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  • 1. Ask, Maria
    et al.
    Beslier, Marie-Odile
    Boillot, Gilbert
    Ocean-continent boundary in the Iberia Abyssal Plain from multichannel seismic data1993In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 218, no 4, p. 383-393Article in journal (Refereed)
    Abstract [en]

    The Ocean-Continent Boundary of the West Iberia margin is marked by a basement ridge trending N-S. Four segments of this ridge are recognized, each of them being progressively offset westward from 40°N to 43°N. Because the setting and seismic character of the ridge in the Iberia Abyssal Plain are similar to those of the Galicia margin ridge, which is made of serpentinized peridotite, we think that the southern segments of the ridge are also made of the same mantle material. The segmentation of the ridge suggests that the northward propagation of the continental break-up during the North Atlantic opening in Early Cretaceous times was discontinuous, each segment possibly corresponding to a propagation step. East of the ridge, the basement of the whole Iberia Abyssal Plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the S reflector previously recognized off Galicia. By analogy with the Galicia margin, we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle, the S reflector corresponding to the contact between crustal rocks and serpentinite.

  • 2.
    Ask, Maria
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Morgan, Julia K.
    Rice University, Houston.
    Projection of mechanical properties from shallow to greater depths seaward of the Nankai accretionary prism2010In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 482, no 1-4, p. 50-64Article in journal (Refereed)
    Abstract [en]

    Deformation processes in sediments at accretionary prisms are directly controlled by the state of in situ effective stresses, the mechanical, physical and geochemical properties of the materials of the fault zone and surrounding wall rocks, as well as time. Measurements of these properties and their evolution in space and time, are therefore needed for a full understanding of the process of earthquake generation within subduction zones.Reconsolidation tests have been carried out on Ocean Drilling Program cores collected from a reference site seaward of the active Nankai décollement zone off the southeast coast of Japan. The reconsolidation stress path subjects the samples to uniaxial strain deformation, which mimics their stress history, however at much higher loading rates than in the natural system. We have conducted two tests each from two mudstone samples within Lower Shikoku Basin. The samples were collected at 361 and 476 meter below seafloor, on either side of the protodécollement horizon.The objectives for mechanical testing are to probe the yield and failure surfaces of these shallow sediments (Considering their large scale behavior, our tests show that the samples collected above the protodécollement have higher strength than those below. We propose that cementation, microfabric and mineralogy of the sediments above the protodécollement result in a higher effective yield stress than predicted from in situ effective vertical stress at hydrostatic pore pressures. Sediments below the protodécollement, in contrast, are slightly underconsolidated, and provide an upper constraint on the magnitude of in situ effective vertical stress and pore-fluid pressure. We also used the test results to make initial predictions for the yield surface in 2D and 3D for subdécollement samples across the margin. The construction of the 2D yield surface is the first attempt to quantify the model of sediment deformation proposed by Morgan et al. (2007). These results hint that the presence of cement has a strong, and increasing, influence on sediment behavior. Further testing is needed to verify these findings.

  • 3.
    Behrens, K.
    et al.
    Institute of Geophysics, University of Hamburg.
    Goldflam, S.
    Institute of Geophysics, University of Hamburg.
    Heikkinen, P.
    Institute of Seismology, University of Helsinki.
    Hirschleber, H.
    Institute of Geophysics, University of Hamburg.
    Lindqvist, Gustaf
    Luleå tekniska universitet.
    Lund, C-E-
    Department of Geophysics, University of Uppsala.
    Reflection seismic measurements across the Granulite Belt of the POLAR Profile in the northern Baltic Shield, Northern Finland1989In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 162, no 1-2, p. 101-111Article in journal (Refereed)
    Abstract [en]

    Reflection seismic measurements were carried out in 1985 on the central part of the POLAR refraction seismic profile in Northern Finland. The survey was planned and executed jointly by the Universities of Helsinki, Uppsala and Hamburg, using digital equipment with a total of 144 channels. By repeating the shots and moving the geophone spreads every day we were able to observe a 42-50 km recording spread. Referred to reflection elements, a line of 84 km total length through the source-receiver midpoints was recorded. We processed the results up to normal moveout-corrected time sections.A number of reflectors dipping 8°-15° to the northeast were recorded in the Lapland Granulite Belt range. These represent a system of sheared granulites which were observed on the surface in the southern part of the profile. From gravity modelling, the bottom of the dipping layers coincides with the lower boundary of the granulites. Between the depths of 22 and 35 km the crust in this area seems to be transparent to seismic signals. This leads to the assumption that the middle part of the crust is characterized by gentle velocity and density gradients. The crust-mantle boundary seems to be a layered Moho with good reflectors lying at depths between 40 and 44 km.

