| 1. |
- Elming, Sten-åke, et al.
(författare)
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Cooling history and age of magnetization of a deep intrusion : A new 1.7 Ga key pole and Svecofennian-post Svecofennian APWP for Baltica
- 2019
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Ingår i: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 329, s. 182-194
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Tidskriftsartikel (refereegranskat)abstract
- A paleomagnetic and chronogical study has been performed on the Turinge gabbro-diabase formation and on a cross cutting basic dyke in central Sweden and on the Joulovaara gabbro intrusion in northern Sweden in the Fennoscandian Shield. U-Pb age of baddeleyite and 40Ar/ 39Ar ages of hornblende and biotite reveal a cooling history of the deep gabbro-diabase intrusion in Turinge. The cooling is suggested to have taken place in two stages, one related to the time of intrusion in temperature down to ca 500 °C with a cooling rate up to 46 - 59°/Ma and another at a lower rate of ca 2.9 °C/Ma, which is suggested to be related with uplift. From this cooling history it can be concluded that the magnetization age of the diabase, ca 1695 – 1700 Ma is close to the crystallization age and the 40Ar/39Ar age of hornblende. Applying a similar cooling history for the other studied deep intrusion, the ca 1800 Ma gabbro of Joulovaara gabbro, it is estimated that the magnetization age of the gabbro should be close to that of the U-Pb age of the formation, although the pole of the Joulovaara gabbro is less reliable.The cooling history presented here for the Turinge gabbro-diabase has implications for estimations of magnetization ages also for other deep intrusions.The new pole (Plat. = 51.6°, Plon. = 220.2°; A95= 4.8°) of the Turinge gabbro-diabase passes most of the reliability criteria and is considered a new key pole for Fennoscandia.The Basic dyke that cuts the Turinge gabbro-diabase was here dated at ca 1200 Ma (whole rock, 40Ar/ 39Ar) and the virtual geomagnetic pole calculated from its primary magnetization falls into the expected trend of APWP for Baltica.The new Turinge key pole prolong the time of overlapping poles for Fennoscandia, indicating only small movements of the shield between ca. 1870 to 1700 Ma.
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| 2. |
- Eriksson, Per, et al.
(författare)
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Magma flow and palaeo-stress deduced from magnetic fabric analysis of the Álftafjörður dyke swarm : implications for shallow crustal magma transport in Icelandic volcanic systems
- 2015
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Ingår i: Geological Society Special Publication. - 0305-8719 .- 2041-4927. ; 396, s. 107-132
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Tidskriftsartikel (refereegranskat)abstract
- Neogene regional mafic dykes extending north of the Álftafjörður central volcano in east Iceland are studied to test models of dyke swarm emplacement at spreading ridges. This is accomplished by using anisotropy of magnetic susceptibility to define fossilized magma flow regimes. The imbrication of the foliation plane, defined by the minor susceptibility axis, is used as an indicator of the flow direction. Contemporaneous shear resolved on the dyke walls may modify a pure flow-induced fabric and such shear regimes are therefore retracted. The magma flow and palaeo-stress resolved on the dykes are determined in 13 of 24 dykes. The magma flow is interpreted as subhorizontal and northwards directed away from the central volcano for nine dykes, and found to be vertical in three cases. The preferentially subhorizontal magma flow in the Álftafjörður swarm suggests that dyke propagation in this type of Icelandic volcanic system originates in shallow crustal magma chambers. The regional tectonic palaeo-stress field is deduced to cause oblique spreading across the Álftafjörður dyke swarm and govern a subhorizontal dextral shear component on the dyke planes during propagation. This interpretation is not in conflict with the left-stepping en echelon trend distribution of individual dykes relative to the trend of the swarm
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| 3. |
- Gong, Zheng, et al.
