| 1. |
- Routh, Joyanto, 1968-
(författare)
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Metamorphism and Structural Interpretation of the Zanskar Shear Zone, PJW Himalaya, India
- 1993
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Ingår i: Journal of the Geological Society of India. - : Geological society of India. - 0016-7622 .- 0974-6889. ; 41:3, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- Herren in 1987, reported normal faults resulted in the telescoping of metamorphic isograds within a 200 meter zone between Sumche Topko to Mulung Topko in the Zanskar Shear Zone NW Himalaya. However petrographic study-of sections obtained from Mulung Topko and surrounding areas (pensila-padarn section) indicate that the rocks belong to the kyanite-sillimanite-starurolite grade only. Extension crenulation cleavage and other shear criteria show an initial NESW movement after which a layer parallel extension occurred and the shear zone developed. Minieralogical assemblage and mapping indicate that the isograds run paralleI to each other before they possibly truncate against the shear zone along the Pensila-Padam section. A probable model has been proposed to explain the features
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| 2. |
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| 3. |
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| 4. |
- Öhlander, Björn, et al.
(författare)
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Delineation and character of the Archaean-Proterozoic boundary in northern Sweden
- 1993
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 64:1-4, s. 67-84
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Tidskriftsartikel (refereegranskat)abstract
- Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used Sm---Nd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative εNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ≈ 100 km farther to the north. The ≈ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ≈ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ≈ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.
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| 5. |
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| 6. |
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| 7. |
- Muiuane, Elónio
(författare)
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Hydrogeophysics of Tropical Africa : Recent advances and perspectives
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the present work an attempt is made to put into perspective the hydrogeophysical investigations for rural water supply carried out over the last two decades in tropical Africa, paying special attention to groundwater targets in the weathered rocks in and above the crystalline basement. The applicability of geophysical techniques in groundwater investigations in tropical Africa, particularly the DC resistivity and electromagnetic techniques is discussed in the light of recent developments in instrumentation, data collection and interpretation procedures.Although there has been an increase in the use of 2D resistivity surveying systems, standard VES is still the main tool of geophysical surveys for groundwater in Africa, where it is often used to characterise the lithology of the weathered layer. In the present work, two automatic 1D inversion schemes based on iterative least squares procedures with singular value decomposition are presented. In order to reinforce the convergence of the inversion schemes towards a global minimum, the standard least squares procedure is combined with other measures of robustness and search strategies in the parameter space. Tests with synthetic and field data prove their usefulness for fast interpretation of DC resistivity data. Compared with plane wave RMT data DC resistivity data generally seem to have less resolving power for the models tested.
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| 8. |
- Sirat, Manhal
(författare)
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Structural and neural network analyses of fracture systems at the Äspö Hard Rock Laboratory, SE Sweden
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The > 10,000 fractures documented in the 450m deep Äspö Hard Rock Laboratory (HRL) provide a unique opportunity to study brittle deformation of a Swedish bedrock mass. The fracture population consists of six major sets, one subhorizontal and five sub-vertical. A classical structural analysis explored the interrelations between geometry and frequency of both dry and wet fractures with respect to depth and in-situ stresses. Three main findings are:In-situ stresses govern frequency distributions of dilated, hence water-bearingfractures. About 68.5% of subhorizontal fractures are dilated in the thrustregime above a depth of ca. 230m while 53% of sub-vertical fractures aredilated in the underlying wrench regime.Fractures curve both horizontally and vertically, a finding confirmed by theapplication of artificial neural networks that included Back-Propagation andSelf-Organizing (Kohonen) networks.The asymmetry of the total fracture population and tilts of the sub-Cambrianpeneplain demonstrates that multiple reactivations of fractures have tilted theÄspö rock mass 6° to the west.The potential space problem raised by this tilt is negated by systematic curvature of steep fractures, some of which sole out to gently dipping fracture zones. Fractures probably developed their curvature when they formed deep in crystalline crust in Precambrian times but have since reactivated at shallow depths. These findings add significantly to the conceptual model of Äspö and should be taken into account in future studies regarding the isolation of Sweden's high-grade radioactive waste in crystalline bedrock.
