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Steady state creep under constant stress has been measured in a fine grain granite (aplite) from Schauinsland, Germany for the temperature range of partial melting 860°–1060°C, under a hydrostatic pressure of 4.2 kbar, and at low shear stresses of 5–50 bar. The apparatus used is described briefly. Rheological measurements are complemented by microscopic investigations. With a melt fraction of up to about 20%, creep can be described by a power law with a stress exponent of 3-4 and an activation energy of 80 kcal/mole, typical for creep in solids. Above 1010°C or 20% melt, the activation energy increases rapidly to a value of 200 kcal/mole simultaneously with a rapid increase of the melt fraction and a decrease of feldspar content. From the grain structure and from etching tests it is concluded that quartz contributes little to the plastic deformation which is governed mainly by the stress and temperature induced recrystallization of feldspar. The large temperature dependence of the creep rate above 1010° C may be caused by the decreasing area of grain contacts and consequent rise in local stress. These results support those of Arzi (1974) and Roscoe (1952).
ARK: https://n2t.net/ark:/88439/y060550
Permalink: https://geophysicsjournal.com/article/258
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The parameters of the auroral electrojet are described by using an ionospheric sheet current model. Data from a line of magnetometers between 62 and 67 degrees magnetic latitude are the input for the computation of the parameters during electrojet activity. The parameters are depicted in the parameter-time diagram which furnishes mainly the following information: 1. Applicability of the two dimensional current model. 2. Latitudinal extent of the electrojets as a function of time. 3. Location of centre of the current as a function of time. 4. Current flow direction.Two case studies are carried out. The first comprises DMSP (Defense Meteorological Satellite Program) data, the second makes use of data collected simultaneously by the Scandinavian Twin Auroral Radar Experiment (STARE). The results of the parameter-time diagram and the DMSP as well as the STARE data are in good agreement.
ARK: https://n2t.net/ark:/88439/y059691
Permalink: https://geophysicsjournal.com/article/257
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Palaeomagnetic data from the Osterhorngruppe in the Northern Calcareous Alps (NCA) southeast of Salzburg are presented. The investigations were concentrated on the red nodular Adneter Kalk (limestone) of Liassic age which carries a stable natural remanent magnetisation. Rockmagnetic investigations revealed magnetite as the carrier of the remanent magnetization. The obtained palaeomagnetic results do not contradict a northward shift of the NCA of the order of several hundred kilometres and indicate a clockwise rotation of about 45° of the Northern Calcareous Alps with respect to Eurasia, since the Jurassic. Time of possible shift and rotation are briefly discussed.
ARK: https://n2t.net/ark:/88439/y040372
Permalink: https://geophysicsjournal.com/article/256
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In this paper the origin of precursors to P' P' with lead times greater than 50 s is investigated. Good NORSAR records of P'650P' and P'400P' for these arrivals as well as corresponding slowness estimates are presented. These phases are interpreted as done by others in terms of underside reflections from discontinuities or sharp transition zones in the upper mantle. An extensive search of 5 years of NORSAR records did not produce any significant evidence on P' P' precursor arrivals with lead times greater than 50 s other than those mentioned above.
ARK: https://n2t.net/ark:/88439/y030873
Permalink: https://geophysicsjournal.com/article/255
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A narrow heat flow transition zone between the Fennoscandian Shield and the North Sea Basin has been investigated along a profile from the Precambrian of South Norway to the Danish Embayment in North Jylland. Along this profile the surface heat flow varies from about 42 mWm-2 (measured) in South Norway to 60–70 mWm-2 (estimated) in Denmark. Geothermal, seismic, gravity and other geophysical and geological data have formed the basis for construction of heat production and thermal conductivity models in the depth interval 0–50 km. The related steady-state temperatures and heat flow distributions are calculated by a numerical solution of the heat conduction equation in two dimensions. Three models are presented, a preferred model and two others which yield temperatures assumed to be close to the lowest and highest possible values. The preferred model gives temperatures of about 350 °C at the crust-mantle boundary in the Shield and approximately 700 °C beneath the Danish Embayment. These differences are associated with considerable variations in the heat flow from the mantle. In the main model variations from 16-17 mWm-2 in the shield region to about 40 mWm-2 in the Danish Embayment have been found. Some geophysical and petrological implications are discussed. In the sedimentary basin partial melting in the lower crust and at shallow depth in the uppermost mantle seems to be likely.
ARK: https://n2t.net/ark:/88439/y020204
Permalink: https://geophysicsjournal.com/article/254
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Eighteen sites (342 samples) from Upper Cretaceous Wadi Natash volcanics (24.5° N, 34.5° E) yield a mean direction of magnetization D = 345.4°, I = 16.7° with α95 = 8.5°, k = 21.4, N = 15 after AF cleaning resulting in a pole at 69.3° N, 258° E with A95 = 5.8° All sites have normal polarity consistent with their age and the magnetic stratigraphy in the Cretaceous. From 5 sites (85 samples) from Upper Cretaceous Nubian sandstone at Wadi Natasha mean direction D = 358.1°, I = 32.0° with α95 = 8.7°, k = 143 (mixed polarity) was obtained after thermal demagnetization. Combined with previous investigations on Nubian sandstone at other locations in Southern Egypt (Schult et al. 1978) this yields a pole at 81.8° N, 223° E with A95 = 3.3°, N = 23. 9 sites from Eocene Baharia iron ores (27.5° N, 29.0° E) yield a mean direction D = 188.0°, I = –43.6° and α95 = 6.4°, k = 65 with a pole at 83.5° N, 139° E and A95 = 7° In addition the palaeomagnetism of some Tertiary basaltic rocks in Northern Egypt was studied. The polar wander path for Africa in Mesozoic time is presented showing more mobility than in earlier papers. For appropriate reconstructions of South America with respect to Africa the polar wander paths of both continents are substantially in agreement.