  • 4.
    Beka, Thomas I.
    et al.
    Department of Physics and Technology, UiT-The Arctic University of Norway.
    Smirnov, Maxim
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Birkelund, Yngve
    Department of Physics and Technology, UiT-The Arctic University of Norway.
    Senger, Kim
    The University Centre in Svalbard.
    Bergh, Steffen G.
    Department of Geology, UiT-The Arctic University of Norway, N-9037 Tromsø.
    Analysis and 3D inversion of magnetotelluric crooked profile data from central Svalbard for geothermal application2016In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 686, p. 98-115Article in journal (Refereed)
    Abstract [en]

    Broadband (0.001–1000 s) magnetotelluric (MT) data along a crooked profile collected to investigate the geothermal potential on Spitsbergen could not be fully explained by two-dimensional (2D) models; hence we interpret the data with three-dimensional (3D) inversion herein. To better accommodate 3D features and nearby off profile resistivity structures, the full MT impedance tensor data together with the tipper were inverted. As a model control, a detailed bathymetry is systematically incorporated in the inversion. Our results from testing different inversion settings emphasised that appropriately choosing and tuning the starting model, data error floor and the model regularization together are crucial to obtain optimum benefit from MT field data. Through the 3D inversion, we reproduced out of quadrant impedance components and obtained an overall satisfactory data fit (RMS = 1.05). The final 3D resistivity model displays a complex geology of the near surface region (< 1.5 km), which suggests fractures, localized and regional fault systems and igneous intrusions in the Mesozoic platform cover deposits. The Billefjorden fault zone is revealed as a consistent and deep rooted (> 2 km) conductive anomaly, confirming the regional nature of the fault. The fault zone is positioned between two uplifted basement blocks (> 1000 Ωm) of presumably pre-Devonian (Caledonian) metamorphic rocks, and the fault may have been responsible for deformation in the overlying Paleozoic-Mesozoic unit. Upper crustal conductive anomalies (< 10 Ωm) below the Paleozoic-Mesozoic succession in the western part of the 3D model are interpreted as part of a Devonian basin fill. These conductors are laterally and vertically bounded by resistive rocks, suggesting a conducive environment for deep geothermal heat storage. Having this scenario in an area of a known high heat-flow, deep faults and a thinned lithosphere makes the hypothesis on finding a technologically exploitable geothermal resource close to human settlement in the area stronger.

  • 5.
    Brethes, Anais
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Geological Survey of Denmark and Greenland (GEUS), Department of Petrology and Economic Geology, Denmark.
    Guarnieri, Pierpaolo
    Geological Survey of Denmark and Greenland (GEUS), Department of Petrology and Economic Geology.
    Rasmussen, Thorkild Maack
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Bauer, Tobias Erich
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Interpretation of aeromagnetic data in the Jameson Land Basin, central East Greenland: Structures and related mineralized systems2018In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 724-725, p. 116-136Article in journal (Refereed)
    Abstract [en]

    This paper provides a detailed interpretation of several aeromagnetic datasets over the Jameson Land Basin in central East Greenland. The interpretation is based on texture and lineament analysis of magnetic data and derivatives of these, in combination with geological field observations. Numerous faults and Cenozoic intrusions were identified and a chronological interpretation of the events responsible for the magnetic features is proposed built on crosscutting relationships and correlated with absolute ages. Lineaments identified in enhanced magnetic data are compared with structures controlling the mineralized systems occurring in the area and form the basis for the interpretations presented in this paper. Several structures associated with base metal mineralization systems that were known at a local scale are here delineated at a larger scale; allowing the identification of areas displaying favorable geological settings for mineralization. This study demonstrates the usefulness of high-resolution airborne magnetic data for detailed structural interpretation and mineral exploration in geological contexts such as the Jameson Land Basin.

  • 6.
    Buchan, K. L.
    et al.
    Geological Survey of Canada.
    Mertanen, S.
    Geological Survey of Finland.
    Park, R. G.
    Department of Earth Sciences, Keele University.
    Pesonen, L. J.
    Geological Survey of Finland.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Abrahamsen, N.
    University of Aarhus.
    Bylund, G.
    Lunds universitet.
    Comparing the drift of Laurentia and baltica in the Proterozoic: the importance of key palaeomagnetic poles2000In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 319, no 3, p. 167-198Article in journal (Refereed)
    Abstract [en]

    Key palaeomagnetic poles are defined as those which pass basic reliability criteria and are precisely and accurately dated. They allow a more rigorous analysis of Precambrian continental drift and continental reconstructions than the traditional apparent polar wander path (APWP) approach using mostly non-key poles. Between ca. 2.45 and 2.00 Ga in the early Palaeoproterozoic, key poles define the drift of the Archaean Superior craton of Laurentia, yielding a result that is quite unlike the drift interpreted in earlier studies using the APWP method. There are no early Palaeoproterozoic key poles for the other Archaean cratons that amalgamated to form Laurentia and Baltica prior to 1.8 Ga, so that a rigorous test of early Palaeoproterozoic reconstruction models is not possible. Key poles from Laurentia between ca. 1.46 and 1.267 Ga and Baltica between 1.63 and 1.265 Ga help to define, in a preliminary fashion, the early Mesoproterozoic drift of the two shields. The key pole age match at ca. 1.265 Ga is consistent with Baltica located adjacent to eastern Greenland, and geological considerations suggest that the most reasonable fit aligns the Labradorian belt of Laurentia with the Gothian belt of Baltica. Although there is limited support from non-key poles and key poles that are not matched in age for such a fit as early as ca. 1.8 Ga, no rigorous assessment will be possible until a match in key pole ages is achieved. In the late Mesoproterozoic to Neoproterozoic, Laurentia's drift is reasonably well documented by seven key poles between 1.235 and 0.73 Ga. There are no key poles in this period from Baltica, however, so that a ≈90° clockwise rotation of Baltica relative to Laurentia between 1.265 and 1.0 Ga, widely used in the literature, cannot be confirmed.