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| 9. |
- Yao, Zhengsheng
(författare)
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Seismic tomography : Algorithms and applications
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A preferred algorithm for seismic tomography needs to have capabilities including computational efficiency, minimal computer memory requirements, easy regularization and nonuniqueness analysis. In this thesis, regularised recursive least squares for seismic tomography is developed. This algorithm is equivalent to full matrix inverse techniques but is preferable because the computational costs can be reduced by avoiding operations with zeros. Subjective choice of the a priori model covariance matrix based on ray density is conceptually similar to reparameterization of the model. The optimal weight, in a statistical sense of the minimum variance estimate, leads to a dynamic regularization. The main advantage of using RRLS is that it allows us to calculate the updated covariance matrix automatically. Because in RRLS storage for the updating covariance matrix is necessary, it is prefered for medium size seismic tomographic problems. The formulae derived for calculating the resolution and covariance matrices for the LSQR solver, which require only small changes in LSQR computer code, make LSQR become even more powerful in very large scale seismic tomography. Because with LSQR the inversion is usually carried out in a Krylov space the components of the solution are inevitably correlated to some extent. Therefore, it is necessary to examine the resolution matrix corresponding to increasing iteration number until an acceptable correlation level has been reached. Based on the Lanczos process, the algorithm for solving regularised tomographic problems in Krylov space was developed for facilitating the choice of the regularization parameter, which makes the regularization very practical in real applications. The two techniques considered for choosing the regularization level work well in our modeling. Because in real cases the errors in the observed data may not be known, the cross validation technique may be a good candidate for the choice of the regularization parameter. Finally, the regularized recursive least squares inversion method in combination with a finite difference travel time calculation technique was applied to the travel time data from the network in Costa Rica and the three-dimensional P and S wave velocity structure below Costa Rica were deduced. The tomographic images together with the resolution matrices provide interesting information regarding the geology and tectonics within this area. Comparing the results with that from the other methods shows that both the method and the results are reliable.
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| 10. |
- Wang, Yanmin, et al.
(författare)
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Magnetic aggregation in dispersions of mineral ultrafines
- 1995
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Ingår i: Journal of Dispersion Science and Technology. - : Informa UK Limited. - 0193-2691 .- 1532-2351. ; 16:2, s. 137-154
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Tidskriftsartikel (refereegranskat)abstract
- Particle aggregation can be enhanced in two ways; (a) field-induced magnetic moment and (b) magnetic moment due to the remanent magnetisation. Investigation of the magnetic field-induced aggregation of hematite and chromite in aqueous suspension with the use of a laboratory scale electromagnetic solenoid related the aggregation process to particle size and external magnetic field in the natural pH value of the dispersions. This study have shown that hematite ultrafines in a well-dispersed slurry are selectively aggregated with sized magnetite in the absence of aggregating reagents, high shear rates or an external magnetic field.
