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Andersson, M. & Malehmir, A. (2018). Internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) revealed using 3D models of gravity and magnetic data. Tectonophysics, 740-741, 53-71
Open this publication in new window or tab >>Internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) revealed using 3D models of gravity and magnetic data
2018 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 740-741, p. 53-71Article in journal (Refereed) Published
Abstract [en]

The Alnö Complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the world. Presented here is the 3D models of ground gravity and aeromagnetic data that confirm some of the previous ideas about the 3D geometry of the complex but also suggest that the complex may continue laterally further to north than previously expected. The data show the complex as (i) a strong positive Bouguer anomaly, around 20 mGal, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT. Magnetic structures are clearly discernible within the complex and surrounding area. Both gravity and magnetic inversion models suggest that dense (> 2850 kg/m(3)) and magnetic ( > 0.05 SI) rocks extend down to about 3.5-4 km depth. Previous studies have suggested a solidified magma reservoir at this approximate depth. The inversion models further suggest that two apparently separate regions within the complex are likely connected at depth, starting from 800 to 1000 m, implying a common source for the rocks observed in these two regions. Modelling of the aeromagnetic data indicates that a > 3 km wide ring-shaped magnetic high situated in the sea north of Alnö Island may be a part of the complex. This could link a smaller satellite intrusion in Soraker on mainland to the larger intrusion on Alnö Island. While the rim of the ring must consist of highly magnetic rocks to support the anomaly, the centre has relatively low magnetisation and is probably made up of low-magnetic wall-rocks or metasomatised wall-rocks down to about 2 km depth. Below this depth the 3D susceptibility model suggests higher magnetic susceptibility values. Worldwide alkaline and carbonatite complexes are the main resources for rare earth elements (REEs), and owing to the size of the Alnö Complex, it can be highly prospective for REEs at depth.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Carbonatite, Volcanism, Gravity, Magnetic, 3D modelling, Rare earth elements
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-359865 (URN)10.1016/j.tecto.2018.05.008 (DOI)000436218900005 ()
Funder
Swedish Research Council, 621-2009-4439
Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2018-09-07Bibliographically approved
Yan, P., Andersson, M., Kalscheuer, T., Garcı́a Juanatey, M. A., Malehmir, A., Shan, C., . . . Almqvist, B. S. G. (2016). 3D magnetotelluric modelling of the Alnö alkaline and carbonatite ring complex, central Sweden. Tectonophysics, 679, 218-234
Open this publication in new window or tab >>3D magnetotelluric modelling of the Alnö alkaline and carbonatite ring complex, central Sweden
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2016 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 679, p. 218-234Article in journal (Refereed) Published
Abstract [en]

Thirty-four broadband magnetotelluric stations were deployed across the Alno alkaline and carbonatite ring intrusion in central Sweden. The measurements were designed such that both 2D models along existing seismic profiles and a 3D model can be constructed. Alno Island and surrounding areas are densely populated and industrialized and in order to reduce the effect of noise, the remote reference technique was utilized in time series processing. Strike and dimensionality analyses together with the induction arrows show that there is no homogeneous regional strike direction in this area. Therefore, only the determinant of the impedance tensor was used for 2D inversion whereas all elements of the impedance tensor were used for 3D inversion. Representative rock samples were collected from existing outcrops and their resistivities were measured in the laboratory to facilitate interpretation of the inversion models. The results from these measurements show that coarse grained (sovite, white color) and fine-grained (dark color) carbonatites are the most conductive and resistive rock types, respectively. In accordance with the interpretation of the reflection seismic images, the 2D and 3D resistivity models depict the caldera-related ring-type fault system and updoming faulted and fractured systems as major 10-500 Omega m conductors, extending down to about 3 km depth. A central similar to 4000 Omega m resistive unit at about 3 km depth appears to correspond to a solidified fossil magma chamber as speculated from the reflection seismic data and earlier field geological studies.

Keywords
Alnö; Carbonatite; Alkaline; 3D; Magnetotelluric; Resistivity
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-292604 (URN)10.1016/j.tecto.2016.05.002 (DOI)000378177700017 ()
Funder
Swedish Research Council, 621-2009-4439
Available from: 2016-05-04 Created: 2016-05-04 Last updated: 2017-11-30Bibliographically approved
Andersson, M., Almqvist, B. S. G., Burchardt, S., Troll, V. R., Malehmir, A., Snowball, I. & Kubler, L. (2016). Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden. Scientific Reports, 6, Article ID 27635.
Open this publication in new window or tab >>Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden
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2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 27635Article in journal (Refereed) Published
Abstract [en]

