| 1. |
- Elming, Sten-åke, et al.
(författare)
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A palaeomagnetic and Ar-40/Ar-39 study of late precambrian sills in the SW part of the Amazonian craton: Amazonia in the Rodinia reconstruction
- 2009
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 178:1, s. 106-122
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Forskningsöversikt (refereegranskat)abstract
- A new key palaeomagnetic pole (Plat. = 64.3 degrees S, Plon. = 271.0 degrees E, N = 14, A(95) = 9.2 degrees; Q = 5) is calculated from a primary magnetization isolated in early Neoproterozoic Aguapei basic sills and dykes hosted by 1.3-1.0 Ga sedimentary rocks from the southwestern part of the Amazon craton. The characteristic remanence carried by stable, pseudo-single domain titanomagnetite shows two antipodal polarities that pass a reversals test. Magnetic anisotropy for most sites shows fabric orientations that are typical of sills, with horizontal magnetic foliations concordant to the flat-lying bedding of the host sedimentary rocks. Ar-40/Ar-39 analyses for one of the sills reveal a well-defined plateau age at 981 +/- 2 Myr. A tectonic reconstruction for Amazonia relative Laurentia based on this new pole 'is consistent with' a position of the present northwestern part of Amazonia attached with eastern Laurentia close to Greenland at ca. 981 Ma. On basis of palaeomagnetic and geological data, we propose a scenario where Amazonia moved northeastwards along the present southeast coast of Laurentia from ca. 1200 to 980 Ma. By 980 Ma, Amazonia is placed alongside Laurentia and Baltica, in a position similar to other reconstructions of Rodinia but with a significantly different orientation.
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| 2. |
- Eneroth, Erik
(författare)
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Preservation of a magnetization carried by magnetite-inclusions during amphibolite grade metamorphic conditions? An example from the Sarek Dykeswarm, Northern Swedish Caledonides
- 2002
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Ingår i: Geophysical Journal International. - 0956-540X. ; 150:1, s. 65-78
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Tidskriftsartikel (refereegranskat)abstract
- An outcrop consisting of a 608 Myr old sheeted dyke complex and dyke-intruded metasediments has been investigated palaeomagnetically. It is located in the interior of a nappe the basal and upper parts of which suffered penetrative deformation and amphibolite grade metamorphism during the Caledonian orogeny. The only trace from the orogeny at the outcrop is a widely spaced network of brittle and ductile shear zones crosscutting dyke contacts. Foliated amphibolite consisting of hornblende, quartz, plagioclase, garnet and sphene occur locally within them. The opaque mineralogy of the dykes differs from that commonly found in continental dykes, with ilmenite and pyrrhotite being the major visible opaque phases, and there are indications that magnetite inclusions exsolved from silicates carry a primary remanence even though Ar-39-(40) Ar mica ages were reset at ca. 430 Myr during the orogeny. A secondary component carried by pyrrhotite is less prominent but falls on the Baltican APW path at 430 Myr. The inferred primary component has a large directional uncertainty, but indicates low palaeolatitudes for the Baltscandian margin during rifting.
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| 3. |
- Greve, Annika, et al.
(författare)
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The geomagnetic field intensity in New Zealand : Palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre
- 2017
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Ingår i: Geophysical Journal International. - 0956-540X. ; 211:2, s. 814-830
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Tidskriftsartikel (refereegranskat)abstract
- Very few absolute palaeointensity data exist from Holocene-aged rocks in New Zealand. Here we present a new suite of high-quality palaeointensities, supported by detailed rock magnetic investigations. Samples from 23 sites representing 10 distinct eruptive units of the Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand, were studied. Both traditional double heating and microwave palaeointensity methods were employed. The reliability of the palaeointensity data varies with rock magnetic properties of the samples, corresponding, in particular, to their positions within the lava flows. The highest success rates are from samples obtained from near the flow tops where a significant proportion of the remanence unblocked at intermediate temperatures (200-350 °C). By contrast, samples from flow centres, particularly the parts showing platey fracturing, have the lowest success rates. Reliable, high-quality palaeointensity results ranging from 32.4 ± 5.1 μT to 72.1 ± 4.7 μT were obtained from six flows with ages between c. 12 800 yr BP and the present. These correspond to virtual dipole moments that increase from 52 ± 10 ZAm2 in the early Holocene and peak at 112 ± 14 ZAm2 about 300 yr ago. The data agree well with calibrated relative palaeointensities from New Zealand lake sediments. The volcanic and sedimentary data together yield a Holocene virtual axial dipole moment curve that fits the global average variation well in the early Holocene, but which differs significantly in recent millennia. This difference is associated with recent migration of the southern high latitude core-mantle boundary flux lobe towards New Zealand, as is seen in global field models.
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| 4. |
- Hall, Stuart, et al.