  • 7.
    Chouliaras, Gerassimos
    et al.
    Department of Seismology, University of Uppsala.
    Rasmussen, Thorkild Maack
    Department of Solid Earth Physics, University of Uppsala.
    The application of the magnetotelluric impedance tensor to earthquake prediction research in Greece1988In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 152, no 1-2, p. 119-135Article in journal (Refereed)
    Abstract [en]

    Continuous short period (0.1-3600 s) measurements of the magnetotelluric field components were conducted simultaneously at two sites in Greece for a period of 5 weeks. The estimation of the impedance tensor elements from 8-hr windows of recorded data, at each station, is found to describe the local electromagnetic induction with great accuracy. The computation of the residual electric field, obtained as the difference between the observed and estimated inductive part of the electric fields, clearly improves the detection of any local electric field anomaly. This method was used in an attempt to detect precursory Sci smic electrical signals (SES) that have been reported to precede earthquakes in Greece. The results indicate the success of this method in resolving ambiguities about the nature of the reported SES, i.e. whether it has an external or internal source. Although during the recording period no large earthquake occurred, five with a magnitude of between ML = 4.0 and ML = 4.3 occurred within a radius of 150 km from one of the stations. The magnetotelluric recordings and the computed residual electric fields for the time intervals reported to contain the SES precursors to these events were analysed in some detail. In two of the cases it was found that the sources could not be related to earthquake processes. In the other three cases the nature of the source of the suspected SES, although electric, could not be established with certainty. These results indicate that for earthquakes of small magnitude (ML ≤ 4.3) or of large epicentral distance (r ≥ 100 km), the detection of a SES is extremely difficult.

  • 8.
    Dehghannejad, Mahdieh
    et al.
    Department of Earth Sciences, Uppsala University.
    Bauer, Tobias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Malehmir, Alireza
    Department of Earth Sciences, Uppsala University.
    Juhlin, Christopher
    Department of Earth Sciences, Uppsala University.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Crustal geometry of the central Skellefte district, northern Sweden: constraints from reflection seismic investigations2012In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 524-525, p. 87-99Article in journal (Refereed)
    Abstract [en]

    The Palaeoproterozoic Skellefte mining district in Sweden is one of the most important mining districts in Europe. As a part of a 4D geologic modeling project, three new sub-parallel reflection seismic profiles, with a total length of about 95 km, were acquired in the central part of the district. Processed seismic data reveal a series of gentle- to steeply- dipping reflections and a series of diffraction packages. The majority of reflections that extend to the surface can be correlated with geological features either observed in the field or interpreted from the aeromagnetic map. A set of south-dipping reflections represent inferred syn-extensional listric extensional faults that were inverted during subsequent crustal-shortening. Cross-cutting north-dipping reflections are correlated to late-compressional break-back faults. Flat-lying reflections in the central parts of the study area could represent lithological contacts within the Skellefte Group, or the contact between Skellefte Group rocks and their unknown basement. Flat-lying reflections occurring further north are inferred to originate from the top of the Jörn intrusive complex or an intrusive contact within it. So far unknown south- and north-dipping faults have been identified in the vicinity of the Maurliden deposit. Based on the seismic results, a preliminary 3D-model has been created in order to visualize the fault pattern and to provide a base for future 3D/4D modeling in the Skellefte district.

  • 9.
    Eaton, David W.S.
    et al.
    Department of Earth Sciences, University of Western Ontario.
    Dineva, Savka
    Mereu, Robert F.
    Department of Earth Sciences, University of Western Ontario.
    Crustal thickness and V P/V S variations in the Grenville orogen (Ontario, Canada) from analysis of teleseismic receiver functions2006In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 420, no 1-2, p. 223-238Article in journal (Refereed)
    Abstract [en]

    We have developed a simple semblance-weighted stacking technique to estimate crustal thickness and average V P/V S ratio using teleseismic receiver functions. We have applied our method to data from 32 broadband seismograph stations that cover a 700 × 400 km 2 region of the Grenville orogen, a 1.2-0.98 Ga Himalayan-scale collisional belt in eastern North America. Our seismograph network partly overlaps with Lithoprobe and other crustal refraction surveys. In 8 out of 9 cases where a crustal-refraction profile passes within 30 km of a seismograph station, the two independent crustal thickness estimates agree to within 7%. Our regional crustal-thickness model, constructed using both teleseismic and refraction observations, ranges between 34.0 and 52.4 km. Crustal-thickness trends show a strong correlation with geological belts, but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium. The thickest crust (52.4 ± 1.7 km) was found at a station located within the 1.1 Ga mid-continent (failed) rift. The Central Gneiss Belt, which contains rocks exhumed from deep levels of the crust, is characterized by V P/V S ranging from 1.78 to 1.85. In other parts of the Grenville orogen, V P/V S is found to be generally less than 1.80. The thinnest crust (34.5-37.0 km) occurs northeast of the 0.7 Ga Ottawa-Bonnechere graben and correlates with areas of high intraplate seismicity