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| 11. |
- Ayele, Atalay
(författare)
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Seismicity and earthquake source mechanism study in the East African rift
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A new earthquake catalogue of the Horn of Africa has been compiled for the period from 1960 to 1993. The threshold magnitude of this catalogue is found to be 4.5 (mb). Spatial and temporal variation of seismic energy release and b-value are investigated for this catalogue. The b-value is lower in the continental part of the Afro-Arabian rift system (Afar and southern Sudan) than in the southern Red Sea and Gulf of Aden rifts which is in agreement with the increasing fault segmentation from north to south. From the recent spatial distribution of earthquakes in the East African Rift, Danakil & Aisha horsts and the Tanzanian Craton are behaving as prototypes of microplates.Earthquake source parameters in some parts of the East African Rift are studied for both the pre-WWSSN (World-Wide Seismographic Station Network) eras as well as afterwards. Digitized old seismograms from Wiechert-type instruments and bulletins are used to analyze source parameters for three pre-WWSSN events. The old analog seismograms are scanned and digitized using the nXscan software. P-wave first-motion readings and waveform data from short-period, broadband and long-period instruments are employed to investigate the earthquake source mechanisms.A total of 10 earthquakes in the magnitude range from 4.8 (mb) to 6.9 (Mw) are studied for fault mechanisms using P-wave first-motion readings and waveform inversion techniques. The results show both normal and oblique slip deformations with left-lateral strike-slip components. This indicates sinistral motion of the Somalia plate with respect to the Nubia plate, in agreement with the general E-W extension of the East African Rift.
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| 12. |
- Chouliaras, Gerasimos
(författare)
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Seismic quiescence patterns and earthquake prediction research : Testing hypotheses for Greece and China
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main aim of this investigation is to evaluate the most well known hypotheses and models in earthquake prediction research. For this reason we investigate the "localized" seismic quiescence hypothesis using the Z value griding method as well as the recently introduced Seismolap hypothesis of "extended" seismic quiescence.The results indicate that precursory seismic quiescence has preceded crustal main shocks in China and Greece and also that aseismic quiescence can be attributed to "silent earthquakes " such as creep events in subduction zones. The application of the Z-value method in investigating the recent seismic quiescence regime in Greece has given indications of three areas in Greece where significant quiescence is observed and which also are candidate areas for strong earthquakes according to their previous seismic history.Recently, the Gulf of Corinth in Central Greece has been appointed as an official multiparameter site for earthquake prediction research by the European Council and in this framework, magnetotelluric and seismological investigations revealed the presence of fluids in the Earth's crust that are mainly responsible for the observed seismic and electromagnetic anisotropy which correlates well with the local stress field as revealed from GPS measurements. In addition, the spectral fitting method was used on digital short period data from this region in order toobtain dynamic source parameters from local earthquakes and also to determine scaling relations using two different earthquake source models.
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| 13. |
- Quintero, Ronnie
(författare)
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Seismotectonics and lithospheric structure of Costa Rica
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interaction of different tectonic features on a short-distance scale of 100's of km produces the complexity of the Costa Rica region. The intraplate Caribbean earthquake activity of shallow-depth nature is mainly concentrated along an E-W axis in the central part of the country. All types of faults are presented in this region, although a strike-slip fault system, with NB-SW direction is proposed. Intraplate Caribbean esrthquakes in northwestem Panama are shallow and dominated by thrust fault motion. The seismicity along the Pacific coast is mainly associated with the subduction of the Cocos under theCaribbean plate, with predominant thrust fault mechanisms. The interaction of the Cocos and Nazca plates in southeastem Costa Rica, along the Panama fracture fault system produces events with strike-slip mechanism.A proposed 1D P-velocity model for the region is presented. Relocation of earthquakes of this area made through this ID-velocity model provides a reliable geometry of the Wadati-Benioff zone, with a slab dipping at 45° to a maximum depth of approximately 250 km.Station corrections and P- and S-velocity distribution correlate well with the major upper crust geological features of the area. The P- and S-velocity distribution for depth below 30 km shows a lateral velocity variation, which is associated with the subductionslab.
