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In 1975, a rifting episode started in the neovolcanic zone of northern Iceland, consisting of a succession of slow inflation periods and rapid subsidence events, which is still going on. The center of activity is situated below the Krafla caldera, and the rifting process is affecting the 80-km-long fissure swarm associated with this central volcano. Gravity and height variations associated with this process have been investigated by re-observing profiles earlier established in the Namafjall and in the Gjastikki area, situated nearly 10 km south and north of Krafla respectively, as well as by the re-observation of a number of gravity stations in the northern part of the fissure zone, in 1976, 1977, and 1978. By repeated observations with 2 or 3 LaCoste-Romberg gravity meters, the accuracy obtained in each gravity survey is of the order of ± 10 x 10-8 ms-2. In the profiles crossing the fissure zones, a rate of gravity increase of more than 100 x 10-8 ms-2/a has been found in the central part, while gravity at the flanks decreases at the same order. These variations are correlated with subsidence and elevation rates of the order of 0.5 m/a.
ARK: https://n2t.net/ark:/88439/y060150
Permalink: https://geophysicsjournal.com/article/268
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The complex geological-geophysical expedition of the Russian Academy of Sciences jointly with the National Energy Authority (Orkustofnun) of Iceland conducted in 1977 a detailed seismic investigation of the flood basalt and rift zones in northern Iceland, similar to studies in southwestern Iceland. Continuous seismic profiling with 4–6 shot points was carried out across a total length of 90 km. In the flood basalt zone shallow refracting horizons were observed with Vp = 4.3–4.5 km/s and tilted toward the rift zone. They closely correspond to dense sheets of flood basalts mapped geologically. Refracting and reflecting horizons at 2 to 12 or 15 km depth are tilted more gently in the same direction. A slight depression filled with neovolcanic formations with VP < 3 km/s exists near the surface in the rift zone. Reflectors with steep tilt can be traced to depths of 15 km. The deepest part of the depression is west of Myvatn in the region of current volcanism and rifting. A seismically homogeneous body without reflectors was detected under this region at 10–15 km depth. On the whole, the structure of the rift zone in northern Iceland is similar to, but more complex than, that of southwestern Iceland.
ARK: https://n2t.net/ark:/88439/y050551
Permalink: https://geophysicsjournal.com/article/267
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The low-temperature oxidation process in titanomagnetite has been investigated. Using samples with controlled grain sizes, it has been shown that there is a critical grain size in the oxidation behaviour of titanomagnetites; smaller grains undergo low-temperature oxidation, while larger ones separate to Fe-rich titanomagnetite and Ti-rich hemo-ilmenite (high-temperature oxidation). The difference between the results of similar experiments by Ozima and Sakamoto (1971) and by Readman and O'Reilly (1972) can be explained in terms of differences in partide sizes of titanomagnetites. Comparable results were obtained from natural subaerial and submarine basalts; when they are heated to temperatures between 150° C and 375° C, oxidation proceeds in submarine basalts, but it does not in subaerially erupted basalts and a high-temperature oxidation process occurs. This fact indicates that the grain size of the magnetic minerals is one of the most important controlling factors in low-temperature oxidation of titanomagnetites.
ARK: https://n2t.net/ark:/88439/y041502
Permalink: https://geophysicsjournal.com/article/266
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Converted phases from the mantle transition zone have been observed as precursors about 1 min before the main S-wave phases S, SKS and ScS in long-period records of the Grafenberg array (GRF) and a few European WWSSN stations, at epicentral distances from 70-90°. The 23 earthquakes used were located along the west coast of America from Alaska to Ecuador, with a concentration of events in Central America, and in East Asia from the Aleutians through Japan to Sumatra. Relatively strong converted energy was observed for the American earthquakes, except for two events in the South Mexico/Guatemala region. The East Asian earthquakes produced significantly less precursor energy. The interference of conversions from P to SV below the focus and from SV to P below the stations is studied with theoretical-seismogram calculations. Due to interference precursors are normally stronger on the horizontal-radial than on the vertical component; this is in agreement with the observations. In special cases with either maximum or minimum P radiation towards the station conversion takes place only on one side, and precursor observations can be related directly to structure either below the focus or the station. The data set includes such favorable cases. The interpretation of observed precursors in terms of the fine structure in the conversion zone is difficult, even in favorable cases, because of the low resolving power of long-period converted phases. Nevertheless, the following conclusions can be drawn from the observations presented. Most of the precursor observations for the American events are compatible with typical models of the transition zone between upper and lower mantle, having two discontinuities at depths of about 400 and 670 km. Such a structure applies for western Europe and for the Caribbean Sea/Gulf of Mexico region, in the latter case with a possible local interruption by a smoother transition zone. A relatively smooth transition zone below East Asia from about Korea to the Sea of Okhotsk can also explain the lack of precursor energy for a few earthquakes in and close to Japan. These results indicate large-scale lateral variations in the sharpness of the mantle transition zone.
