Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite-skarn deposits

Malehmir, Alireza; Dahlin, Peter; Lundberg, Emil; Juhlin, Christopher; Sjöström, Håkan; Högdahl, Karin

doi:10.1029/2011jb008643

Cited by 54 publications

(52 citation statements)

References 50 publications

(102 reference statements)

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“…A 3D approach to seismic processing (similar to e.g. Malehmir et al 2011) could help to clarify the nature of some of the complex reflectivity patterns. A magnetotelluric survey to complement the rather sparse regional data presented by Korja et al (2008), with the potential to allow tracing of the décollement and the middle and lower allochthons to be distinguished, could help to refine our interpretation and model.…”

Section: Discussionmentioning

confidence: 99%

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Hedin¹,

Malehmir²,

Gee³

et al. 2013

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The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

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Section: Discussionmentioning

confidence: 99%

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Hedin¹,

Malehmir²,

Gee³

et al. 2013

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“…It consists of an hydraulic hammer mounted on a tractor and a computer control system. This type of source has been used in other studies in Sweden with good results (e.g., Juhlin et al, , 2010Juhlin and Palm, 2005;Dehghannejad et al, 2010;Lundberg and Juhlin, 2011;Malehmir et al, 2011). It is generally preferred to explosives due to its lower cost and uniform source signature .…”

Section: Seismic Reflection Data Acquisitionmentioning

confidence: 97%

MT and reflection seismics in northwestern Skellefte Ore District, Sweden

et al. 2013

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A seismic reflection and MT survey was carried out along a 27-km long transect in northwestern Skellefte District, as part of a bigger 3D modeling project. The main motivation for the data acquisition is to elucidate the geologic relationship between the known mineralizations in the Adak mining camp to the north and in the well studied Kristineberg area south of the transect. The seismic reflection data were acquired with a VIBSIST system, and show reflectivity down to 3 s. Apart from the conventional processing for crystalline environments, the seismic data was also subject to an azimuthal binning procedure and crossdip analysis, allowing the orientation of planar reflectors in 3D. Regarding the MT data, it is primarily of good quality along the 17 installed sites. The inversion of the determinant of the impedance tensor yielded a stable 2D resistivity model, dominated by resistors corresponding to the postorogenic intrusions along the transect. Adding the location of the analyzed seismic reflectors in the MT inversion rendered an integrated model that facilitated a preliminary joint interpretation of the data sets. Overall, the results are in good agreement with surface observations and reveal a crude configuration of the geologic units below the transect. The most prominent outcomes are the lateral and depth extent of the large postorogenic intrusions in the area reaching to 5-or 6-km depth, the dimensions of the nearly vertical Brännäs gabbro extending to 6-km depth, and the presence of enhanced conductivities along the transect at about 10 km depth. The latter is probably related to the deep conductor previously identified in the district.

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“…We suggest that there is a relationship between the scarp and the high-conductivity and low-velocity structures observed in the bedrock from ERT, RMT and seismic data. There is a major brittle structure in Malehmir et al, 2011); nevertheless interpreted to be associated with multi-phase deformation zones and not to any post-glacial faulting. It is difficult to explain why the bedrock is shallower on the eastern side of the scarp than on the western side if we expect the fault to be post-glacial and have recent reverse kinematics.…”

Section: Origin Of the Bollnäs Scarp And Role Of Earlier Structuresmentioning

confidence: 99%

Post-glacial reactivation of the Bollnäs fault, central Sweden – a multidisciplinary geophysical investigation

et al. 2016

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Abstract. 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 Bollnäs. The Bollnäs scarp strikes approximately northsouth for about 12 km. The maximum vertical offset in the sediments across the scarp is 4-5 m 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 highconductivity in the bedrock associated with a magnetic lineament that is offset horizontally about 50 m to the west of the scarp. The top of the bedrock is found ∼ 10 m below the surface on the eastern side of the scarp and about ∼ 20 m 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 Bollnäs 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.

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Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite-skarn deposits

Cited by 54 publications

References 50 publications

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

MT and reflection seismics in northwestern Skellefte Ore District, Sweden

Post-glacial reactivation of the Bollnäs fault, central Sweden – a multidisciplinary geophysical investigation

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