(författare)
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Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden
- 2018
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 317, s. 14-32
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Tidskriftsartikel (refereegranskat)abstract
- Paleogeographic proximity of Baltica and Laurentia in the supercontinent Rodinia has been widely accepted. However, robust paleomagnetic poles are still scarce, hampering quantitative tests of proposed relative positions of the two cratons. A recent paleomagnetic study of the early Neoproterozoic Blekinge-Dalarna dolerite (BDD) dykes in Sweden provided a 946–935 Ma key pole for Baltica, but earlier studies on other BDD dykes discerned large variances in paleomagnetic directions that appeared to indicate more complicated motion of Baltica, or alternatively, unusual geodynamo behavior in early Neoproterozoic time. We present combined paleomagnetic, rock magnetic, magnetic fabric and geochronological studies on BDD dykes in the Dalarna region, southern Sweden. Positive baked-contact and paleosecular variation tests support the reliability of the 951–935 Ma key pole (Plat = −2.6°N, Plon = 239.6°E, A95 = 5.8° N = 12 dykes); and the ancient magnetic field was likely a stable geocentric axial dipole at that time, based on a positive reversal test. Detailed analysis of the 947 Ma Nornäs dyke, one of the dykes previously showing anomalous directions, suggests a partial viscous remagnetization. Therefore, the observed large variances in nearly coeval BDD dykes are suspected to result from present-day overprints that were not adequately removed in earlier studies. In addition, we obtained a 971 Ma virtual geomagnetic pole (Plat = −27.0°N, Plon = 230.4°E, A95 = 14.9° N = 4 dykes) for Baltica. Comparing similar-aged poles from Laurentia, we suggest that Baltica and Laurentia drifted together from high to low latitude between 970–960 Ma and 950–935 Ma, and returned back to high latitude by 920–870 Ma. In this scenario, the apparent polar wander paths of Baltica and Laurentia may be more complicated than the previously proposed, solitary Sveconorwegian and Grenville loops. The new U-Pb baddeleyite ages do not support BDD dykes as a giant circumferential swarm generated by a mantle plume, and the prolonged timespan of dyke intrusion is likely associated with the plate boundary forces as causing gravitational extension at the waning stage of the Sveconorwegian orogeny.
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| 4. |
- Singsoupho, Sounthone, et al.
(författare)
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Deformation in Jurassic-Cretaceous redbeds from Champasak and Khammouane, Lao PDR, revealed by anisotropy of magnetic susceptibility
- 2018
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Ingår i: Chang Mai journal of science. - Chīang Mai : Mahāwitthayālai Chīang Mai. - 0125-2526. ; 45:1, s. 576-587
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Tidskriftsartikel (refereegranskat)abstract
- The anisotropy of magnetic susceptibility measurement technique was applied to the Jurassic-Cretaceous redbeds in the Champasak and Khammouane provinces of the southern and central part of Lao PDR, in order to define the correlation between the magnetic anisotropic data and degree of deformation in redbeds due to tectonic stress field focusing on the mountain belt and fault areas. The results show that the alignment of Kmax axes is predominantly parallel to the trending of the mountain belts (or fold axis) and the strike of faults. The orientation patterns of the principal axes of the susceptibility ellipsoid did not show the typical pattern usually found in undeformed sedimentary rocks. It can be concluded that the development of the triaxial and pencil structure patterns of secondary magnetic fabric correlates well with the degree of rock deformation which is caused by tectonic compression acting on the rocks.
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| 5. |
- Singsoupho, Sounthone, et al.
(författare)
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Palaeocurrent direction estimated in Mesozoic redbeds of the Khorat Plateau, Lao PDR, Indochina Block using anisotropy of magnetic susceptibility
- 2015
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 106, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- Mesozoic redbeds in the western part of central and southern Laos are the most abundant rocks of the Khorat Plateau. In earlier studies, palaeocurrent directions of the Mesozoic redbeds in the Khorat Plateau were mapped, but for the Upper Cretaceous sediments no palaeocurrent data was presented. The purpose of the present study is to test previous models of sedimentary source areas and propose palaeocurrent flow directions for Upper Cretaceous sediments in the Khorat Plateau. This we do by characterization of the rocks on the basis of their magnetic properties and anisotropy parameters. Integrated magnetic properties, including susceptibility, NRM, Q-value, and S-ratio, have been used for classification of the stratigraphies of the redbeds. A clear difference in magnetic properties was observed between the Jurassic and Cretaceous rocks, while some sites of the Lower and Upper Cretaceous were similar. From magnetic analyses the Champasak redbeds could be defined statigraphically as an unconformity beneath the Khorat Group. On the basis of the AMS interpretation, the rocks can be divided into undeformed and deformed rocks. The palaeocurrent directions in the undeformed redbeds are predominantly from the NNE and NE of the Khorat Basin. For the Upper Jurassic-Lower Cretaceous redbeds, we argue that the sources rocks of the sediments were located in the northern and eastern margin of the Khorat Plateau, probably in the Truong Son Belt zone in the northeastern Laos and central Vietnam. While, the source rock of the Upper Cretaceous redbeds in the Khorat Plateau was located in the Qinling Belt in China or in the Loei-Luang Prabang Fold Belt in northeastern Thailand and northern Laos.