(författare)
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Palaeomagnetic palaeolatitudes of the Ontong Java Plateau from 120 to 55 Ma: implications for the apparent polar wander path of the Pacific Plate
- 2007
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Ingår i: Geophysical Journal International. - 0956-540X. ; 169:2, s. 455-470
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Tidskriftsartikel (refereegranskat)abstract
- We present palaeomagnetic inclinations from Late Cretaceous-Early Tertiary deep-water carbonates obtained during ODP Leg 192 drilling at the Ontong Java Plateau (OJP) for the following periods: magnetochrons C33n (73.6-79.1 Ma), C32 (71.1-73.6 Ma), C27r-C31n (61.3-68.7 Ma) and C25-C26 (55.9-60.9 Ma). Compaction-induced inclination shallowing is considered to be negligible for the OJP sediments examined here because: (i) their palaeomagnetic inclinations are in excellent accord with those recently obtained from underlying OJP basement rocks, (ii) the studied sediments have anisotropy of anhysteretic remanent magnetization fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations and (iii) in the few cases where we observe a significant difference between our new OJP sedimentary palaeomagnetic data and data from other parts of the Pacific Plate, obtained mainly from seamount magnetization studies and skewness analyses, the sedimentary inclinations are not systematically lower, and therefore, cannot be explained in terms of inclination shallowing. Combining our new data with existing OJP palaeomagnetic data we obtain an internally consistent data set that we interpret to indicate northward motion of the plateau from its formation at similar to 120 Ma until 55 Ma. At 120 Ma the central plateau was located at 24 degrees +/- 4 degrees S. Approximately 45 Myr later (similar to 76 Ma) the plateau is located near 21 degrees S. Our data, therefore, indicate that the OJP was either stationary or exhibited a slow northward latitudinal drift during this interval. In contrast, from similar to 76 Ma until 68 Ma the plateau moved rapidly through approximately 10 degrees of latitude to similar to 10 degrees S. These intervals of slow and rapid motion track similar motions previously proposed for the entire Pacific Plate during the Cretaceous but extends the interval of slow motion into the Late Cretaceous. From 68 Ma until 56 Ma the plateau was again almost latitudinally stationary before moving slowly northward. More direct palaeomagnetic data are necessary to better define Pacific Plate motions during this time interval.
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| 5. |
- Ji, Yuntao, et al.
(författare)
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Characterization of pore structure and strain localization in Majella limestone by X-ray computed tomography and digital image correlation
- 2015
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 200:2, s. 699-717
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Tidskriftsartikel (refereegranskat)abstract
- Standard techniques for computed tomography imaging are not directly applicable to a carbonate rock because of the geometric complexity of its pore space. In this study, we first characterized the pore structure in Majella limestone with 30 per cent porosity. Microtomography data acquired on this rock was partitioned into three distinct domains: macropores, solid grains, and an intermediate domain made up of voxels of solid embedded with micropores below the resolution. A morphological analysis of the microtomography images shows that in Majella limestone both the solid and intermediate domains are interconnected in a manner similar to that reported previously in a less porous limestone. We however show that the macroporosity in Majella limestone is fundamentally different, in that it has a percolative backbone which may contribute significantly to its permeability. We then applied for the first time 3-D-volumetric digital image correlation (DIC) to characterize the mode of mechanical failure in this limestone. Samples were triaxially deformed over a wide range of confining pressures. Tomography imaging was performed on these samples before and after deformation. Inelastic compaction was observed at all tested pressures associated with both brittle and ductile behaviors. Our DIC analysis reveals the structure of compacting shear bands in Majella limestone deformed in the transitional regime. It also indicates an increase of geometric complexity with increasing confinement-from a planar shear band, to a curvilinear band, and ultimately to a diffuse multiplicity of bands, before shear localization is inhibited as the failure mode completes the transition to delocalized cataclastic flow.
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| 6. |
- Johansson, Sara, et al.
