Electrical conductivity structure of the north-western Fennoscandia (MaSca project)
Abstract
The Magnetotellurics in the Scandia (MaSca) project is aimed on the investigation of the crust, upper mantle and lithospheric structure of the north-western Fennoscandian Shield. The measurements are located in the area composed of the Precambrian rocks of ancient Baltica (in Sweden and Finland) and Silurian to Devonian rocks of Caledonian orogeny and post-orogenic extension (in Norway). The array covers an area of 500x400 square kilometers and stretches from Tromsø and Bodø (Norway) in the west to Kiruna and Skellefteå (Sweden) in the east. The project started in the summer 2011 and data collection continued till summer 2013, resulting all together in 70 synchronous long period (LMT) and 236 broad-band (BMT) sites. All LMT sites were occupied for about two months, while most of the BMT sites were measured during one day. The average distance between LMT sites is 30 km, and 10 km between BMT, however there are gaps of 50-80 km in the region of the Scandinavian Mountains. All the transfer functions were estimated using multi-remote reference technique as well as novel multivariate analysis technique, thus providing stable transfer function in the period range from 0.001 - 40000 s. Here we present the results of 2-D inversions along seven parallel profiles with an average distance of 60 km between profile lines. The strike determination from phase tensor and Q-function analyses showed a dominant direction of the regional structure being 40 degrees. However, the dimensionality analyses highlights 3-D effects in the data and presence of galvanic effects. Therefore, we inverted the determinant of the impedance tensor which is proved to be superior in such circumstances. The inverse solutions reveal extensive areas of resistive Precambrian lithosphere in south proper. Thick Cambro-Silurian successions occur in Nordland and Tromsø and presented as conductive upper crust layers in the west most likely composed of alum shales. A vast area of high conductivity in the upper mantle is revealed along two profiles. However, the nature of the conductor is not obvious, since there is no other deep geophysical information in this region. The thickness of stable Proterozoic lithosphere is estimated to be about 300 km. From the other hand in the northen part of the project results shows no clear indication for Archean lithosphere thickness. We observe that upper mantle in Archean province is almost an order of magnitude more conductive compared to Proterozoic lithosphere. We also present results of 3-D inversion of MaSca data set. We inverted full impedance tensor including all the data in the period range from 10 to 40000s. Also we selected several areas and ran independent 3D inversions with data at shorter periods from 0.003 to 10s. The results are well agrees with 2-D models for the crust and upper mantle. However, the thickness of the lithosphere is not well resolved and its resistivity is one decade higher than in 2-D. An upper mantle conductor that well seen in 2-D models, are not supported by 3-D results. A new data acquired in the summer 2013 will bring an additional information and updated 3-D inversion will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMGP22A..08C
- Keywords:
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- 1515 GEOMAGNETISM AND PALEOMAGNETISM Geomagnetic induction