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| 14. |
- Tryggvason, Ari
(författare)
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Seismic tomography : Inversion for P- and S-wave velocities
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Seismology is undoubtedly the most powerful method for determining the structure of the Earth below the deepest borehole. This thesis describes the, use of seismic signals from both man-made and earthquake sources to model seismic velocities within a volume of the Earth. Based on data from explosions, a two-dimensional model for the P-wave velocities from the Baltic Shield in southern Sweden, across Denmark and into the North GermanBasin is presented. The observed crustal structures show clear signs of extension, estimated to be at least 20% along the profile. However, the emphasis of my work has been on modeling seismic velocities in three dimensions using local earthquake travel times. Two studies have been conducted, one using data from the Long Valley caldera, California. Originally formed in a catastrophic eruption 0.76 My ago, the caldera is today characterized by intense seismic activity. To better understand this volcanic system we have imaged the three-dimensional P- and S-wave velocity structure beneath the caldera. Low seismic velocities are observed below the resurgent dome down to depths of 9 km, which we believe are caused by hydrothermal circulation of fluids rather than by a large magma chamber. The observed lower than normal Vp/Vs ratios (<1.7) are associated with fractured rock containing compressible and low density fluids. Partially molten or hotmaterial will reduce the S- more than the P-wave velocities, thus producing elevated Vp/Vs ratios. The second study exploits the earthquake data catalog from southwest Iceland. Within the study region are several volcanic centers, active and extinct. The seismically most active one is the Hengill volcanic complex, within 30 km of Reykjavík. The large fissure system is characterized by lower than normal velocities and, similar to the Long Valley caldera, low Vp/Vs ratios. We conclude that supercritical magmaticfluids, such as CO2 and H20, circulate in the fissure system and cause the reduced velocities. These findings are important for understanding the nature of the volcanism and volcanic hazard in both study regions.
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| 15. |
- Persson, Kjell
(författare)
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Archaeological prospection
- 1998
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Ingår i: Laborativ Arkeologi. ; :10-11, s. 17-26
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Tidskriftsartikel (refereegranskat)
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| 16. |
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| 17. |
- Vassiljev, Jüri
(författare)
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Simulating the palaeorecord of northern European lakes using a coupled lake-catchment model
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A coupled lake-catchment model was developed to examine the controls on lake-level changes in currently overflowing lakes during Holocene. The lake energy-balance is simulated as the one-dimensional vertical heat transfer by eddy diffusion and convective mixing. The accumulation and ablation of lake ice, and snow cover on the ice, is simulated thermodynamically. The lake water-balance is given by the balance between precipitation over the lake, evaporation from the water surface, catchment runoff, and lake outflow. Runoff is calculated using a one-dimensional, two layer soil covered by vegetation and a snowpack. Snow accumulation and ablation are controlled by air temperature. Outflow is controlled by the outlet size and outflow velocity, using the Manning equation. The coupled model was validated for Lake Bysjön (southern Sweden) and Lakes Karujärv and Viljandi (Estonia). The simulated monthly lake level matched observations of lake-level changes between 1944-1956 at Lake Viljandi (r=0.78), between 1976-1987 at Lake Karujärv (r=0.78) and between 1973-1977 at Lake Bysjön (r=0.7). The model was used to examine lake-level sensitivity to changes in individual climatic parameters. Changes in radiation, temperature, vapour pressure and wind strength produce lake-level changes of <0.5 m. Changes in mean annual and winter precipitation produce changes an order of magnitude larger. The lake-level sensitivity to precipitation changes is greatest when winter temperatures were higher than present. The magnitude of the response to a specific climatic change is strongly affected by the ratio of the lake area to the catchment area.
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| 18. |
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| 19. |
- Basokur, Ahmet Tuǧrul, et al.