  • 10.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mikhailova, N. P.
    Geophysical Institute, Academy of Sciences of Ukraine, Kiev.
    Kravchenko, S.
    Geophysical Institute, Academy of Sciences of Ukraine, Kiev.
    Palaeomagnetism of Proterozoic rocks from the Ukrainian shield: new tectonic reconstructions of the Ukrainian and Fennoscandian shields2001In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 339, no 1-2, p. 19-38Article in journal (Refereed)
    Abstract [en]

    A palaeomagnetic study has been performed on Palaeo-Mesoproterozoic rocks from three crustal blocks of the Ukrainian Shield, southern Sarmatia. Primary remanent magnetizations have been isolated in 2.0 Ga monzonite, 2.0-1.8 Ga sandstone, 1.77-1.72 Ga anorthosite and from mafic dykes of probably Palaeo-Mesoproterozoic ages. On basis of these results a sequence of 2.0-1.72 Ga apparent polar wander has for the first time been defined for the Ukrainian Shield. Palaeomagnetic and geological data indicate that there has probably not been any large scale tectonic movements within Sarmatia since the Mesoproterozoic. This suggests that tectonic reconstructions for the Ukrainian Shield may also include Sarmatia. The calculated pole positions for the Ukrainian Shield are significantly different from poles of similar age from the Fennoscandian Shield. The tectonic reconstructions demonstrate that the relative position and orientation of the Ukrainian Shield as a part of Sarmatia in the time interval 2.0-1.78 Ga was different from its present position relative to Fennoscandia. One pole from the Ukrainian Shield falls on the ca. 1.6 or 1.3 Ga part of the Fennoscandian APWP. This pole may represent a time when Fennoscandia was already accreted to Ukrainia. Contemporaneous rifting of the two cratons at ca. 1.35 Ga indicates that they were already joined to each other at that time, which means that the final accretion should have taken place sometimes after ca. 1.8 Ga ago.

  • 11.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Pesonen, L. J.
    Leino, M. A. H.
    Khramov, A. N.
    Mikhailova, N. P.
    Krasnova, A. F.
    Merlanen, S.
    Bylund, G.
    Terho, M.
    The drift of the Fennoscandian and Ukrainian Shields during the Precambrian: a Palaeomagnetic analysis1993In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 223, no 3-4, p. 177-198Article in journal (Refereed)
    Abstract [en]

    A revised Precambrian (2.85-0.6 Ga) Apparent Polar Wander Path (APWP) for the Fennoscandian Shield, based on a new compilation and analysis of data, is presented. In fitting the APW path to successive Grand Mean Palaeomagnetic poles (GMPs), we applied the spherical spline technique originally developed by Jupp and Kent in 1987. The position and orientation of the Fennoscandian Shield during 2.85-0.6 Ga was determined from the GMPs. Major palaeoclimatological findings are used to constrain the palaeomagnetic interpretation of palaeolatitudes. The general drift of Fennoscandia, from relatively high latitudes in the late Archaean-Early Proterozoic to nearly equatorial latitudes in the Middle Proterozoic, correlates with palaeoclimatological indications that a period of cold climate was followed by one of warm climate during this time interval. From the continuous APWP the APW velocities and latitudinal drift velocities of the shield were calculated. An accumulated APW curve was also calculated. The palaeomagnetic data are irregularly distributed and some periods are rather poorly represented. This means that the calculated velocities can sometimes be artifacts of sampling. Late Archaean and Early Proterozoic (2.85-1.90 Ga) data are too sparse to make these calculations meaningful and velocity calculations are therefore restricted to data of 1.90 Ga and younger ages. The accumulated APW curve shows a number of linear segments with varying slopes, indicating sudden changes in drift rate. During the Middle Proterozoic (1.90-1.35 Ga) there was a period when the rate of APW was constant and low and that of latitudinal drift also was low. This pattern changed at ca. 1.35 Ga, and the following Middle-Late Proterozoic period can be described by rapid APW and strongly fluctuating drift velocities. Jotnian rifting and the intrusion of numerous dyke swarms (at ca. 1.25 Ga) correlate with this shift in rate. These changes are attributed to changes in plate configuration. A new database for the Ukrainian Shield is also presented, and GMPs in the 2.32-1.20 Ga range are defined. The database is still inadequate and the comparison of the Ukrainian and Fennoscandian drift histories is therefore tentative. Similarities in position, latitudinal drift and rotation during the Early-Middle Proterozoic are, nevertheless, evident. A close relationship between the shields in this period is consistent with the low APW rate of Fennoscandia, indicating that Fennoscandia may have been part of a larger continent, including the Ukraine, at that time. At ca. 1.2 Ga, the latitudinal position of Ukraine differed significantly from that of Fennoscandia, suggesting that the large shield split up between ca. 1.35 and 1.2 Ga. This would explain the change in APW rate at 1.35 Ga. The subsequent increase in rate was due to a reduction in the size of the shield. The discrepancy in palaeopositions of Fennoscandia and Ukraine at 1.2 Ga led Mikhailova and Kravchenko to suggest a late Precambrian time (1.07-0.57 Ga) for the accreation of Fennoscandia to the East European Platform (EEP). This may be correct as the rate of APW for Fennoscandia decreased in the late Precambrian, reflecting such a consolidation.