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| 6. |
- Suteerasak, Thongchai, et al.
(författare)
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Deposition rates and C-14 apparent ages of Holocene sediments in the Bothnian Bay of the Gulf of Bothnia using paleomagnetic dating as a reference
- 2017
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Ingår i: Marine Geology. - : Elsevier BV. - 0025-3227 .- 1872-6151. ; 383, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Three 6-m-long cores of sediments were collected in the northern, middle and southern part of the Bothnian Bay. The sediments were dated by paleomagnetic dating techniques, constrained by magnetic properties and geochemical data. The results indicate the ages of the sediments in the bottom part of the cores in the northern, middle and southern parts of the Bothnian Bay to be approximately 5300 years BP, 5350 years BP and 3500 years BP, respectively. The deposition rate calculated from the estimated ages at various depths show that the deposition rate was generally in the range 0.5-1.5 mm/year but it was higher in the southern part than in the middle and northern parts of the bay. There was a significant increase in the deposition rate at ca 2200 years BP, recorded in all three cores, a rate varying between 2.47 and 3.07 mm/year and lasting until ca 1840 years BP. A proposed constant uplift rate of the crust during the period ca 5500 years BP to present is thus not reflected by a constant deposition rate. The peaks in deposition rates at ca 2200-1840 years BP were followed by a decrease in salinity. This phenomenon is suggested to be caused by crustal uplift, with a threshold being formed in the southern part of the bay, thereby increasing the reactivation of bottom sediments and reducing the inflow of brackish water from the Bothnian Sea. The C-14 ages of the sediments reveal differences in age compared with the paleomagnetic ages. In the southern core, the C-14 ages are ca 1350 years older, and in the north, the age offset is mixed. The reactivation and re-deposition of sediments is argued to be the reason for the apparent C-14 age differences. This finding demonstrates that C-14 cannot be used for the dating of Bothnian Bay sediments unless the radiocarbon age offset has been determined.
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| 7. |
- Tavakoli, Saman, et al.
(författare)
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Deep massive sulphide exploration using 2D and 3D geoelectrical and induced polarization data in Skellefte mining district, northern Sweden
- 2016
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Ingår i: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 64:6, s. 1602-1619
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Tidskriftsartikel (refereegranskat)abstract
- Geoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high-chargeability low-resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high-resolution image of the top ∼450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep-resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ∼1-km depth.
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| 8. |
- Tavakoli, Saman, et al.
(författare)
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Potential Field, Geoelectrical and Reflection Seismic Investigations for Massive Sulphide Exploration in the Skellefte Mining District, Northern Sweden
- 2016
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Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 64:6, s. 2171-2199
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Tidskriftsartikel (refereegranskat)abstract
- Multi-scale geophysical studies were conducted in the central Skellefte district (CSD) in order to delineate the geometry of the upper crust (down to maximum similar to 4.5 km depth) for prospecting volcanic massive sulphide (VMS) mineralization. These geophysical investigations include potential field, resistivity/induced polarization (IP), reflection seismic and magnetotelluric (MT) data which were collected between 2009 and 2010. The interpretations were divided in two scales: (i) shallow (similar to 1.5 km) and (ii) deep (similar to 4.5 km). Physical properties of the rocks, including density, magnetic susceptibility, resistivity and chargeability, were also used to improve interpretations. The study result delineates the geometry of the upper crust in the CSD and new models were suggested based on new and joint geophysical interpretation which can benefit VMS prospecting in the area. The result also indicates that a strongly conductive zone detected by resistivity/IP data may have been missed using other geophysical data.
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