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We report on a 40Ar — 39Ar study of karnaite from the ⁓ 17 km Lappajarvi impact crater, Finland. Four samples from a 3,000 m profile across the crater center give rather well defined age plateaux and indicate complete degassing at the time of the impact event. The mean age is 77 m.y., much younger than geologically derived age estimates.
ARK: https://n2t.net/ark:/88439/y062490
Permalink: https://geophysicsjournal.com/article/186
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Various parameters pertinent to the lower crustal layer under Precambrian regions are listed for locations where seismic, and geomagnetic or geoelectric, studies have been undertaken. The parameters define three distinct types of lower crustal layer with certain dominant characteristics: Type I – "Normal" – typical continental seismic parameters and a high electrical resistivity (103–104 Ωm); Type II – "Intermediate" – high compressional wave velocity (either fixed Vp = 7.0 km s-1 or transitional Vp = 6.7 → 7.3 km s-1) and a moderate resistivity (100–300 Ωm); Type III – "Low" – a low shear wave velocity layer (LVsL), high Poisson's ratio (> 0.30) and low electrical resistivity (10–50 Ωm). Possible conditions and rock types, existing at the P–T environment of the lower crust and which could account for the observations, are suggested. The zoning of Canada into types implies that Type II layers are shield "edge" effects, and that inability to observe what is regarded as the final stage of development of a shield region under certain shields may be due to their being too small.
ARK: https://n2t.net/ark:/88439/y052001
Permalink: https://geophysicsjournal.com/article/185
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Detailed geological and magnetic mapping in an area of Pliocene and Plio-Pleistocene volcanic rocks in Southwestern Iceland has enabled us to correlate a 2,100-m-thick lava succession with similar dated sequences in Iceland and with the ocean-floor geomagnetic polarity time scale. This correlation, supported by additional K-Ar dating, implies (1) that the succession is between 4.2 and 1.8 Ma in age, (2) that at least 13 glaciations occurred in Western and Southwestern Iceland between 3.1 and 1.8 Ma ago, and (3) that at least two geomagnetic events are present in the Lower Matuyama epoch. Paleomagnetic results from 353 igneous units, mostly basalt lavas, are tabulated. Analysis of directions from 258 of these shows them to possess some serial correlation; their mean is very close to a central axial dipole field value, but explanations are proposed for observed systematic departures from this field in other Icelandic paleomagnetic survey results.
ARK: https://n2t.net/ark:/88439/y042132
Permalink: https://geophysicsjournal.com/article/184
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The University of Kiel cosmic ray experiment on board Helios 1 measures nucleons above 1. 7 MeV/nucleon and electrons above 0.3 MeV in the inner solar system between 1.0 and 0.3 AU from the Sun. A first survey is given on quiet time proton and Helium spectra which are compared near Earth and close to perihelion. The anomalous Helium component is also present at radial distances within 0.4 AU. Quiet time Helium spectra from 3.8 to 48 MeV/nucleon gradually increase between December 1974 and June 1975. For the integral radial gradient (protons above 51 MeV) we estimate a value of (11±2.5) %/AU during a period of slowly increasing cosmic ray intensity. We discuss solar particle events on January 5 (at 0.93 AU), March 7 (at 0.41 AU), and March 19, 1975 (at 0.32 AU). The March 19 event was measured closer to the Sun than any other event before. It exhibits sharp temporal structures, differences in the time profiles of various particle species, and a large abundance of Helium 3, with a 3He/4He ratio of 2 to 3 in the range 5 to 7 MeV/nucleon. This event occurred close to the peak of a high speed solar wind stream.
ARK: https://n2t.net/ark:/88439/y032443
Permalink: https://geophysicsjournal.com/article/183
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The effect of stress waves on the magnetization of basalt was studied. The stress waves were generated by impacting cylindrical basalt samples with aluminium projectiles. The 3 mm thick aluminium plates were accelerated in a non-magnetic compressed air gun accelerator to velocities ranging from 20 to 160 m/s, corresponding to peak stresses in the basalt between 2.5 and 10 kbar. The duration of the stress impulse was about several micro-seconds. For the experiments a basalt with well-known magnetic properties was used (Rauher Kulm, Germany). The magnetizing effect of the stress waves was determined as a function of the number of impacts, the intensity and direction of the applied magnetic field ( < 10 Oe) and the peak stress amplitude. In the used stress range the measured shock remanent magnetization (SRM) tends to a final steady value after 5 or 6 impacts. This value is proportional to the intensity of the applied field and increases with the peak stress applied. The produced SRM can be erased with maximum ac-fields of about 150 to 200 Oe. Any dependance of SRM on the direction of the applied magnetic field could not be recognized within the accuracy limits of the experiments. The demagnetizing effect of stress waves on the highfield (1,000 Oe) isothermal remanent magnetization, the low-field (1 Oe) thermoremanent magnetization and the natural remanent magnetization was studied as a function of the number of impacts and the peak stress. A final steady state of magnetization is generally obtained after 4 or 5 impacts. With increasing peak stresses increasingly harder remanent magnetizations can be demagnetized, with stresses of 2.5 kbar corresponding to coercive forces of about 75 Oe, 5.5 kbar to about 125 Oe and 8 kbar to about 175 Oe.