Magma transport through the Earth's crust occurs dominantly via sheet intrusions, such as dykes and cone-sheets, and is fundamental to crustal evolution, volcanic eruptions and geochemical element cycling. However, reliable methods to reconstruct flow direction in solidified sheet intrusions have proved elusive. Anisotropy of magnetic susceptibility (AMS) in magmatic sheets is often interpreted as primary magma flow, but magnetic fabrics can be modified by post-emplacement processes, making interpretation of AMS data ambiguous. Here we present AMS data from cone-sheets in the Alno carbonatite complex, central Sweden. We discuss six scenarios of syn- and post-emplacement processes that can modify AMS fabrics and offer a conceptual framework for systematic interpretation of magma movements in sheet intrusions. The AMS fabrics in the Alno cone-sheets are dominantly oblate with magnetic foliations parallel to sheet orientations. These fabrics may result from primary lateral flow or from sheet closure at the terminal stage of magma transport. As the cone-sheets are discontinuous along their strike direction, sheet closure is the most probable process to explain the observed AMS fabrics. We argue that these fabrics may be common to cone-sheets and an integrated geology, petrology and AMS approach can be used to distinguish them from primary flow fabrics.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-299041 (URN)10.1038/srep27635 (DOI)000377683800001 ()27282420 (PubMedID)
Funder
Swedish Research Council, 621_2009_4439
Available from: 2016-07-13 Created: 2016-07-13 Last updated: 2017-11-28Bibliographically approved
Malehmir, A., Andersson, M., Mehta, S., Brodic, B., Munier, R., Place, J., . . . Lund, B. (2016). Post-glacial reactivation of the Bollnas fault, central Sweden: a multidisciplinary geophysical investigation. Solid Earth, 7(2), 509-527
Open this publication in new window or tab >>Post-glacial reactivation of the Bollnas fault, central Sweden: a multidisciplinary geophysical investigation
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2016 (English)In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 7, no 2, p. 509-527Article in journal (Refereed) Published
Abstract [en]

Glacially induced intraplate faults are conspicuous in Fennoscandia where they reach trace lengths of up to 155 km with estimated magnitudes up to 8 for the associated earthquakes. While they are typically found in northern parts of Fennoscandia, there are a number of published accounts claiming their existence further south and even in northern central Europe. This study focuses on a prominent scarp discovered recently in lidar (light detection and ranging) imagery hypothesized to be from a post-glacial fault and located about 250 km north of Stockholm near the town of Bollnas. The Bollnas scarp strikes approximately north-south for about 12 km. The maximum vertical offset in the sediments across the scarp is 4-5m with the western block being elevated relative to the eastern block. To investigate potential displacement in the bedrock and identify structures in it that are related to the scarp, we conducted a multidisciplinary geophysical investigation that included gravity and magnetic measurements, high-resolution seismics, radio-magnetotellurics (RMT), electrical resistivity tomography (ERT) and ground-penetrating radar (GPR). Results of the investigations suggest a zone of low-velocity and high-conductivity in the bedrock associated with a magnetic lineament that is offset horizontally about 50m to the west of the scarp. The top of the bedrock is found similar to 10m below the surface on the eastern side of the scarp and about similar to 20m below on its western side. This difference is due to the different thicknesses of the overlying sediments accounting for the surface topography, while the bedrock surface is likely to be more or less at the same topographic level on both sides of the scarp; else the difference is not resolvable by the methods used. To explain the difference in the sediment covers, we suggest that the Bollnas scarp is associated with an earlier deformation zone, within a wide (> 150 m), highly fractured, water-bearing zone that became active as a reverse fault after the latest Weichselian deglaciation.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-297571 (URN)10.5194/se-7-509-2016 (DOI)000374543400014 ()
Available from: 2016-06-23 Created: 2016-06-23 Last updated: 2017-11-28Bibliographically approved
Andersson, M. (2015). 3D Structure and Emplacement of the Alnö Alkaline and Carbonatite Complex, Sweden: Integrated Geophysical and Physical Property Investigations. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>3D Structure and Emplacement of the Alnö Alkaline and Carbonatite Complex, Sweden: Integrated Geophysical and Physical Property Investigations
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbonatites are carbonate-rich magmatic rocks that are rare and of great relevance for our understanding of crustal and mantle processes. Although found on all continents and in settings ranging from Archaean to present-day, their deeper plumbing system is still poorly understood. Therefore, the main goal of this thesis is to broaden the existing knowledge of carbonatite systems, often limited to surface geological observations, by providing depth constraints using a number of geophysical methods and petrophysical measurements. The Alnö alkaline and carbonatite complex in central Sweden was chosen for this purpose. Data from three reflection seismic lines, ground gravity and magnetic measurements are presented. These data are complemented by a series of petrophysical measurements, including ultrasonic velocities, density, magnetic bulk susceptibility, anisotropy of magnetic susceptibility (AMS), and magnetic remanence, to aid in the interpretation of the geophysical data. The reflection seismic data indicate a solidified saucer-shaped fossil magma chamber at about 3 km depth. Caldera-style volcanism, constrained by surface geological observations, provides a plausible scenario to explain the emplacement of the complex, suggesting that carbonatite magmas have been stored, transported and erupted in a similar manner to known emplacement mechanisms for silicic calderas, although these are compositionally different. The AMS data from most of the carbonatite sheets in Alnö show a strong degree of anisotropy and oblate-shaped susceptibility ellipsoids. A set of syn- and post-emplacement processes that may control the AMS signature is evaluated based on the dataset. Overprinting of the primary flow patterns by processes related to sheet closure at the terminal stage of magma transport may explain the AMS observations. A complementary study using 3D inversion of ground gravity and aeromagnetic data was then carried out to better delineate the 3D internal architecture of the complex. Resulting models indicate a depth extent of the complex to about 3-4 km, consistent with the interpretation of the reflection seismic data. The modelling results of a ring-shaped magnetic anomaly observed in the Klingefjärden bay adjacent to Alnö Island further suggest that the complex may extend laterally about 3 km towards the north.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. p. 64
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1246
Keywords
Alnö, carbonatite, caldera, reflection seismic, anisotropy of magnetic susceptibility, gravity, magnetic
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-248113 (URN)978-91-554-9224-3 (ISBN)
Public defence
2015-05-22, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-04-27 Created: 2015-03-27 Last updated: 2015-07-07Bibliographically approved
Muhamad, H. A., Juhlin, C., Lehnert, O., Meinhold, G., Andersson, M., Juanatey, M. d. & Malehmir, A. (2015). Analysis of borehole geophysical data from the Mora area of the Siljan Ring impact structure, central Sweden. Journal of Applied Geophysics, 115, 183-196
Open this publication in new window or tab >>Analysis of borehole geophysical data from the Mora area of the Siljan Ring impact structure, central Sweden
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2015 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 115, p. 183-196Article in journal (Refereed) Published
Abstract [en]

