The Fennoscandian Shield: A treasure box for deep electromagnetic studies

Korja, T.; Hjelt, Sven‐Erik

doi:10.1007/bfb0011904

Cited by 4 publications

(4 citation statements)

References 62 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Detailed discussion on the significance of sub-surface conductivity variations can be found from previous synthesis (e.g. Korja and Hjelt, 1998;Zhamaletdinov, 1996). We also wish to emphasise that the map-and the underlying database-is a result of the compilation of all the existing 1D-and 2D-resistivity models obtained before the BEAR experiment.…”

Section: Discussion Of S-modelsmentioning

confidence: 99%

Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield

et al. 2014

View full text Add to dashboard Cite

A priori knowledge on large-scale sub-surface conductivity structure is required in many applications investigating electrical properties of the lithosphere. A map on crustal conductivity for the Fennoscandian Shield and its surrounding oceans, sea basins and continental areas is presented. The map is based on a new database on crustal conductance, i.e. depth integrated conductivity, where all available information on the conductivity of the bedrock, sedimentary cover and seawater are compiled together for the first time for the Fennoscandian Shield. The final model consists of eight separate layers to allow a 3D description of conductivity structures. The first three layers, viz. water, sediments and the first bedrock layer, describe the combined conductance of the uppermost 10 km. The other five bedrock layers contain the data of the crustal conductance from the depth of 10 km to the depth of 60 km. The database covers an area from 0• E to 50• E and 50• N to 85• N. Water conductances are estimated from bathymetric data by converting depths to conductances and taking into account the salinity variations in the Baltic Sea. Conductance of the sedimentary cover includes estimates on the conductance of both marine and continental sediments. Bedrock conductances are extrapolated from 1D-and 2D-models. Extrapolations are based on data from magnetometer array studies, airborne electromagnetic surveys and other electromagnetic investigations as well as on other geophysical and geological data. The crustal conductivity structure appears to be very heterogeneous. Upper crust, in particular, has a very complex structure reflecting a complex geological history. Lower crust seems to be slightly more homogeneous although large regional contrasts are found in both the Archaean and Palaeoproterozoic areas.

show abstract

Section: Discussion Of S-modelsmentioning

confidence: 99%

Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield

et al. 2014

View full text Add to dashboard Cite

show abstract

“…The combination of MT and reflection seismic methods has repeatedly proven to be effective in crustal studies because these two methods provide independent data sets and therefore complement each other. Magnetotellurics is more sensitive to the texture and conductive fillings of the subsurface rather than volume (Hjelt et al 2006) and, in addition to conductivity models, also provides information on the dimensionality and orientation of the structures (Korja & Hjelt 1998). Consequently models, built in particular on reflection seismic data, can be tested by MT method, and vice versa (Jones 1998).…”

Section: Introductionmentioning

confidence: 99%

“…Large‐scale crustal conductors can be studied best by magnetovariational (MV) arrays and MT profiles whereas the mapping of the near‐surface conductivity structures requires methods having denser lateral coverage such as airborne electromagnetic (AEM) measurements made in the area by the Geological Survey of Finland (Airo 2005). The combination of these different data sets provides us a powerful tool that allows us to investigate in detail both the position of conductors with respect to geological and tectonic structures and the internal structure of the conductors themselves (Korja & Hjelt 1998).…”

Section: Introductionmentioning

confidence: 99%

High-resolution magnetotelluric studies of the Archaean-Proterozoic border zone in the Fennoscandian Shield, Finland

