. However, many uncertainties remain in the geological setting of its basement. In summer 2008, two seismic refraction profiles ran across this area to reveal the depth of the volcanic rocks and the underlying geological structure.     

On the application of the coupled finite-infinite element method to geodetic boundary-value problem

Our aim is to introduce the Coupled Finite-Infinite Element Method (CFIEM) as a new alternative approach to the Earth’s gravity field modelling. We show that if the computational domain is large enough in radial direction, one can obtain the qualitatively and quantitatively comparable solution to the solution by the Finite Element Method (FEM). We study the influence of the size of the computational domain on the final CFIEM solution as well as the successive refinement of the discretization and its convergence to the exact solution. As an input data we use the synthetic boundary conditions computed from a Synthetic Earth Gravity Model (SEGM) and we test the CFIEM solution by the data generated directly from SEGM and the solution by the FEM.     

AMS record of brittle dilation,viscous-stretching and gravity-driven magma ascent in area of magma-rich crustal extension (Vosges Mts., NE France)

Orogenic compression-related fabrics (~340–335 Ma) were reworked during regional extensional deformation (~328–325 Ma) in a large anatectic crustal domain of the Central Vosges (NE France). The extension was first accommodated by brittle dilation affecting vertically anisotropic high-grade rocks associated with emplacement of subvertical granitic sheets. The AMS fabric of granitoids is consistent with highly partitioned transtensional deformation marked by alternations of flat and steep foliations and development of orthogonal lineations. This deformation passes to top-to-the-southwest ductile shearing expressed in southerly migmatitic middle crust. The AMS fabric revealed moderately west-dipping foliations bearing subhorizontal NNW–SSE-trending lineations and predominantly plane strain to prolate shapes. This fabric pattern is interpreted as a viscous response of stretched partially molten crust during continuous ductile extension. Vertical ascent of voluminous granites and stoping of the upper crust occurs further south. This gravity ascent triggered by extension leads to development of south-dipping AMS foliations, south-plunging lineations and oblate fabrics in various crustal granites. Vertical shortening related to ascent of these (~325 Ma) granitoids and persistent N–S stretching is responsible for reworking and remelting of originally vertical compression-related fabric in roof supracrustal granites (~340 Ma) and development of highly prolate fabrics in these rocks. This work shows that the finite shape of AMS fabric ellipsoid is highly sensitive to both strain regime and superpositions of orthogonal deformation events.     

A New Bolide Station at the High Tatra Mountains

The European Fireball Network (EN) is operating since 1963 and one of its stable stations, from the very beginning, is the station at the Skalnate Pleso Observatory in the High Tatras. The station is sited at a height of 1788 m. More than 2900 expositions has been made at the Skalnate Pleso station since 1964 and among them one significant and spectacular event was recorded––bolide Turji-Remety in 2001 followed by a fall of about 450 kg meteorite (Spurny and Porubcan [in: Warmbein (ed.) Asteroids Comets Meteors, 2002]). A systematic search for the meteorite was unsuccessful. The new station having an ideal horizon will be operating since July 2007 on the top of Lomnicky Stit (2636 m above the sea level). This station will be equipped with an Autonomous Fireball Observatory of the Astronomical Institute of the Czech Academy of Sciences, which are already utilized in the Czech part of the EN for several years.     

Depth-Recursive Tomography Along the Eger Rift Using the S01 Profile Refraction Data: Tested at the KTB Super Drilling Hole,Structural Interpretation Supported by Magnetic,Gravity and Petrophysical Data

The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01 profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels, were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the “slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity image down to 15 km. Although the contact of Saxothuringian and the Teplá-Barrandian Unit (TBU) is collateral with the S01 profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular, the Moldanubian and TBU contact beneath the Western Krušné hory/Erzgebirge Pluton, the buried contact of the Lusatia unit and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms⁻¹ isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs beneath the Doupovské hory and the České středohoří volcanic complexes. The intermediate mid-crustal P-wave velocity lows are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic and petrophysical data.