Journal of Geophysics
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Paleomagnetism of Jurassic sediments from the western border of the Rheingraben, Alsace (France)
Get PDFUpper Bajocian oolithic limestones and Pliensbachian marls and marly ovoids from 9 sites distributed over 4 exposures at the western border of the Rheingraben (mean coordinates 47.8° N, 7.5° E) were sampled by 92 samples. The carriers of the natural remanent magnetization in the limestones are goethite and magnetite, and in the marls and ovoids the carrier is magnetite. After tectonic correction, the characteristic component of NRM isolated in the cleaning processes was mostly of normal polarity. The mean direction is D = 30.1°, I = 53.2° (N = 7, k = 92, α95 = 6.3°) and the pole position is 63.1° N, 120.1° E. It is close to the Jurassic reference data for the stable European plate.
ARK: https://n2t.net/ark:/88439/y066400
Permalink: https://geophysicsjournal.com/article/122
 
Electromagnetic sounding experiments in the Schwarzwald central gneiss massif
Get PDFGeomagnetic and telluric pulsations were observed at nine stations, partially at the same time, and at two sites with additional recordings of variations. They occupied a 20 x 30 km2 area of high-grade metamorphism. There is a nearly perfect spatial uniformity of the magnetic variation field except for a small local anomaly attributable to the Rheingraben. The telluric field is highly polarized in a N47 ± 7W direction but with local differences in amplitude. In contrast, telluric phases are spatially uniform and, as a function of period, distinctly different for the N47W orientation of the telluric field (= "B-polarisation") and the N43E orientation (= "E-polarisation"). Telluric and magnetic observations are not explainable by one-dimensional (1-D) models for the Schwarzwald alone. Therefore a 2-D model is derived, comprising Schwarzwald and Rheingraben, which can account for the graben Z anomaly and the phase curves in both polarisations. An unscaled 1-D model is obtained from the telluric phases in E-polarisation and then a 2-D model for Schwarzwald and Rheingraben in E- and B-polarisation. This latter model allows the scaling of the 1-D Schwarzwald model and shows a thin conductive layer under the gneiss massif at a depth of 12 km with a conductance of 650 S.
ARK: https://n2t.net/ark:/88439/y056011
Permalink: https://geophysicsjournal.com/article/121
 
An upper bound on lithosphere thickness from glacio-isostatic adjustment in Fennoscandia
Get PDFThe three-layer Maxwell half-space model of the Earth and a disk-load approximation of the Weichselian deglaciation history of Fennoscandia are used to calculate glacio-isostatic adjustment for this region. The calculations include the effects of deglaciation-induced geoid perturbations and eustatic sea-level rise and regard (1) lithosphere thickness, (2) asthenosphere viscosity and (3) ice thickness as the free model parameters. Numerical values of parameters (1)-(3) are estimated by calculating the past land uplift and present land-uplift rate observed in central Sweden (glaciation centre) and the past land uplift and past land tilt observed in southern Finland (glaciation margin). The uniqueness of the estimates and their sensitivity to uncertainties in (4) subasthenosphere viscosity, (5) ice cross-section and (6) deglaciation time are also assessed. The principal result of the investigation is that it suggests an upper bound of 80 km on the thickness of the Fennoscandian lithosphere.
ARK: https://n2t.net/ark:/88439/y082218
Permalink: https://geophysicsjournal.com/article/93
 
The upper-mantle discontinuities underneath the GRF array from P-to-S converted phases
Get PDFA data-processing method is applied which includes a rotation of the three components, normalization and delay-and-sum of broadband records of earthquakes from a large distance and azimuth distribution, recorded at a single station (or an array). Clear P-to-S converted phases at the mantle discontinuities are observed in the Grafenberg records, after this data processing. Theoretical seismograms are computed for the PREM model and processed in the same way as the observed data. A comparison with the data shows that the depth interval between the two discontinuities in the mantle transition zone (those at 400 and 670 km depth in PREM) is around 240 km. The 670-km discontinuity is sharper than the 400-km discontinuity and is comparable in sharpness with the crust-mantle transition, as far as it is possible to judge from the broadband data used. There are indications of pronounced lateral heterogeneity of the 400-km transition, underneath GRF. We have also observed converted and multiply reflected shear waves in the crust, which set sensitive limits to the average crustal model underneath the array. These data suggest that the velocity jump at the Moho is smaller than indicated by refraction studies.
ARK: https://n2t.net/ark:/88439/y067070
Permalink: https://geophysicsjournal.com/article/135
 