(författare)
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An alternative methodology for the analysis of electrical resistivity data from a soil gas study
- 2011
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Ingår i: Geophysical Journal International. - 0956-540X. ; 186:2, s. 632-640
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this paper is to present an alternative method for the analysis of resistivity data. The methodology was developed during a study to evaluate if electrical resistivity can be used as a tool for analysing subsurface gas dynamics and gas emissions from landfills. The main assumption of this study was that variations in time of resistivity data correspond to variations in the relative amount of gas and water in the soil pores. Field measurements of electrical resistivity, static chamber gas flux and weather data were collected at a landfill in Helsingborg, Sweden. The resistivity survey arrangement consisted of nine lines each with 21 electrodes in an investigation area of 16 x20 m. The ABEM Lund Imaging System provided vertical and horizontal resistivity profiles every second hour. The data were inverted in Res3Dinv using L-1-norm-based optimization method with a standard least-squares formulation. Each horizontal soil layer was then represented as a linear interpolated raster model. Different areas underneath the gas flux measurement points were defined in the resistivity model of the uppermost soil layer, and the vertical extension of the zones could be followed at greater depths in deeper layer models. The average resistivity values of the defined areas were calculated and plotted on a time axis, to provide graphs of the variation in resistivity with time in a specific section of the ground. Residual variation of resistivity was calculated by subtracting the resistivity variations caused by the diurnal temperature variations from the measured resistivity data. The resulting residual resistivity graphs were compared with field data of soil moisture, precipitation, soil temperature and methane flux. The results of the study were qualitative, but promising indications of relationships between electrical resistivity and variations in the relative amount of gas and water in the soil pores were found. Even though more research and better data quality is necessary for verification of the results presented here, we conclude that this alternative methodology of working with resistivity data seems to be a valuable and flexible tool for this application.
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| 7. |
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| 8. |
- Johansson, Sara, et al.
(författare)
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Spectral induced polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties
- 2020
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 220:2, s. 928-950
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Tidskriftsartikel (refereegranskat)abstract
- With advances in data acquisition and processing methods, spectral inversion of time domain (TD) induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole–Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain (FD) systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of this study is, thus, to perform a direct comparison of inverted spectral parameters from TD IP field data with FD IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50-m-long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with FD spectral IP in laboratory. The results showed that the inverted Cole–Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous–Palaeogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous–Palaeogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Palaeogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of cylindrical bryozoan fragments could be identified in the inverted Cole–Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole–Cole parameters from the field scale TD IP tomography were comparable to the magnitude and shape of FD IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.
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| 9. |
- Koene, Erik F.M., et al.
(författare)
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Eliminating time dispersion from seismic wave modeling
- 2018
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 213:1, s. 169-180
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Tidskriftsartikel (refereegranskat)abstract
- We derive an expression for the error introduced by the second-order accurate temporal finitedifference (FD) operator, as present in the FD, pseudospectral and spectral element methods for seismic wave modeling applied to time-invariant media. The 'time-dispersion' error speeds up the signal as a function of frequency and time step only. Time dispersion is thus independent of the propagation path, medium or spatial modeling error.We derive two transforms to either add or remove time dispersion from synthetic seismograms after a simulation. The transforms are compared to previous related work and demonstrated on wave modeling in acoustic as well as elastic media. In addition, an application to imaging is shown. The transforms enable accurate computation of synthetic seismograms at reduced cost, benefitting modeling applications in both exploration and global seismology. The Author(s) 2017. Published by Oxford University Press. All rights reserved.
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| 10. |
- Martin, Tina, et al.
(författare)
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Desaturation effects of pyrite–sand mixtures on induced polarization signals
- 2022
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 228:1, s. 275-290
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Tidskriftsartikel (refereegranskat)abstract
- Induced polarization (IP) is an acknowledged method in ore exploration and can be applied to evaluate the metal content in dumps containing the residues of ore processing facilities. Existing models explain the relationships between ore content and grain size of the ore particles with IP parameters. However, the models assume full water saturation of the ore containing samples, which is often not the case in field conditions at dump sites. Hence, our study investigates the effect of desaturation on the resulting IP signal. We used six different sand–pyrite mixtures with varying amount and grain sizes of the pyrite particles. Evaporative drying desaturated the samples. Complex conductivity spectra were recorded in the frequency range between 0.02 and 1000 Hz at certain saturation levels. The resulting spectra indicate an decrease of the conductivity amplitude with progressing desaturation. This effect agrees with the second empirical Archie equation. The saturation exponent of the conductivity amplitude shows values slightly larger than one. The measured spectra were processed by a Debye decomposition. We observe a nearly constant total chargeability during desaturation. This finding is in agreement with existing models that relate the total chargeability to the metal content in the sample. However, the mean relaxation time decreases remarkably during the drying process, whereas the normalized relaxation time, which considers the ratio between the mean relaxation time and the resistivity of the embedding material, does not indicate any dependence on water saturation. This behaviour contradicts existing models that predict a decreasing relaxation time with increasing water salinity, which results from evaporative drying. In order to explain the observed effects, we propose a conceptional model that compares a mixture of pyrite particles in an embedding material (sand, water and air) with an electrical RC circuit. The pyrite grains behave as small condensers that are charged and discharged via the conductive background material. According to this simple physical model, the relaxation time is proportional to the resistivity of the embedding material. A resistivity increase while desaturation causes an increase of relaxation time as observed in our experiments. This conceptional model is in good agreement with other experiments that change the resistivity in the background material by varying water salinity or clay content. The capacitive behaviour of the dispersed particles is characterized by the normalized relaxation time that depends on the grain size and the volume content of the pyrite particles.
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