(författare)
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Comparison of induced polarization and controlled-source audio-magnetotellurics methods for massive chalcopyrite exploration in a volcanic area
- 1997
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Ingår i: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 62:4, s. 1087-1096
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we compare and contrast the results of field experiments with the dipole-dipole-induced polarization (IP) and controlled-source audio-magnetotellurics (CSAMT) methods, along the same survey profiles, at a test area that was subject to extensive drilling and detailed geological investigation. The ore bodies are interbedded between two series of dacitic tuff. The depth and thickness of the massive chalcopyrite-pyritesphalerite body vary between 25 and 100 m and 0.5 and 16 m, respectively. Resistivity and IP phase measurements on the core samples collected from the test area provide some idea of the relative differences between the background rock units and the target. The resistivity of the chalcopyrite samples varies between 0.6 to 2 ohm-m and provides sufficient resistivity contrast with surrounding volcanic rock units for target detection. The results of dipole-dipole IP profiling with a 50-m dipole length conducted along two profiles are presented in the form of apparent resistivity and phase pseudosections. CSAMT measurements were made at 13 frequencies from 2 Hz to 8192 Hz along three profiles. The receiver dipole length was 25 m. The CSAMT data are presented in the form of pseudosections using conventional and new definitions of apparent resistivity. The elliptical contours of low apparent resistivity generated by the transition-field notch can be misleading with respect to the real anomaly of the ore body. These artificial anomaly patterns are suppressed by making use of an alternative apparent resistivity definition derived from the frequency-normalized impedance. The qualitative interpretation based on the IP and CSAMT pseudosections shows that the location and the extension of the ore body are indicated better in the CSAMT apparent resistivity data computed from the alternative definition. The qualitative interpretation of the IP data is difficult because of the 3-D effect arising from the neighboring thicker parts of the ore body and pyrite particles within the basement. At the final stage, the far-field range of the CSAMT apparent resistivity and phase data is identified by the visual inspection of the individual sounding diagrams. The models derived from the 2-D Occam inversion carried out on the far-field data are compared with the drill-hole information and are found to describe the actual geological situation.
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| 20. |
- Belay, R., et al.
(författare)
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One‐dimensional non‐linear inversion of magnetotelluric data : the importance of data errorrs
- 1995
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Ingår i: Geophysical Prospecting. - : Wiley-Blackwell. - 0016-8025 .- 1365-2478. ; 43:7, s. 905-918
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Tidskriftsartikel (refereegranskat)abstract
- Inversion of noisy magnetotelluric data over a horizontally stratified earth has no unique solution. Instead, the resistivities and thicknesses of the layers can only be specified to lie within certain bounds at a particular confidence level. The significance of having correctly estimated data errors when calculating the parameters and parameter bounds is discussed. Emphasis is put on the very corrunon problem of not being able to obtain models which can be considered acceptable with a reasonable certainty from a statistical point of view. A method is presented to obtain a pragmatic data error description by adjustment of wrongly estimated data errors while keeping the suppression of 1D data features at a minimum. A comparison is made between calculation of the parameter bounds based on approximating the confidence surface by hyperellipsoids and a calculation based on a full non‐linear most‐squares analysis. In general it is found that the approximation works very well when the confidence limit is small. However, significant deviations are found in some cases.
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| 21. |
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| 22. |
- Berthelsen, A., et al.
(författare)
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Recording marine airgun shots at offsets between 300 and 700 km
- 1991
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 18:4, s. 645-648
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Tidskriftsartikel (refereegranskat)abstract
- This paper demonstrates that - under favorable conditions - by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up to 700 km, the maximum offset at which the authors attempted to record data.^Besides a powerful airgun array, a low noise environment at the recording site and the elimination of static shifts are the prerequisites to obtain refracted and reflected arrivals from the crust and upper mantle at such large offsets.^Primary arrivals detected at offsets between 400 and 700 km image the upper mantle from 70 to about 120 km depth.^Stacking of neighboring shots and/or receivers successfully increases the signal-to-noise ratio, if the traces have been corrected for offset differences, which requires knowledge of the apparent phase velocities.^The data presented here were collected in autumn 1989 during the BABEL Project on the Baltic Shield.
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| 23. |
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| 24. |
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| 25. |
- Bylund, Göran, et al.