  • 12.
    Juanatey, María A. García
    et al.
    Uppsala University, Sweden.
    Hübert, Juliane
    Uppsala University, Sweden.
    Tryggvason, Ari
    Uppsala University, Sweden.
    Juhlin, Christopher
    Uppsala University, Sweden.
    Pedersen, Laust B.
    Bauer, Tobias E.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Dehghannejad, Mahdieh
    Uppsala University, Sweden. Ramboll Sweden..
    2D and 3D MT in the central Skellefte Ore District, northern Sweden2019In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 764, p. 124-138Article in journal (Refereed)
    Abstract [en]

    New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area.

  • 13.
    Korja, Toivo
    et al.
    Department of Geophysics, University of Oulu.
    Hjelt, Sven-Erik
    Department of Geophysics, University of Oulu.
    Koivukoski, K.
    Department of Geophysics, University of Oulu.
    Rasmussen, Thorkild Maack
    Department of Geophysics, University of Uppsala.
    Roberts, Roland G.
    bDepartment of Geophysics, University of Uppsala.
    The geoelectric model of the POLAR Profile, Northern Finland1989In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 162, no 1-2, p. 113-133Article in journal (Refereed)
    Abstract [en]

    Electromagnetic soundings have been made in order to construct a geoelectrical (conductivity) model of the crust along the European Geotraverse (EGT) POLAR Profile. Forty magnetotelluric (MT) soundings, eighteen audiomagnetotelluric (AMT) soundings and ten magnetohydrodynamic (MHD) soundings were made on the main POLAR Profile (POLAR I) and ten more MT soundings on a parallel profile (POLAR II), 40 km to the southeast of the main profile. Analysis of simultaneous recordings by the EISCAT magnetometer chain, and thin-sheet modelling of the effect of the Barents Sea, indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area. The magnetotelluric data have been modelled with two-dimensional models representing the regional structure along the profiles. In addition to the regional structure, a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions. Although details of the surface electrical structures are poorly resolved, the gross features of the geoelectrical cross section are considered to be reliable. The results divide the POLAR Profile into three different blocks. The better conducting Karasjok-Kittilä Greenstone Belt in the south has an average resistivity of less than 10 Ωm. The more resistant Lapland Granulite Belt, with a resistivity between 100 and 200 Ωm, is underlain by conductive (< 5 Ωm), N-dipping layers. The depth for the uppermost conductive layer varies from a few kilometres in the southwestern part of the granulite belt to 13 km in the northeastern part, from where it rises steeply towards the surface close to the boundary between the Lapland Granulite Belt and the Inari Terrain. These features appear to be continuous between the two parallel MT profiles. Within the Inari Terrain a conductive zone at an approximate depth of 10 km and with a resistivity of about 20 Ωm was identified in a resistive upper crust. The geoelectric cross section agrees, in gross detail, with the corresponding gravity, refraction seismic and reflection seismic cross sections of the POLAR Profile. All methods indicated a similar shape for the southwestern part of the Lapland Granulite Belt i.e., granulites have a gently NE-dipping boundary against the underlying Karelian Province. In the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep S-dipping boundary against the Inari Terrain northeast of the granulite belt.

  • 14.
    Lindqvist, Gustaf
    Luleå tekniska universitet.
    Heat flow density measurements in the sediments of three lakes in northern Sweden1984In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 103, no 1-4, p. 121-140Article in journal (Refereed)
    Abstract [en]

    Heat flow density measurements in the sediments of three lakes, 36 m, 73 m and 99 m deep, were carried out during a period of 3-5 years. The gradient measurements were made from ice during the winter season with a Bullard-type probe with nine thermistors 0.5 m apart. The thermal conductivity was measured in situ. Corrections were made for the disturbing temperature wave in the horizontally stratified sediments caused by seasonal variations in the bottom water temperature. It is clearly shown that the differences in bottom water temperatures between different years can be fully corrected only by very long time series. The heat flow density values obtained are compared with measurements in boreholes and they seem to be correct to within 20-30%

  • 15.
    Liu, H.Y.
    et al.
    Luleå tekniska universitet.
    Kou, S.Q.
    Luleå tekniska universitet.
    Lindqvist, Per-Arne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
    Tang, C.A.
    Center for Rock Instability & Seismicity Research, Northeastern University, Shenyang.
    Numerical studies on the failure process and associated microseismicity in rock under triaxial compression2004In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 384, no 1-4, p. 149-174Article in journal (Refereed)
    Abstract [en]