ARK: https://n2t.net/ark:/88439/y010555
Permalink: https://geophysicsjournal.com/article/252
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Densities of 1294 Precambrian rocks from the Southern Indian Shield, confined to low-grade granite-greenstone (Karnataka) and high-grade granulite (Eastern Ghats Belt) terrains are presented and categorized into 16 metamorphic and igneous varieties. In Karnataka, densities of granites (2.65 g/cm3), gneisses (2.68 g/cm3) and granodiorites (2.73 g/cm3) are considerably less than those of greenstone belts (2.84 g/cm3) and dolerites (3.03 g/cm3). An average density of 2.75 g/cm3 is obtained for surface rocks from this terrain. Over the Eastern Ghats belt, densities of charnockites range from 2.71 g/cm3 to 3.12 g/cm3 as they vary from acidic to ultrabasic composition. An average density of 2.85 g/cm 3 is obtained for surface rocks from this belt. The contrast in the average densities of surface rocks from the lowand high-grade terrains may play a significant role in a proper assessment of the regional gravity fields over these terrains. The results compare well with the data from similar terrains in other shield regions. Magnetic susceptibilities for 482 of these samples are also presented. All the rock types include a wide range of susceptibilities, as is to be anticipated in metamorphic terrains. A plot of density versus magnetic susceptibility for thirteen of these rock types suggests a linear relationship.
ARK: https://n2t.net/ark:/88439/y009916
Permalink: https://geophysicsjournal.com/article/251
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As a contribution to the International Magnetospheric Study the University of Munster has installed an array of 32 Gough-Reitzel type magnetometers located mostly in Northern Scandinavia. Also for the IMS, the Max Planck Institute for Aeronomy at Lindau is operating the Scandinavian Twin Auroral Radar Experiment (STARE) which consists of two nearly identical backscatter radars located near Trondheim (Norway) and Sauvamaki (Finland). For a weak isolated substorm on October 7, 1976 the spatial structure of the electron density irregularities observed by the Trondheim-radar and the equivalent current distribution derived from the magnetic measurements have been compared. A good correspondence has been found between the location and magnitude of the maxima of the horizontal magnetic disturbance and the radar backscatter amplitude for an eastward electrojet. For most of the comparison there appeared also to be good agreement between the direction of the equivalent current and the direction antiparallel to the line-of-sight irregularity drift. This supports the idea that the backscatter irregularities are caused by current driven plasma instabilities and that it is possible to determine auroral ionospheric currents with the backscatter radar technique. However, during periods of enhanced electron precipitation, differences between the drift directions given by the two methods were observed.
ARK: https://n2t.net/ark:/88439/y099827
Permalink: https://geophysicsjournal.com/article/250
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The reflectivity method for the calculation of theoretical body-wave seismograms is extended to include a double-couple point source. Theoretical seismograms of SV waves from this type of source are presented for models of the Earth's crust and the crust-mantle boundary, and for models of the whole Earth. In the models of the crust-mantle boundary, there are up to four SV head waves, depending on the sharpness of the transition. The most remarkable one is slightly slower and later than Sn and has unusually low frequencies. Theoretical SV-wave seismograms for models of the whole Earth for periods from 15 s to 60 s and in the epicentral distance range from 10° to 160° show as prominent phases S, ScS, SKS and SKKS. SKS, and SKKS are different in wave form, in agreement with observations from long-period WWNSS stations. A diffracted wave SPdiffKS + SKPdiffS is found in the theoretical seismograms whose travel-time curve is tangential to that of SKS at a distance of 107 °. The resulting interference causes the wave form of SKS to change markedly around 120°. Since the theoretical seismograms are the complete response of the Earth models from the crust-mantle boundary down to the inner core, they also include many multiple and converted phases. Besides those associated with the crust-mantle boundary there are phases related to the transition zones in the upper mantle. Their amplitudes depend strongly on the sharpness of these zones.
ARK: https://n2t.net/ark:/88439/y089618
Permalink: https://geophysicsjournal.com/article/249
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Palaeosecular variation records have been obtained from three cores from Lac de Joux, Switzerland ( 46°37' N, 6°16' E) on which a parallel biostratigraphy has been constructed. All three cores show a well developed Late-glacial sequence which extends to beyond 13,400 years B.P. and the recorded palaeomagnetic declination and inclination variations can be correlated between these cores. The longer core (no. 3) also shows a well developed Post-glacial sequence and the declination and inclination records can be correlated in detail with the independently dated United Kingdom records which extend back to about 10,000 years B.P. Ages along Lac de Joux core 3, obtained by palaeomagnetic correlation with U.K. cores, are compared with ages based on the local palynology and thus a timescale has been attached to the Lac de Joux record back to beyond 13,400 years B.P. during which time there is no evidence of any geomagnetic excursion or short event.
ARK: https://n2t.net/ark:/88439/y079999
Permalink: https://geophysicsjournal.com/article/247