ARK: https://n2t.net/ark:/88439/y023624
Permalink: https://geophysicsjournal.com/article/182
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ISEE-1 and 2 satellite observations at the dayside magnetopause have enabled important progress to be made on the solar wind/magnetosphere coupling problem. The ISEE results have emphasized the significance of reconnection and have indicated that the process can occur both in a quasi-steady form and in an unsteady (flux transfer event (FTE)) manner. The detection of FTEs on open field lines within the magnetosphere, the discovery of reverse polarity FTEs, and the observation that FTEs can be associated closely with intervals of quasi-steady reconnection flow, all support further the view that FTEs are reconnection phenomena. Indeed, a sharp distinction need not exist between FTEs and the type of reconnection event described as quasi-steady.
ARK: https://n2t.net/ark:/88439/y013595
Permalink: https://geophysicsjournal.com/article/181
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Most of the existing seismic-refraction profiles in the Rhenish Massif/Rhenohercynian zone of Western Germany have been jointly reinterpreted using traveltime and amplitude information. The general pattern of observed phases can be divided into three types; each type corresponds to a distinct kind of velocity structure. Type I: Throughout the central Rhenish Massif and the adjacent Hessische Senke a strong P-phase reflection from the crust-mantle boundary is recorded in regions where no major volcanic features are crossed by the lines of seismic observations. The average crustal thickness is 28-29 km, the average crustal velocity (excepting sediments) is 6.2-6.3 km/sec, and the crust is nearly homogeneous. This structure is here referred to as the Rhenohercynian crustal model. Type II: Beneath the southern part of the Rhenish Massif and two areas in the northeast and southeast some structure within the crust is evident. Both an intracrustal and the Moho discontinuities are evidenced by strong reflected phases, the Moho reflection being the stronger one. Along the profiles crossing major volcanic features such as Vogelsberg and central Westerwald, but not beneath the eastern Eifel, the M-discontinuity is heavily disrupted or "smeared" and an intermediate intracrustal boundary at about 20 km depth forms the main reflector for seismic waves. Beneath this boundary the velocity increases gradually from about 7 km/sec to upper-mantle velocities. Type III: For profiles crossing the northern Rhine Graben area as well as for a line from the Siebengebirge through the Rhenish Massif to the north, east of the Lower Rhine basin, the observed phases indicate only one major seismic boundary at a depth of about 23 km where the velocity increases rapidly to 7.3 km/sec. Below this boundary the velocity increases gradually with depth reaching 8 km/sec at 27-28 km. The occurrence of types I, II, and III can be roughly correlated with tectonic setting. The Pn phase is recorded with variable success and disappears completely on a profile passing the eastern Eifel volcanics, but is clear on the lines through Vogelsberg and central Westerwald. The petrographic differences between these volcanics appear such to be reflected in the behaviour of the seismic waves. Cross sections and areal views are used to display the variations in crustal and upper mantle velocity structure.
ARK: https://n2t.net/ark:/88439/y002726
Permalink: https://geophysicsjournal.com/article/180
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On October 26, 1975, data from two parallel meridian chains of densely spaced magnetometers in Northern Scandinavia indicated the presence of an evening sector eastward electrojet which between 1606 and 1710 UT was two-dimensional. By separating the relevant components of the magnetic disturbance fields into internal and external parts, and by subsequent upward continuation (towards the source) of the external horizontal component, equivalent height-integrated ionospheric current densities were derived as a function of latitude and time. Whereas the demarcation line between the eastward and westward ionospheric electrojet currents remained stationary, the sharp southern border of the eastward electrojet moved towards the south with a speed of about 50 ms-1 , possibly indicating an earthward movement of the inner edge of the magnetospheric plasma sheet. The maximum eastward height-integrated current density was of the order of 0.5 Am-1 , the corresponding large-scale field-aligned current density has been estimated to have been of the order of 1.0 μAm-2 . After 1630 UT there was evidence for a superposed stationary small-scale structure (wavelength of the order of 250 km) in the eastward flow, which would imply a pair of local field-aligned current sheets possibly related to an auroral arc.
ARK: https://n2t.net/ark:/88439/y092617
Permalink: https://geophysicsjournal.com/article/179
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Mathematical methods for linearized geophysical inverse problems are reviewed, in cases with and in cases without constraints. The role of constraints receives particular attention, both in linear convex problems and in an exactly solvable non-linear example.
ARK: https://n2t.net/ark:/88439/y081128
Permalink: https://geophysicsjournal.com/article/178
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Observations of seismic arrivals from the lower lithosphere in France revealed a low mean velocity of observed first arrivals of about 8.1 km/s between 150 and more than 1,000 km distance and several high velocity phases with apparent velocities between about 8.2 and 8.6 km/s. Each one of these high velocity phases is only observed over a range of about 100 or 200 km. The possibility of interpreting these phenomena in terms of fine structure of the lower lithosphere is investigated. The resulting model consists of alternating high and low velocity layers. No first order discontinuities seem to exist between these layers. A reliable quantitative estimate of the thickness of these high and low velocity zones can only be obtained by a joint travel time and amplitude study.
ARK: https://n2t.net/ark:/88439/y070109
Permalink: https://geophysicsjournal.com/article/177
