| 1. |
- Chew, D M, et al.
(författare)
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The tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?
- 2008
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 2041-479X .- 0016-7649. ; 165:3, s. 675-685
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Tidskriftsartikel (refereegranskat)abstract
- The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head-Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing 14 metamorphic assemblages (c. 670 degrees C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average Pb-207/Ph-206 zircon age of 467 +/- 12 Ma whereas the main fabric yields a Ar-40-Ar-39 biotite cooling age of 468 +/- 1.4 Ma. The pegmatites yield 457 +/- 7 Ma and 458 +/- 7 Ma Rb-Sr muscovite-feldspar ages and Ar-40-Ar-39 step-heating plateaux of 466 +/- 1 Ma and 468 +/- 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm-Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U-Pb analyses from a psammitic gneiss yield age populations at 1.05-1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.
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| 2. |
- Deng, X. H., et al.
(författare)
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Dynamics and waves near multiple magnetic null points in reconnection diffusion region
- 2009
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Ingår i: Journal of Geophysical Research. - : Blackwell Publishing. - 0148-0227 .- 2156-2202. ; 114:7
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Tidskriftsartikel (refereegranskat)abstract
- Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.
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| 3. |
- Kirkland, C. L., et al.
(författare)
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The Finnmarkian Orogeny revisited: An isotopic investigation in eastern Finnmark, Arctic Norway
- 2008
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951. ; 460:1-4, s. 158-177
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Forskningsöversikt (refereegranskat)abstract
- The Scandinavian Caleclonides have been viewed as resulting from either a single Silurian (i.e. Scandian) event or from polycyclic orogenies involving several collisions on the margin of Baltica. Early studies of the Kalak Nappe Complex (KNC) in Finnmark, Arctic Norway, led to the hypothesis of an Early Cambrian-Early Ordovician (520-480 Ma) Finnmarkian Crogeny, though the nature of this tectonic event remains enigmatic. In this contribution we have employed in situ UV laser ablation Ar-Ar dating of fine-grained phyllite and schist from the eastern Caledonides of Arctic Norway to investigate the presence of pre-Scandian tectonometamorphic events. U-Th-Pb detrital zircon and whole rock Sm-Nd analyses have been used to test the regional stratigraphic correlations of these metasedimentary rocks. These results indicate that the Berlevag Formation within the Tanafjord Nappe, previously assumed to be part of the KNC, was deposited after 1872 Ma and prior to a low temperature hydrothermal event at 555 +/- 15 Ma. It has a likely provenance on the Baltica continent, lacks any Grenville-Sveconorwegian detrital zircons, and thus cannot be part of the KNC which contains abundant detritus in this age range. Instead the Berlevag Formation is interpreted as part of the Laksefjord Nappe Complex, which structurally underlies the KNC. Laser-ablation argon-argon dating also shows that late Caledonian (i.e. Scandian) tectonometamorphism affected both the KNC and its immediate footwall at c. 425 +/- 15 Ma. This is corroborated by a step-heating argon-argon muscovite age of 424 +/- 3 Ma which is interpreted as dating cooling. However, within two samples from the KNC, an earlier (Middle-Late Cambrian) metamorphic event is also recorded. A biotite-grade schist yielded an Ar-Ar inverse isochron age of 506 +/- 17 Ma from whole rock surfaces, in which the mineral domains are too fine-grained to date individually. An early generation of muscovite from a coarser-grained amphibolite-facies sample yielded an inverse isochron of 498 +/- 13 Ma. Both isochron ages have atmospheric argon intercept values. Previous studies have documented similar Cambrian ages in the Caledonian nappes below the KNC. These results suggest correlative tectonometamorphic events in the eastern KNC and its footwall at c. 500 Ma. This Cambrian event may reflect the arrival of the Kalak Nappe Complex as a previously constructed exotic mobile belt onto the margin of Baltica. Combined with recent studies from the western Kalak Nappe Complex, the results do not support the traditional constraint on the Finnmarkian Orogeny sensu stricto. However they vindicate classic tectonic models involving a Cambrian accretion event. (c) 2008 Elsevier B.V. All rights reserved.
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| 4. |
- Pease, Victoria, et al.
(författare)
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Baltica in the Cryogenian, 850-650 Ma
- 2008
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 160:1-2, s. 46-65
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Forskningsöversikt (refereegranskat)abstract
- This new tectonic synthesis provides a framework for understanding the dynamic evolution of Baltica and for constraining tectonic correlations within the context of the Neoproterozoic break-up of Rodinia–Pannotia. Cryogenian Baltica is described with respect to five geographic regions: the northwest, northeast, east, south, and southwest (modern coordinates). These geographic regions define three principal Cryogenian tectonic margins: a rifting northwestern margin, a passive northeastern margin, and a poorly understood southern margin. The northwest region is characterized by Neoproterozoic to lower Ordovician sedimentary successions deposited on Archean to late Mesoproterozoic crystalline complexes, reworked during Caledonian orogenesis. Lare Neoproterozoic to lower Ordovician sedimentary strata record the change from an alluvial setting to a marine environment, and eventually to a partially starved (?) turbidite basin. They document rifting from the Rodinian-Pannotian supercontinent, which was unsuccessful until ca. 620–550 Ma when voluminous dikes and mafic/ultramafic complexes were intruded. Baltica's northeastern and eastern regions document episodic intracratonic rifting throughout the Mesoproterozoic, followed by pericontinental passive margin deposition throughout the Cryogenian. In the northeast platformal and deeper-water basin deposits are preserved, whereas the eastern region was later affected by Paleozoic rifting and preserves only shelf deposits. The northeastern and eastern regions define Baltica's Cryogenian northeastern tectonic margin, which was an ocean-facing passive margin of the Rodinia–Pannotia supercontinent. It remained a passive margin until the onset of Timanian orogenesis at ca. 615 Ma, approximately synchronous with the time of Rodinia–Pannotia rifting. Baltica's southern and southwestern regions remain enigmatic and controversial. Precambrian basement is generally hidden beneath thick successions of Ediacaran and younger platform sediments. Similarities between these regions exist, however, and suggest that they may share a similar tectonic evolution in the Cryogenian and therefore define the southern tectonic margin of Baltica at this time. Paleo- to Mesoproterozic basement was affected by Neoproterozoic and younger tectonism, including Cryogenian (?) and Ediacaran rifting. This was followed by Ediacaran (ca. 550 Ma) passive margin sediment deposition at the time of Rodinia–Pannotia break-up, until Early Paleozoic accretion of allochthonous terranes record the transition from rifting to a compressional regime. Paleomagnetic and paleontological data are consistent with Baltica and Laurentia drifting together between ca. 750 and 550 Ma, when they had similar apparent polar wander paths. Microfossil assemblages along the eastern margin of Laurentia and the western margin of Baltica (modern coordinates), suggest proximity between these two margins at this time. At ca. 550 Ma, Laurentia and Baltica separated, consistent with paleomagnetic, paleontological, and geological data, and a late break-up for Rodinia–Pannotia.
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