(författare)
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The Dala dolerites, central Sweden, and their palaeomagnetic signature
- 1992
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 114:1, s. 143-153
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Tidskriftsartikel (refereegranskat)abstract
- New palaeomagnetic results are presented from nine dolerites from central Sweden. Data are also presented from four porphyry sites and one granite site. The data obtained are compared with previously published data from the area. Based on the directions of the characteristic magnetizations isolated in the various rock units a division into five groups, A-E, has been made. Group A consists of dolerites of late Sveconorwegian (c. 1050-850 Ma) age and group B comprises dolerites belonging to the Central Scandinavian Dolerite Group (c. 1250-1150 Ma). Groups C and D are less clearly defined. Group C, with a shallow negative northerly remanence inclination represents the Tuna dykes (c. 1370 Ma). The D direction, with a shallow positive northerly inclination, may represent an older generation of dolerites. The border between groups C and D is diffuse and may be the result of a prolonged mid-Proterozoic period with dolerite intrusions in Dalarna during which significant apparent polar wander occurred. Group E comprises dolerites with a steep positive inclination close to the direction of the Earth's present field. The porphyries and the granite studied carry group D and E remanence directions. This strengthens the evidence for a D-group generation of dolerites contemporary with the porphyries and the granite, c.1650 Ma ago.
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
- Elming, Sten-åke
(författare)
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Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic Shield
- 1990
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 348, s. 34-38
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Tidskriftsartikel (refereegranskat)abstract
- Plate tectonics provides the linking framework for all tectonic and magmatic activity seen today, but it is not known when plate tectonics first developed on Earth. New deep seismic reflection and coincident refraction profiles across an exposed, 1.89-Gyr-old volcanic arc complex show a 10-km-thick offset in the Moho and bivergent reflectors in the crust, which were most probably created by plate convergence, subduction and accretion during the Early Proterozoic. Hence, plate tectonic models seem to be applicable for at least the second half of Earth's history.
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| 31. |
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| 32. |
- Elming, Sten-åke, et al.
(författare)
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Palaeomagnetism and the Siljan impact structure, central Sweden
- 1991
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 105:3, s. 757-770
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Tidskriftsartikel (refereegranskat)abstract
- A palaeomagnetic study of rocks, from inside and outside the impact structure has been carried out, with the aim of identifying natural remanent magnetizations (NRMs) related to the impact event. Three different directions of magnetizations, not recognized elsewhere, were identified within the central part of the structure. These three magnetizations are defined in terms of coercivity and blocking temperature. The implications of the results are discussed in relation to ages obtained from Ar-Ar dating of impact melt
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| 33. |
- Elming, Sten-åke
(författare)
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Palaeomagnetism of Precambrian rocks in northern Sweden and its correlation to radiometric data
- 1994
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 69:1-4, s. 61-79
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Tidskriftsartikel (refereegranskat)abstract
- A palaeomagnetic study has been performed on Palaeo- to Mesoproterozoic basic intrusions and volcanic rocks from the Fennoscandian shield in northern Sweden. Three, possibly four, different generations of magnetizations were identified, the oldest assigned to a Svecofennian age (1.86-1.89 Ga). A second generation is related to the intrusion of granitoids of 1.80-1.76 Ga. In this geological event probably also the third group of directions has its origin. These different magnetizations may indicate that there are at least two different generations of basic intrusions in northern Sweden. A fourth group of directions is isolated as overprints. This magnetization is interpreted to be a Subjotnian magnetization, reflecting a previously unrecognized regional Subjotnian metamorphic event in the northwestern part of the Fennoscandian shield. The drift history for the Fennoscandian shield during the period 1.88-1.50 Ga has been defined based on these new palaeomagnetic data
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| 34. |
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| 35. |
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| 36. |
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| 37. |
- Elming, Sten-åke, et al.
