| 1. |
- Elming, Sten-åke, et al.
(author)
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A palaeomagnetic study and age determinations of Tertiary rocks in Nicaragua, Central America
- 2001
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 147:2, s. 294-309
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Journal article (peer-reviewed)abstract
- Palaeomagnetic, K-Ar and Ar40/Ar39 measurements have been made to elucidate Nicaragua's plate tectonic history. These show that the Nicaraguan Highland rotated 90° counterclockwise between 30 and 18 Myr ago, but the Pacific Coastal Plain has not rotated.A lack of data has prevented a direct comparison with the tectonic history of adjacent areas, in particular, the Chortis Block, which is the part of the Caribbean Plate that is geologically most closely related to the Nicaraguan Highland. However, our data suggest that the Caribbean Plate in this part of Central America was not consolidated until c. 15 Ma.
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| 2. |
- Cherevatova, M., et al.
(author)
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A multi-resolution approach to electromagnetic modelling
- 2018
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In: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 214:1, s. 656-671
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Journal article (peer-reviewed)abstract
- We present a multi-resolution approach for 3-D magnetotelluric forward modelling. Our approach is motivated by the fact that fine-grid resolution is typically required at shallow levels to adequately represent near surface inhomogeneities, topography and bathymetry, while a much coarser grid may be adequate at depth where the diffusively propagating electromagnetic fields are much smoother. With a conventional structured finite difference grid, the fine discretization required to adequately represent rapid variations near the surface is continued to all depths, resulting in higher computational costs. Increasing the computational efficiency of the forward modelling is especially important for solving regularized inversion problems. We implement a multi-resolution finite difference scheme that allows us to decrease the horizontal grid resolution with depth, as is done with vertical discretization. In our implementation, the multi-resolution grid is represented as a vertical stack of subgrids, with each subgrid being a standard Cartesian tensor product staggered grid. Thus, our approach is similar to the octree discretization previously used for electromagnetic modelling, but simpler in that we allow refinement only with depth. The major difficulty arose in deriving the forward modelling operators on interfaces between adjacent subgrids. We considered three ways of handling the interface layers and suggest a preferable one, which results in similar accuracy as the staggered grid solution, while retaining the symmetry of coefficient matrix. A comparison between multi-resolution and staggered solvers for various models shows that multi-resolution approach improves on computational efficiency without compromising the accuracy of the solution.
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| 3. |
- Elming, Sten-åke, et al.
(author)
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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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In: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 178:1, s. 106-122
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Research review (peer-reviewed)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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| 4. |
- Elming, Sten-åke, et al.
(author)
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Palaeomagnetism and the Siljan impact structure, central Sweden
- 1991
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In: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 105:3, s. 757-770
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Journal article (peer-reviewed)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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| 5. |
- Elming, Sten-åke, et al.
(author)
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Results of magnetotelluric and gravimetric measurements in western Nicaragua, central America
- 1997
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In: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 128:3, s. 647-658
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Journal article (peer-reviewed)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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| 6. |
- Elming, Sten-Åke, et al.
(author)
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Uplift deduced from remanent magnetization of a proterozoic basic dyke and the baked country rock in the Hoting area, Central Sweden: a palaeomagnetic and Ar-40/Ar-39 study
- 2009
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In: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 179:1, s. 59-78
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Journal article (peer-reviewed)abstract
- Remanent magnetization in dyke contact zones in the Hoting area of the Fennoscandian Shield in the central part of Sweden has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked contact test for two Subjotnian dykes demonstrates the primary nature of the dyke magnetization. From the magnetic properties and the palaeomagnetic data, it can be concluded that the overprinting in the hybrid zone of one of the basic dykes is a partial thermoremanent magnetization. Reliable results were also obtained from a palaeointensity study of samples from the hybrid zone in the baked host rock. The study was performed in the laboratories at Luleå and at Scripps and a mean intensity of the Earth's magnetic field of 5.8 ± 1.9 μT was determined. The maximum temperature due to the dyke intrusion in the hybrid zone has been defined and from that an ambient temperature of 375 °C in the host rock at the time of intrusion has been calculated. This calculated temperature is not contradicted by the 40Ar/39Ar data. A palaeothermal gradient in the crust at ca. 1.6 Ga is calculated at ca. 34 °C km-1 and yields a depth of burial of the present erosion surface at ca. 10.4 km. This implies an uplift rate of 0.65 km (100 Ma)-1. A slow cooling of the gabbroic host rock (ca. 3.5 °C Ma-1) has been calculated from the difference in the U-Pb age of zircon (1.786 ± 0.010 Ga) and the 40Ar/39Ar biotite ages (1.648 ± 0.012 Ga; 1614 ± 0.024 Ga) of this study. This slow cooling resulted in a palaeomagnetic age of ca. 1.7 Ga for the gabbro, which is also the age of the determined palaeointensity. The calculated ca. 1.614 Ga palaeomagnetic pole from the basic dykes fulfils most of the criteria for a reliable pole and may be regarded as a new key-pole for Fennoscandia.
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| 7. |
- Hobbs, R. W., et al.
(author)
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Integrated seismic studies of the Baltic shield using data in the Gulf of Bothnia region
- 1993
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In: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 112:3, s. 305-324
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Journal article (peer-reviewed)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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| 8. |
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| 9. |
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| 10. |
- Ju, Ma, et al.
(author)
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Moment tensor inversion with three-dimensional sensor configuration of mining induced seismicity (Kiruna mine, Sweden)
- 2018
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In: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 213:3, s. 2147-2160
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Journal article (peer-reviewed)abstract
- Mining induced seismicity is an undesired consequence of mining operations, which poses significant hazard to miners and infrastructures and requires an accurate analysis of the rupture process. Seismic moment tensors of mining-induced events help to understand the nature of mining-induced seismicity by providing information about the relationship between the mining, stress redistribution and instabilities in the rock mass. In this work, we adapt and test a waveform-based inversion method on high frequency data recorded by a dense underground seismic system in one of the largest underground mines in the world (Kiruna mine, Sweden). A stable algorithm for moment tensor inversion for comparatively small mining induced earthquakes, resolving both the double-couple and full moment tensor with high frequency data, is very challenging. Moreover, the application to underground mining system requires accounting for the 3-D geometry of the monitoring system. We construct a Green's function database using a homogeneous velocity model, but assuming a 3-D distribution of potential sources and receivers. We first perform a set of moment tensor inversions using synthetic data to test the effects of different factors on moment tensor inversion stability and source parameters accuracy, including the network spatial coverage, the number of sensors and the signal-tonoise ratio. The influence of the accuracy of the input source parameters on the inversion results is also tested. Those tests show that an accurate selection of the inversion parameters allows resolving the moment tensor also in the presence of realistic seismic noise conditions. Finally, the moment tensor inversion methodology is applied to eight events chosen from mining block #33/34 at Kiruna mine. Source parameters including scalar moment, magnitude, double-couple, compensated linear vector dipole and isotropic contributions as well as the strike, dip and rake configurations of the double-couple term were obtained. The orientations of the nodal planes of the double-couple component in most cases vary from NNW to NNE with a dip along the ore body or in the opposite direction.
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