The Siljan impact structure is the largest known impact structure in Europe, the result of a Late Devonian meteorite impact (380.9 +/- 4.6 Ma). It is outlined mainly by a ring of lakes and Palaeozoic sedimentary rocks of Ordovician to Devonian age. The Palaeozoic successions are generally poorly exposed, but often well preserved with clear stratigraphy. At some locations they are strongly tectonised with sharply inclined or nearly overturned packages of crystalline basement and/or sediments. Down-hole logging data were acquired in the western part of the Siljan impact structure to determine some of the physical properties of the Palaeozoic sedimentary rocks in the area. Boreholes Mora 001 (356 m logged depth), Vattumyra Production (420 m logged depth), Mora VM 2 (94 m logged depth) and Mobillyft (437 m logged depth) were logged for temperature, sonic velocity and electrical resistivity. Logging data were compared to the lithology in the Mora 001 core, which had been mapped in detail, and information from other cores in the area. Good agreement between the natural gamma log and the core lithology was found. The sonic log shows a marked difference in velocity for the more clastic Silurian succession compared to the Ordovician succession and the Precambrian basement. A synthetic seismogram shows that a high amplitude reflection is expected at the Silurian-Ordovician boundary, raising some questions concerning interpretation of a seismic profile located about 6-7 km north of the study area. Correlation of the borehole logs shows that the thickness of the Silurian succession varies rapidly in the area and that its composition differs over distances of less than 1 km. These rapid variations suggest that the study area may be located in a megablock zone that was highly influenced by the impact Caledonian tectonics and changing depositional environments may also play a role in explaining the present-day borehole lithologies. Even though the boreholes are relatively far from the seismic profile and the geology is complex, the new data confirm that the Silurian has significant thickness along parts of the seismic profile. Potentially, the Silurian can be up to 450 m thick on parts of the profile. Further geophysical investigations in the area, including seismic surveying and gravity measurements, may help in mapping the complex structures away from the boreholes and discriminating between possible geological models.