    In this paper, firstly the mesoscopic elemental mechanical model for elastic damage is developed and implemented into the rock and tool interaction code (R-T (super 2D) ). Then the failure processes of a heterogeneous rock specimen subjected to a wide variety of confining pressures (0-80 MPa) are numerically investigated using the R-T (super 2D) code. According to the simulated results, on the one hand, the numerical simulation reproduced some of the well-known phenomena observed by previous researchers in triaxial tests. Under uniaxial compression, rock failure is caused by a combination of axial splitting and shearing. Dilatancy and a post-failure stage with a descending load bearing capacity are the prominent characteristics of the failure. As the confining pressure increases, the extension of the failed sites is suppressed, but the individual failure sites become dense and link with each other to form a shear fracture plane. Correspondingly, the peak strength, the residual strength and the shear fracture plane angle increase, but the brittleness decreases. When the confining pressure is high enough, the specimen behaves in a plastic manner and a narrow shear fracture plane leads to its failure. The prominent characteristics are volume condensation, ductile cataclastic failure, and a constant load bearing capacity with increasing strain. On the other hand, the numerical simulation revealed some new phenomena. The highest microseismicity events occur in the post-failure stage instead of the maximal stress, and most of the microseismicity energies are released in the failure localization process. As the confining pressure increases, the microseismicity events in the non-linear deformation stage increase dramatically and the ratio between the energies dissipated at the non-linear deformation stage and those dissipated in the whole loading process increases correspondingly. Therefore, it is concluded that the developed mesoscopic elemental mechanical model for elastic damage is able to reproduce accurately the failure characteristics in loading rock specimens under triaxial conditions, and the numerical modelling can furthermore obtain some new clarifications of the rock fracture process.

  • 16.
    Lyngsie, Stig B.
    et al.
    Geological Institute, University of Copenhagen.
    Thybo, H.
    Geological Institute, University of Copenhagen.
    Rasmussen, Thorkild Maack
    bGeological Survey of Denmark and Greenland.
    Erratum to "Regional geological and tectonic structures of the North Sea area from potential field modelling" [Tectonophysics Volume 413/3-4 (2006) 147-170] (DOI:10.1016/j.tecto.2005.10.045)(2006In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 413, no 3-4, p. 303-306Article in journal (Refereed)
  • 17.
    Lyngsie, Stig B.
    et al.
    Geological Institute, University of Copenhagen.
    Thybo, H.
    Geological Institute, University of Copenhagen.
    Rasmussen, Thorkild Maack
    bGeological Survey of Denmark and Greenland.
    Regional geological and tectonic structures of the North Sea area from potential field modelling2006In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 413, no 3-4, p. 147-170Article in journal (Refereed)
    Abstract [en]

    The spatial distribution of large-scale crustal domains and their boundaries are investigated in the North Sea area by combining gravity, magnetic and seismic data. The North Sea is situated on the plates of three continents, Avalonia, Laurentia and Baltica, which collided during the Caledonian orogeny in the middle Palaeozoic. The location and continuation of the collisional sutures are debated. We apply filters and transformations to potential field data to focus on the crystalline crust and uppermost mantle on a regional scale in order to extract new information on continental sutures. The transformations reveal intrinsic features of crustal transitions between the Caledonian plates and their relation to later extensional structures. The transformations include the Hough Transform applied to the gravity field, calculation of fractional derivatives and integrals of the gravity and magnetic fields, the pseudogravity field and the horizontal gradient field as well as upward continuation. The results indicate a fundamental difference between the lithosphere of Avalonia, Laurentia and Baltica. The location of the Mesozoic rift system (the Central Graben and Viking Graben), may have been partly determined by the presence of the sutures between these three plate, indicative of extensional reactivation of compressional structures. A significant lineament across the entire North Sea between Scotland and North Germany indicates that the lower crust of Baltica provenance may extend as far south-westward as to this lineament. Comparison of the power spectra of the gravity field in five selected areas shows significant differences in the long wavelength components between the areas north and south of the lineament corresponding to differences in crustal properties. This lineament could represent the suture between lithosphere of Caledonian origin (Avalonia) versus lithosphere of Precambrian origin (Baltica) in the lower crust and upper mantle. If this is the case, the lineament is the missing link in the reconstruction of the triple plate collision

  • 18.
    Malehmir, A.
    et al.
    Uppsala University.
    Tryggvason, A.
    Uppsala University.
    Juhlin, C.
    Uppsala University.
    Rodriguez-Tablante, J.
    Uppsala University.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Seismic imaging and potential field modelling to delineate structures hosting VHMS deposits in the Skellefte Ore District, northern Sweden2006In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 426, no 3-4, p. 319-334Article in journal (Refereed)
    Abstract [en]

    Skellefte District in northern Sweden is a roughly 150 by 50 km2 large early Proterozoic massive sulphide belt. Based on high-resolution reflection seismic data along two parallel seismic profiles, potential field modeling has been carried out and two geologic cross sections have been constructed that are consistent with the available geophysical data as well as surface geologic observations. The combined modelling suggests that the Kristineberg deposit occurs on the northern limb of a regional E–W striking syncline. The interpretations help to identify new prospective areas, both down-plunge from known ores, and on the ore-bearing horizon on the southern limb of the syncline. The new results suggest that the post-orogenic Revsund granites can be divided into two major types of intrusives, those which are intruded as domes/stokes with a maximum present day thickness of about 3–3.5 km and those which are intruded as thin sheets, with a maximum thickness of a few hundred meters. The margins of the intrusions are generally inclined inwards, suggesting that the current erosion level is near the middle, or toward the base, of the granites. The contact between the Skellefte volcanic rocks and the Bothnian Basin has been interpreted as a thrust fault. We also suggest that crustal thickening predates the Skellefte volcanism and that the interpreted Bothnian Basin rocks are either a structural basement or a separate terrane to the Skellefte volcanism. Diffraction patterns in the reflection seismic data can be interpreted as originating from either a mafic–ultramafic intrusion or a mineralization zone, similar to observations elsewhere in the world. The results obtained in this study have greatly improved our understanding of the tectonostratigraphic framework and architecture of the poly-deformed c. 1.9 Ga Skellefte VHMS belt and is a key step towards building a 3D geological model in the area.