Vaittinen

Korja

Kaikkonen

et al. 2012

Geophysical Journal International

Self Cite

View full text Add to dashboard Cite

S U M M A R YThe Archaean-Proterozoic collisional zone is a complex mixture of the Archaean complexes [e.g. Iisalmi Complex (IC)], Proterozoic supracrustal belts [e.g. Kainuu Belt (KB) and Savo Belt (SB)] and oceanic arc lithologies in the central Fennoscandian Shield. The zone was formed in the Savo orogeny when the Keitele microcontinent collided with the Archaean Karelian craton in the Palaeoproterozoic time. The crustal architecture of this palaeosuture is studied using new broad-band magnetotelluric data from 104 sites. 2-D conductivity models across the border zone between the Palaeoproterozoic Svecofennian Domain and the Archaean Karelian province are constrained using the recent, partly collocated reflection seismic data from the Finnish Reflection Experiment (FIRE). Dimensionality analyses, in particular the Q-function analysis, show that magnetotelluric data represent reasonably well regional 2-D structure at periods <100 s, which is the longest period used in this study. Strike determinations gave a stable strike of N15W. For the inversions, the data are projected into three parallel profiles with an azimuth of N75E. The determinant inversion is selected as the most suitable method for the data set. Especially the phase data are useable only from the determinant since one of the polarizations have the out-of-quadrant phase at several sites. The interpreted final, geological more appropriate models, where smoother thick conductive areas are replaced by thinner layers, are constructed from the results of the unconstrained smooth inversions with the help of forward modelling, synthetic and prior model inversions and reflection seismic models. The two major sets of crustal conductors are identified. They have an opposite dip and together they form a bowl-shaped conductor. In the west, the eastward dipping SB conductors are located at the bottom of the formation underlain by the Keitele microcontinent. The SB conductors extend to the east possibly cutting the westward dipping conductors of the KB. The conductive KB is flanked above and below by the resistive Archaean IC/Rautavaara and Eastern Finland Complexes and represents the remnants of a basin that was squeezed between two Archaean blocks in the collision. The crustal conductors revealed by the inversion extend to the surface, where they can be associated with the near-surface conductors mapped by airborne electromagnetic surveys. Geological mapping has shown that the near-surface conductors are caused by graphite and/or sulphide-bearing metasedimentary rocks. Thus the deep conductors related with the Proterozoic KB and SB were formed on the surface and transported into the upper and middle crustal levels in the Savo orogeny. The crust in the Karelian Craton is highly resistive having only minor resistivity variations in our research area. Also the Archaean lower crust is resistive on the contrary to the conductive lower crust of the Proterozoic SB and the Keitele microcontinent.

show abstract

“…Several authors have reported on investigations by EM methods of crustal structures in the Fennoscandian Shield. A review of crustal EM and GDS studies in previous three decades is given by Korja & Hjelt (1998). The EM studies suggest strongly heterogeneous (e.g.…”

Section: Introductionmentioning

confidence: 99%

Decomposition analysis of the BEAR magnetotelluric data: implications for the upper mantle conductivity in the Fennoscandian Shield

Lahti

Korja

Kaikkonen

et al. 2005

Geophysical Journal International

Self Cite

View full text Add to dashboard Cite

S U M M A R YLong-period magnetotelluric (MT) and geomagnetic depth sounding data (GDS) have been acquired on the Fennoscandian Shield under the framework of the Baltic Electromagnetic Array Research (BEAR) project. The field campaign was carried out in the summer of 1998 when variations of the natural electromagnetic field were recorded simultaneously at 46 MT and 20 GDS stations. The key targets of the project are to investigate the electrical properties of the upper mantle and to determine the depth to the lithosphere-asthenosphere boundary in the Fennoscandian craton.A challenging task emerges from the fact that numerous highly conductive crustal bodies and local conductivity contrasts generate galvanic and inductive distortions to the calculated transfer functions in the research area. We present here a systematic decomposition and dimensionality analysis of the BEAR data and use the results of this analysis to verify regions for which 1-D inversion is justified. We argue that most of the BEAR data represent regional 2-D and 3-D structures with local galvanic distortion. The decomposition of the long-period (T > 3000 s) MT impedance tensors yield a set of smoothly varying regional strike directions. Yet strike angles vary significantly in the scale of the BEAR array and have abrupt regional changes in some areas. The spatial behaviour of strike angles cannot be connected with largescale geological units. Moreover, strong variation of strike azimuths over the BEAR array convincingly shows that the strike angles cannot be associated with present day plate motion or mantle convection, because that would require a consistent strike azimuth over the whole array. Observed long-period strike angles indicate mainly upper mantle 2-D and 3-D structures or frozen in anisotropy induced by several Palaeoproterozoic and Archaean events.The dimensionality analysis of the BEAR data shows that in the northeastern part of the array the regional structure is approximately 1-D. 1-D inversion of selected data from the western Lapland-Kola Domain reveals a conducting layer in the middle crust. An increase of conductivity is required also at depths greater than 170 km providing a minimum estimate of the lithosphere thickness beneath the target area. Partial melts or dissolved water in olivine are most plausible sources for increased conductivity at such depths.

show abstract

The Fennoscandian Shield: A treasure box for deep electromagnetic studies

Cited by 4 publications

References 62 publications

Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield

Crustal conductivity in Fennoscandia—a compilation of a database on crustal conductance in the Fennoscandian Shield

High-resolution magnetotelluric studies of the Archaean-Proterozoic border zone in the Fennoscandian Shield, Finland

Decomposition analysis of the BEAR magnetotelluric data: implications for the upper mantle conductivity in the Fennoscandian Shield

Contact Info

Product

Resources

About