(författare)
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Results of magnetotelluric and gravimetric measurements in western Nicaragua, central America
- 1997
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 128:3, s. 647-658
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Tidskriftsartikel (refereegranskat)abstract
- Studies a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. The purpose of this study is to provide electric conductivity and density constraints on geological crustal structures along the transect. This may then serve as a base for the understanding of the tectonic evolution of this part of Central America. Questions to be answered are: (1) is the Pacific coastal province an accreted terrane? (2) What is the character of the depression? Is this a hidden contact between an accreted terrane and the continental Chortis block? (3) If so, is there a different physical signature for the crust of the Pacific province compared to that of the Highlands?
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| 38. |
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Elming, Sten-åke, et al.
(författare)
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The drift of the Fennoscandian and Ukrainian Shields during the Precambrian : a Palaeomagnetic analysis
- 1993
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 223:3-4, s. 177-198
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Tidskriftsartikel (refereegranskat)abstract
- A revised Precambrian (2.85-0.6 Ga) Apparent Polar Wander Path (APWP) for the Fennoscandian Shield, based on a new compilation and analysis of data, is presented. In fitting the APW path to successive Grand Mean Palaeomagnetic poles (GMPs), we applied the spherical spline technique originally developed by Jupp and Kent in 1987. The position and orientation of the Fennoscandian Shield during 2.85-0.6 Ga was determined from the GMPs. Major palaeoclimatological findings are used to constrain the palaeomagnetic interpretation of palaeolatitudes. The general drift of Fennoscandia, from relatively high latitudes in the late Archaean-Early Proterozoic to nearly equatorial latitudes in the Middle Proterozoic, correlates with palaeoclimatological indications that a period of cold climate was followed by one of warm climate during this time interval. From the continuous APWP the APW velocities and latitudinal drift velocities of the shield were calculated. An accumulated APW curve was also calculated. The palaeomagnetic data are irregularly distributed and some periods are rather poorly represented. This means that the calculated velocities can sometimes be artifacts of sampling. Late Archaean and Early Proterozoic (2.85-1.90 Ga) data are too sparse to make these calculations meaningful and velocity calculations are therefore restricted to data of 1.90 Ga and younger ages. The accumulated APW curve shows a number of linear segments with varying slopes, indicating sudden changes in drift rate. During the Middle Proterozoic (1.90-1.35 Ga) there was a period when the rate of APW was constant and low and that of latitudinal drift also was low. This pattern changed at ca. 1.35 Ga, and the following Middle-Late Proterozoic period can be described by rapid APW and strongly fluctuating drift velocities. Jotnian rifting and the intrusion of numerous dyke swarms (at ca. 1.25 Ga) correlate with this shift in rate. These changes are attributed to changes in plate configuration. A new database for the Ukrainian Shield is also presented, and GMPs in the 2.32-1.20 Ga range are defined. The database is still inadequate and the comparison of the Ukrainian and Fennoscandian drift histories is therefore tentative. Similarities in position, latitudinal drift and rotation during the Early-Middle Proterozoic are, nevertheless, evident. A close relationship between the shields in this period is consistent with the low APW rate of Fennoscandia, indicating that Fennoscandia may have been part of a larger continent, including the Ukraine, at that time. At ca. 1.2 Ga, the latitudinal position of Ukraine differed significantly from that of Fennoscandia, suggesting that the large shield split up between ca. 1.35 and 1.2 Ga. This would explain the change in APW rate at 1.35 Ga. The subsequent increase in rate was due to a reduction in the size of the shield. The discrepancy in palaeopositions of Fennoscandia and Ukraine at 1.2 Ga led Mikhailova and Kravchenko to suggest a late Precambrian time (1.07-0.57 Ga) for the accreation of Fennoscandia to the East European Platform (EEP). This may be correct as the rate of APW for Fennoscandia decreased in the late Precambrian, reflecting such a consolidation.