  • 19.
    Pesonen, L. J.
    et al.
    Laboratory for Palaeomagnetism, Geological Survey of Finland.
    Bylund, G.
    Department of Geology, University of Lund.
    Torsvik, T. H.
    Department of Earth Sciences, University of Oxford.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mertanen, S.
    Laboratory for Palaeomagnetism, Geological Survey of Finland.
    Catalogue of palaeomagnetic directions and poles from Fennoscandia: Archaean to tertiary1991In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 195, no 2-4, p. 151-207Article in journal (Refereed)
    Abstract [en]

    Palaeomagnetic data from Fennoscandia ranging from the Archaean to the Tertiary have been compiled into a catalogue. The data are presented in table format, listing Precambrian data according to tectonomagmatic blocks and Late Precambrian-Phanerozoic data according to geological periods. Each pole is graded with the modified Briden-Duff classification scheme. The catalogue (complete to the end of 1988) contains 350 entries from 31 tectonomagmatic blocks and/or geological periods. Normal and reversed polarity data are listed separately to allow polarity asymmetries to be studied. Each entry also has an indexed abstract summarizing relevant information, such as the age of the rock, the age of the natural remanent magnetization and the basis for the assigned reliability grade. All the data are stored in the palaeomagnetic data bank, which will be updated annually with new data. The catalogue is the basic source of data for the microcomputer-based palaeomagnetic database for Fennoscandia now being compiled.

  • 20.
    Pesonen, L. J.
    et al.
    Division of Geophysics, Department of Physical Sciences, University of Helsinki.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mertanen, S.
    Geological Survey of Finland.
    Pisarevsky, S.
    Tectonics Special Research Centre, The University of Western Australia.
    D'Agrella-Filho, M. S.
    Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo.
    Meert, J. G.
    Department of Geological Sciences, University of Florida.
    Abrahamsen, N.
    Department of Earth Sciences, University of Aarhus.
    Bylund, G.
    Lunds universitet.
    Palaeomagnetic configuration of continents during the Proterozoic2003In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 375, no 1-4, p. 289-324Article in journal (Refereed)
    Abstract [en]

    Palaeomagnetic data are used to study the configurations of continents during the Proterozoic. Applying stringent reliability criteria, the positions of the continents at 12 times in the 2.45- to 1.00-Ga period have been constructed. The continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents. The Proterozoic continental configurations are generally in agreement with current geological models of the evolution of the continents. The data suggest that three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland, which comprised at least Laurentia, Baltica, Australia and the Kalahari craton. The protracted breakup of Kenorland during the 2.45- to 2.10-Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The second ‘supercontinental' landmass is Hudsonland (also known as Columbia). On the basis of purely palaeomagnetic data, this supercontinent consisted of Laurentia, Baltica, Ukraine, Amazonia and Australia and perhaps also Siberia, North China and Kalahari. Hudsonland existed from 1.83 to ca. 1.50-1.25 Ga. The youngest assembly is the Neoproterozoic supercontinent of Rodinia, which was formed by continent-continent collisions during ≈1.10-1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggests that multiple Grenvillian age collisions took place during 1.10-1.00 Ga. The configurations of Kenorland, Hudsonland and Rodinia depart from each other and also from the Pangaea assembly. The tectonic styles of their amalgamation are also different reflecting probable changes in sizes and thicknesses of the cratonic blocks as well as changes in the thermal conditions of the mantle through time.

  • 21.
    Pesonen, L. J.
    et al.
    Geological Survey of Finland.
    Torsvik, T. H.
    Department of Earth Sciences, University of Oxford.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Bylund, G.
    Department of Geology, University of Lund.
    Crustal evolution of Fennoscandia: palaeomagnetic constraints1989In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 162, no 1-2, p. 27-49Article in journal (Refereed)
    Abstract [en]

    Palaeomagnetic poles from Fennoscandia, ranging in age from Archaean to Tertiary, are compiled and graded using a modified Briden-Duff classification scale. An new "filtering" technique is applied to select only the most reliable poles for analysis. The filtering takes into account the following information: 1. (1) source block of rock unit,2. (2) age of rock,3. (3) age of magnetization component,4. (4) scatter of palaeomagnetic directions,5. (5) information from multicomponent analysis of natural remanent magnetization (NRM),6. (6) whether the pole considered belongs to a cluster or subcluster of poles,7. (7) magnetic polarity and8. (8) the author's original assignment of results.Data are still insufficient for the drawing of separate Apparent Polar Wander Paths (APWP) for different blocks or cratons of Fennoscandia. Treating Fennoscandia as a single plate, a new APWP from Archaean to Permian is constructed. From the five previously drawn APWP loops (or "hairpins"), only one, the Jatulian loop (2200-2000 Ma), disappears in filtering. The loops during 1925-1700 Ma and during 1100-800 Ma ago are linked to Svecofennian and Sveconorwegian orogenies, respectively. Palaeomagnetic data support the concept that these orogenies took place episodically; three distinct orogenic pulses (early, middle and late) can be distinguished in the cluster plots of palaeopoles. The drift history of Fennoscandia from Archaean to Permian is presented. During most of geological history, Fennoscandia has occupied low to moderate latitudes and undergone considerable latitudinal shifts and rotations. The Svecofennian and Sveconorwegian orogenies have different kinematic characteristics. During the Svecofennian orogeny, Fennoscandia drifted slowly while rotating a large amount in an anticlockwise sense. During the Sveconorwegian orogeny, it drifted rapidly and rotated first clockwise and then anticlockwise. The most striking feature in the drift velocity curves is, however, the pronounced maxima in the latitudinal drift and rotation rates (˜ 9 cm/yr and ˜ 0.8°/Ma, respectively) during the late Subjotnian-Jotnian anorogenic magmatism and rifting phase (˜1450-1250 Ma ago), possibly reflecting the passage of Fennoscandia across a thermal upwelling (hotspot) at equatorial latitudes. The use of palaeomagnetism in delineating and dating movements between blocks is demonstrated with three examples from the POLAR Profile area, the northernmost section of the European Geotraverse.