ARK: https://n2t.net/ark:/88439/y031913
Permalink: https://geophysicsjournal.com/article/265
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Material for paleomagnetic research has been collected from sedimentary rocks of Mesozoic age from four localities in Central Iran. Detailed paleomagnetic analyses have been carried out using partial progressive demagnetization procedures both with alternating magnetic fields and with heating in order to isolate the characteristic remanence. The following results were obtained: Early Triassic Sorkh Shales with D = 289°, I = 21°, α95= 14°; Late Jurassic Garedu Red Beds with D = 4°, I = 42°, α95 = 14°; Late Jurassic - Early Cretaceous Bidou Beds with D = 48°, I = 32.5°, α95 = 19°; Middle Cretaceous Dehuk Sandstones with D = 326°, I = 38.5°, α95 = 21°. The large α95 values are due to the rather small collections of specimens, roughly 50 each. From the paleomagnetic data we conclude that in Early Triassic times Central Iran, which forms part of the Iranian-Afghan micro-continent, belonged to Gondwana; since Early Jurassic times the area has been positioned close to the Eurasian continent. The scatter in the declination of the remanence directions can be explained in terms of rotations of individual blocks along the main Central-Iranian fault systems.
ARK: https://n2t.net/ark:/88439/y021734
Permalink: https://geophysicsjournal.com/article/264
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Regional type curves depicting secular variations of declination and inclination through the last 10,000 calendar years constructed for north-western Europe (356° E, 55° N) and east-central North America (270° E, 46° N) by stacking palaeomagnetic data derived from lake sediment cores are described and analysed. The spectral content and phase relationships of the two pairs of curves show that they have a complex origin with both drifting and standing geomagnetic sources contributing to them. The strongest evidence of drifting sources is provided by the inclination type-curves which exhibit maximum correlation for a phase shift of ~650 years suggestive of westward drift at a rate of about 0.13 degrees a year. At the same time, comparison of the declination type-curves strongly suggests that waxing and waning standing sources were dominant. We show that the difference in relative importance of drifting as compared to standing geomagnetic sources implied by the patterns of correlation deduced respectively for declination and inclination can, at least in principle, be attributed to observation point/geomagnetic source geometry by modelling the secular variation that would be produced by standing but oscillating equatorial dipoles and radial dipoles located deep within the outer core, by a pair of drifting deep-seated radial dipoles of constant intensity, and by drifting sheets of radial dipoles (taken to represent current-loops) located at shallow depth within the outer core. Each of these model sources produces secular variation curves with distinctive shapes and phase relationships. Hence, an attempt is made to identify qualitatively the types and locations of the sources which dominated the secular variations as recorded by our type-curves. One of our most important conclusions is that there appears to be a' turning-point' at ~ 4 750 years before present when the relative amplitudes of the active' standing' sources changed but the characteristics of the drifting sources appear to have remained relatively unchanged.