Paleomagnetic investigation of Quaternary West Eifel volcanics (Germany): indication for increased volcanic activity during geomagnetic excursion/event?
No full textEighty-five sites of West Eifel volcanoes were investigated paleomagnetically, giving 64 independent virtual geomagnetic poles (VGP). The VGP distribution is strongly asymmetric: about 30% of the VGPs have latitudes below 60° N and are confined to the longitude sector between 30° E and 120° E. This leads to a mean VGP situated at 74.0° N/63.9° E, deviating significantly from the north geographical pole. The VGP distribution is non-Fisherian, but the radial component is rather similar to that observed for Tertiary to Quaternary Icelandic lavas. Tectonic, petrographic, rock magnetic properties and secular variation do not seem to be responsible for the anomalous VGP positions. We propose that the volcanoes with anomalous VGPs erupted in a short period during an excursion or event of the Earth's magnetic field.
ARK: https://n2t.net/ark:/88439/y098537
Permalink: https://geophysicsjournal.com/article/138
 
Interpretation of the magnetotelluric impedance tensor: regional induction and local telluric distortion
Get PDFA method for the interpretation of the magnetotelluric (MT) impedance tensor, the telluric-vector technique, is presented. The phase information of all impedance tensor elements is used to distinguish between local telluric distortion and regional induction. A model incorporating a superposition of the effects of local surface anomalies and a regional 1-D, 2-D, or 3-D conductivity distribution is applied. In 2-D regional structures, a complete separation of the contributions of local and regional anomalies is possible if additional information from geomagnetic depth sounding (GDS) is used. A new skewness parameter derived from phases alone is introduced to measure the three-dimensionality of the regional structure independent of local distortions.
ARK: https://n2t.net/ark:/88439/y031103
Permalink: https://geophysicsjournal.com/article/129
 
Analytical model calculations for heat exchange in a confined aquifer
Get PDFModel calculations are carried out for the temperature distribution in an aquifer, into which water is injected. The models are simplified so that the temperature distribution can be given by an analytical expression. The one-dimensional simulation shows the well-known fact that effects of conduction can generally be neglected. In two-dimensional models the case of a doublet, in particular, is analysed. A new approach is used for solving the heat transport equation. The distributions of temperature, streamlines and potential, even if a natural uniform flow of groundwater exists, can be easily calculated and plotted. The computation of the temperature in the extraction well shows that the economical working time of a doublet can be longer than the thermal breakthrough time.
ARK: https://n2t.net/ark:/88439/y088418
Permalink: https://geophysicsjournal.com/article/124
 
A study of diffraction-like events on DEKORP2-S by Kirchhoff theory
Get PDFA method is described for calculating the seismic response of an arbitrarily shaped interface below a homogeneous medium by the Kirchhoff integral in the time domain. It is shown, by comparison with other numerical techniques, that this method yields accurate results for reflections. The errors in calculating diffractions are tolerable if the distance of the receiver from the shadow boundary of reflection is not too large; this is usually the case in horizontal seismic profiling. The method has been applied to model qualitatively some typical features in record sections of the deep seismic reflection profile DEKORP2-S. This profile is characterized by numerous strongly curved events that are concentrated mainly in two areas of the profile. These signals can be addressed as diffractions from an interpretation of the travel times. Dynamic calculations, however, show that the surprisingly high amplitudes cannot be explained by diffracting elements like fault edges or small-scale inhomogeneities; instead, one has to assume cylindrically or spherically curved reflectors with a radius of at least 4 km. Some possible geological explanations for these structures, like diapiric intrusions or antiformal stacks, are discussed in view of the tectonic evolution of the Central European Variscides.
ARK: https://n2t.net/ark:/88439/y087788
Permalink: https://geophysicsjournal.com/article/137
 
On the stress tensor in Vrancea region
Get PDFPresented is a new application of Gephart-Forsyth inverse technique for predicting directions and senses of slip. In Vrancea region, the compression axis is thus found to act in a NE-SW direction and not in a SE-NW direction, as is commonly thought from focal mechanism results. In addition, evidence is provided that the stresses in the crust are decoupled from those in the subducted slab.
ARK: https://n2t.net/ark:/88439/y054021
Permalink: https://geophysicsjournal.com/article/101
 
Paleomagnetic study of upper carboniferous volcanics from Sudetes (Poland)
Get PDFUpper Carboniferous and Permian volcanites from Sudetes were sampled in 11 localities (mean site coordinates: 50.8°N, 16.3°E). Thermal demagnetizations of the samples showed several magnetic components. But in one site, all high-temperature components are reversed. The interpretation of the results shows a first, primary magnetization acquired probably in the Upper Carboniferous before tilting (N = 8, D = 192°, I = -2°, k = 27, α95 = 11°, pole: 39°N, 181°E) and a remagnetization acquired later, during or after tilting (N = 5, D = 190°, I = -19°, k = 18, α95 = 18°, pole: 48°N, 181°E).
ARK: https://n2t.net/ark:/88439/y097077
Permalink: https://geophysicsjournal.com/article/125