  • 22.
    Stephansson, Ove
    et al.
    Luleå tekniska universitet.
    Ljunggren, Christer
    Luleå tekniska universitet.
    Jing, Lanru
    Luleå tekniska universitet.
    Stress measurements and tectonic implications for Fennoscandia1991In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 189, no 1-4, p. 317-322Article in journal (Refereed)
    Abstract [en]

    The main results from the Fennoscandian Rock Stress Data Base (FRSDB) are presented. It is suggested that ridge push from the Mid-Atlantic Ridge is the major stress-generating mechanism. The scatter in stress magnitude and direction for Fennoscandia is due to the change in the stress field near faults. This is illustrated by means of simple 2-dimensional distinct element analyses of blocky rock masses subjected to a homogeneous far-field stress regime. Earthquakes are assumed to appear at fault intersections where stresses are concentrated

  • 23.
    Stephens, Michael
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Follin, Sven
    SF GeoLogic AB, Domherrevägen 9, SE-183 51 Täby.
    Petersson, Jesper
    Norconsult AB, Box 8774, SE-402 76 Göteborg.
    Isaksson, Hans
    Geovista AB.
    Juhlin, Christopher
    Department of Earth Sciences, Uppsala University, Uppsala universitet.
    Simenonov, Assen
    Swedish Nuclear Fuel and Waste Management Company.
    Review of the deterministic modelling of deformation zones and fracture domains at the site proposed for a spent nuclear fuel repository, Sweden, and consequences of structural anisotropy2015In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 653, p. 68-94Article in journal (Refereed)
    Abstract [en]

    This paper presents a review of the data sets and methodologies used to construct deterministic models for the spatial distribution of deformation zones and intervening fracture domains in 3-D space at Forsmark, Fennoscandian Shield, Sweden. These models formed part of the investigations to characterize this site, recently proposed as a repository for the storage of spent nuclear fuel in Sweden. The pronounced spatial variability in the distribution of bedrock structures, formed under ductile (lower amphibolite- or greenschist-facies) and subsequently brittle conditions, was controlled by two factors; firstly, the multiphase reactivation, around and after 1.8 Ga, of older ductile structures with a strong anisotropy formed under higher-temperature conditions at 1.87–1.86 Ga; and, secondly, by the release of rock stresses in connection with loading and unloading cycles, after 1.6 Ga. The spatial variability in bedrock structures is accompanied by a significant heterogeneity in the hydraulic flow properties, the most transmissive fractures being sub-horizontal or gently dipping. Although the bedrock structures at Forsmark are ancient features, the present-day aperture of fractures and their hydraulic tranmissivity are inferred to be influenced by the current stress state. It is apparent that the aperture of fractures can change throughout geological time as the stress field evolves. For this reason, the assessment of the long-term (up to 100,000 years) safety of a site for the storage of spent nuclear fuel in crystalline bedrock requires an evaluation of all fractures at the site, not only the currently open fractures that are connected and conductive to groundwater flow. This study also highlights the need for an integration of structural data from the ground surface and boreholes with magnetic field and seismic reflection data with high spatial resolution, during the characterization of structures at a possible site for the storage of spent nuclear fuel in crystalline bedrock.

  • 24.
    Vaněk, J.
    et al.
    Geophysical Institute, Czechoslovak Academy of Sciences.
    Kondorskaya, N.V.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Fedorova, I.V.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Christoskov, L.
    Geophysical Institute, Bulgarian Academy of Sciences.
    Zakharova, A.I.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Dineva, Savka
    Nosova, O.V.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Kireev, I.A.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Chepkunas, L.S.
    Institute of Physics of the Earth, Academy of Sciences, the U.S.S.R.
    Determination of the homogeneous magnitude system magnitudes in seismological practice1985In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 118, no 3-4, p. 359-363Article in journal (Refereed)
    Abstract [en]

    Routine determination of network magnitudes by means of the Eurasian homogeneous magnitude system (HMS) is described and tested on a selection of recent shallow earthquakes. The HMS earthquake magnitudes appear to be more accurate and more reliable than the conventional earthquake magnitudes used in the present seismological practice. A preliminary investigation of correlations between magnitudes determined from different wave types shows a possible dependence of the correlation on the type of focal mechanism of earthquakes

1 - 24 of 24
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