ARK: https://n2t.net/ark:/88439/y011535
Permalink: https://geophysicsjournal.com/article/263
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The dependence of auroral backscatter amplitude on different ionospheric parameters (conductivity, current density, electric field) is studied by means of data recorded by STARE (Scandinavian Twin Auroral Radar Experiment), the two-dimensional Scandinavian Magnetometer Array (SMA), and auroral all-sky cameras. The observations were made on 16 March 1978 during pre-substorm conditions in the region of the eastward polar electrojet. The paper shows that in this event the auroral backscatter amplitudes in the 140 MHz frequency band were controlled mainly by spatial variations in the electron density or conductivity inside the back-scatter volume. To a certain extent also a linear relationship between backscatter amplitude and ionospheric current density was found but it is regarded as a special case of a more general relationship between electron density and backscatter amplitude. A stable discontinuity in the Hall conductivity over the most equatorward auroral arc was deduced from the data: On the equatorward side the conductivity was 3-5 times higher than on the nearby poleward side. Our conclusions are discussed in the light of some previously published results on the same subject.
ARK: https://n2t.net/ark:/88439/y000016
Permalink: https://geophysicsjournal.com/article/262
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Long-range seismic experiments carried out in different regions of the USSR show strong variations of the mantle wave velocities and attenuation. Relatively high mantle velocities, more than 8.2–8.3 km/s, are observed in the old platform areas. To exclude the crustal influence on the mantle waves and to separate the vertical and horizontal inhomogeneity effect the relations between the overlapping travel-time curves are analyzed. As a result, a mantle stratification with positive average velocity gradient in the strata is revealed up to depths of 200 km. Local inclusions with very high (more than 8.5 km/s) and relatively low (less 8.0 km/s) velocities are typical of the mantle.
ARK: https://n2t.net/ark:/88439/y090547
Permalink: https://geophysicsjournal.com/article/261
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Compressional and shear wave velocities have been measured in the laboratory at up to 1.0 GPa effective pressure on a suite of granulite facies rocks from the Seiland Petrographic Province. The suite may be of Precambrian age and the measurements were made to test a proposal that the suite represents lower continental crust. Compressional wave velocities at 0.4 GPa effective pressure range from 6.41–6.97 km s-1 with a mean of 6.71 km s-1 . Measurements of the parameter (∂V/∂Tp) suggest a value of a –0.8 x 10-3 km s-1 ºC-1 , and the temperature corrected mean velocity of about 6.5 km s-1 at 20 km depth is comparable to that found in parts of the lower crust in Scandinavia. The mean Poisson's ratio of 0.29 at 0.4 GPa is slightly higher than that found (0.276), but could be reduced if heterogeneity in the suite and the effect of higher pressures are taken into account. The occurrence of lower continental crust as a thrust slice in the Province is consistent with current ideas on the geological evolution of this area.
ARK: https://n2t.net/ark:/88439/y080958
Permalink: https://geophysicsjournal.com/article/260
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Sanidines from 16 tuffs of the first volcanic phase in the Siebengebirge and the Siebengebirge-Graben were dated by the K/Ar-method. Ages range from 24.1 (Nachtigallental) to 22.9 M.y. (core Rott). Three mineral ages from surface outcrops in the Central Siebengebirge (23.9 ± 0.5 M.y.) are statistically indistinguishable from those of the sub-surface samples Stieldorf-1, Stieldorf-2, and Rott (23.0 ± 0.5 M.y.). These ages confirm the Uppermost Oligocene biostratigraphic age of the 'Blatterkohle Rott' which is interlayered with the trachyte tuff. Sanidine and biotite mineral ages from five trachyte samples give similar apparent ages from 26.4 to 24.6 M.y. These results show the trachytes to be older than the tuffs, in disagreement with the geologically established sequence. The sequence of eruptions in the Siebengebirge area however (trachyte-tuff, trachyte, latite and alkaline-basalt) was confirmed wherever outcrops allow observations. This discrepancy is discussed. The apparent ages of three dated latites are in the same range (26.2 to 25.1 M.y.), in agreement with the geological sequence. The latite ages show that the time span between the eruptions of the trachytes and the latites must have been very small. Isolated basalts belonging to a fourth phase yield K/Ar ages from 25 to 19 M.y. Pliocene volcanism could not be verified in the Siebengebirge. In addition, stratigraphically well-dated glauconites from the Tertiary of the Niederrhein area were used to correlate our isotopic age date with the Cenozoic time scale.
ARK: https://n2t.net/ark:/88439/y070559
Permalink: https://geophysicsjournal.com/article/259