Keywords
Siljan impact structure, Sweden, Down-hole logging, Palaeozoic rocks
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-258831 (URN)10.1016/j.jappgeo.2015.02.019 (DOI)000351646900018 ()
Available from: 2015-07-23 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved
Lundberg, E., Malehmir, A., Juhlin, C., Bastani, M. & Andersson, M. (2014). High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden. Geophysics, 79(2), B97-B107
Open this publication in new window or tab >>High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden
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2014 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 2, p. B97-B107Article in journal (Refereed) Published
Abstract [en]

Quick-clay landslides often occur in the northern hemisphere in areas that were covered by Pleistocene glaciation. They are particularly common along the shorelines of the Göta River in southwestern Sweden. Characterization of potential landslide areas and identification of features that indicate high risk are necessary to better understand the triggering mechanisms of these events. Therefore, an intensive characterization project has been initiated at the location of the Fråstad landslide in Sweden. Part of the characterization program included the acquisition of 3D reflection seismic data to image structures in the normally consolidated sediments, as well as the bedrock topography below the landslide scar. Two seismic horizons within the glacial and post-glacial sediments were observed. The shallowest seismic horizon (here referred to as S1) corresponds to a coarse-grained layer that was previously detected by eight geotechnical boreholes located within the 3D survey area. Discontinuities in S1, mapped by the 3D reflection seismic data, occur across a zone that correlates with the landslide scar boundary, suggesting that this zone may have played a role in triggering and/or in limiting the extension of the landslide. If S1 is truncated by or mixed with clays in this zone, then the outflow of water from the permeable S1 into the clays above may have increased the amount of quick-clays above this zone. The increased outflow of water may also have caused a higher pore-water pressure south of the zone, which in turn could have acted as a trigger for the landslide. This study shows the potential of using the 3D reflection seismic method as a complement to drilling and other geophysical methods when performing landslide site investigations. It also demonstrates the importance of further investigating the relationship between 3D subsurface geometries and landslide development.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-211211 (URN)10.1190/GEO2013-0225.1 (DOI)000336918700007 ()
Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved
Andersson, M., Malehmir, A., Troll, V. R., Dehghannejad, M., Juhlin, C. & Ask, M. (2013). Carbonatite ring-complexes explained by caldera-style volcanism. Scientific Reports, 3, 1677
Open this publication in new window or tab >>Carbonatite ring-complexes explained by caldera-style volcanism
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2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, p. 1677-Article in journal (Refereed) Published
Abstract [en]

Carbonatites are rare, carbonate-rich magmatic rocks that make up a minute portion of the crust only, yet they are of great relevance for our understanding of crustal and mantle processes. Although they occur in all continents and from Archaean to present, the deeper plumbing system of carbonatite ring-complexes is usually poorly constrained. Here, we show that carbonatite ring-complexes can be explained by caldera-style volcanism. Our geophysical investigation of the Alno carbonatite ring-complex in central Sweden identifies a solidified saucer-shaped magma chamber at similar to 3 km depth that links to surface exposures through a ring fault system. Caldera subsidence during final stages of activity caused carbonatite eruptions north of the main complex, providing the crucial element to connect plutonic and eruptive features of carbonatite magmatism. The way carbonatite magmas are stored, transported and erupt at the surface is thus comparable to known emplacement styles from silicic calderas.

National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-200348 (URN)10.1038/srep01677 (DOI)000317581800010 ()
Available from: 2013-05-28 Created: 2013-05-27 Last updated: 2017-12-06
Malehmir, A., Andersson, M., Lebedev, M., Urosevic, M. & Mikhaltsevitch, V. (2013). Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores. Geophysical Prospecting, 61(1), 153-167
Open this publication in new window or tab >>Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores
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2013 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 61, no 1, p. 153-167Article in journal (Refereed) Published
Abstract [en]

To provide a guide for future deep (<1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta-volcanic and meta-intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P- and S-wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P- and S-wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P- and S-wave velocities. The low P- and S-wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P- and S-wave velocities. Later, an iron ore sample from the central part of Sweden was measured using a low-frequency (0.1-50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low-frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser-interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity-anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser-interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone.

Keywords
Anisotropy, Deformation, Mining
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-192027 (URN)10.1111/j.1365-2478.2012.01063.x (DOI)000312548300012 ()
Available from: 2013-01-21 Created: 2013-01-15 Last updated: 2017-12-06Bibliographically approved
Andersson, M. (2011). Berggrund och Kvicklera mäts med vibrationer. HUSBYGGAREN, 6, 24-25
Open this publication in new window or tab >>Berggrund och Kvicklera mäts med vibrationer
2011 (Swedish)In: HUSBYGGAREN, Vol. 6, p. 24-25Article in journal (Other (popular science, discussion, etc.)) Published
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-165246 (URN)
Available from: 2012-01-04 Created: 2012-01-04 Last updated: 2012-02-10Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-6526-8062

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