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| 44. |
- Friborg, Johan
(författare)
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Experimental and theoretical investigations into the streaming potential phenomenon with special reference to applications in glaciated terrain
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The occurrence of an electrical potential difference between the ends of a capillary tube when a fluid flows through is known as the streaming potential phenomenon. It was reported by Quincke in 1859 and was studied by Helmholtz, among others, in the nineteenth century. Its geophysical manifestation is the development of electrical potential differences in the ground when groundwater flows through porous rocks or soils. The phenomenon has been comparatively little studied in a geophysical context. The present thesis is the outcome of the author's experimental and theoretical research in the phenomenon. Natural streaming potentials, along with other electrical potentials in the ground that are present even in the absence of an artificially injected current, are also known as self-potentials (SP) or self-potential anomalies. Small-scale field measurements in the present work have demonstrated that SP observations within areas of size 0.5 m by 0.5 m appear to be approximately normally distributed. Hence a mean of such observations can be accepted as a representative value of the potential of the "point". The experimental work in the thesis was undertaken to simulate the natural phenomenon in the laboratory. An equipment to measure the streaming potentials developed across soil samples as a function of the applied pressure was designed. The total applied pressure could be varied between approximately 15 and 400 kPa. Pressure differences and electrical potential differences could be measured with an accuracy of about 0.1 kPa and 1 mV, respectively. The streaming potential developed across a sample is generally observed to be proportional to the pressure difference and the constant of proportionality is called the streaming potential coefficient C. This was determined for a number of sand and moraine samples. Two methods to estimate C from field observations of SP have been developed. The first can be regarded as a correction to observations of potentials due to water flow in slopes ("topographic SP") and the second is an active method where pumping from a well is used as a controlled source of streaming potentials. A comparison of values of C obtained from laboratory measurements and from estimates based on field observations showed that laboratory data can give reasonable estimates of the in situ value of C. Field observations of SP at several different sites have been used to illustrate the stability of the potentials over extended periods of time as long as conditions in the ground stay the same. When the appearance of an SP-anomaly changes this generally reflects significant changes in the conditions in the ground. It appears that anomalies with an amplitude exceeding about 10 mV, and probably even smaller ones, are significant. A case history illustrates the occurrence of streaming potentials in a practical field situation. It is shown that the removal of a topographic trend enhances the appearance of any local anomaly patterns present in the data. In the case under consideration these patterns reflect both variations in the electrical resistivity and presence of self-potentials not of a streaming origin. The apparent streaming potential coefficient can be obtained from a plot of SP versus elevation but it was found to vary with time due to variation in the near-surface resistivity. The streaming potential phenomenon can be described by means of the theory of coupled flows which expresses the flow (of, e.g., charge, matter or heat) as a linear combination of driving forces (gradients of , e.g., electric potential, pressure or temperature). The formulation is well suited to numerical modelling, and a detailed examination of the generation of sources of conduction current in the streaming potential problem has been made. A numerical study illustrates the calculation of conduction current source terms in a practical example. A qualitative discussion of the generation of sources of conduction current, by flow of ground water, for some simple geological models has been made to further illustrate the physical mechanisms behind the streaming potential phenomenon. Although not strictly a modelling tool, a method to estimate the limiting depth to a streaming potential source region has also been devised using the formal analogy between streaming potentials and magnetostatics and following Smith's analysis for the determination of the maximum depth to the top of a magnetised body.
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| 45. |
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| 46. |
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| 49. |
- Hobbs, R. W., et al.
(författare)
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Integrated seismic studies of the Baltic shield using data in the Gulf of Bothnia region
- 1993
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Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 112:3, s. 305-324
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Tidskriftsartikel (refereegranskat)abstract
- In the autumn of 1989 a co-operative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. the 121 litre airgun array used for this profiling was also recorded by 62 muiticomponent land stations to provide coincident refraction surveys, fan-spreads, and 3-D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high-resolution impedance contrast images as well as more regional 3-D velocity models in both P- and S-waves. In the Bothnian Bay a south-dipping, non-reflective zone coincides with the conductive Archaean-Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic-arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide-angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Åland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non-reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70-1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. the c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near- and far-offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.
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