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  • 1
    Electronic Resource
    Electronic Resource
    Implications of palaeomagnetic data from the Tortworth Silurian inlier (southern Britain) to palaeogeography and Variscan tectonism (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Palaeomagnetic data are presented from early Silurian (Upper Llandovery) lavas from the Tortworth Silurian inlier of south-west England. Two magnetization components are recognized on the basis of contrasting demagnetization characteristics. A lower unblocking-temperature component (〈350°C) is oriented parallel to the Earth's present magnetic field at the sampling site (N= 46, declination = 352°, inclination = 69°, k= 52.1, α95= 2.9°). A higher unblocking-temperature component (350–600°C) is inferred to be primary on the basis of a stratigraphy-parallel reversal pattern (combined tilt-corrected mean: N= 42, declination = 056°, inclination = -30°, k= 22.6, α95= 4.7°). Geological implications of these new data are as follows.(1) the calculated Upper Llandovery (c. 430 Ma) palaeolatitude for the site (16°S) is consistent with tectonic models invoking pre-Wenlock closure of the Iapetus Ocean across Britain.(2) the declination of the primary magnetization indicates substantial clockwise rotation within this sector of the Variscan thrust belt. Rotation is most likely to be linked to early Variscan northward thrust transport with components of dextral transpression. the amount of clockwise rotation is approximately 85° in the south (Mendips) and decreases to 33-63° in the north (Tortworth).(3) the polarity of the primary magnetization (Lower Trap lava: reversely polarized; Upper Trap lava: normally polarized is consistent with that expected from a preliminary analysis of the Silurian magnetic-field reversal pattern.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1994.tb00915.x
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  • 2
    Electronic Resource
    Electronic Resource
    Palaeomagnetic dating of fault rocks: evidence for Permian and Mesozoic movements and brittle deformation along the extensional Dalsfjord Fault, western Norway (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Palaeomagnetic and structural studies of the Dalsfjord Nappe, western Norway, show that the basal low-angle detachment (Dalsfjord Fault) is a long-lived fault zone, and that the most important phase of faulting was of Devonian extension, probably nucleated on an earlier Silurian (Scandian) thrust. Fault rocks produced during subsequent movements indicate that the Dalsfjord Fault underwent periods of brittle low-angle extensional reactivation during the Permian (250–260 Ma) and Upper Jurassic/Lower Cretaceous (c. 150 Ma), corresponding with stages of major extensional movements on the continental shelf. Palaeomagnetic studies may be of great importance for dating faults and major movement stages in long-lived fault systems. The particular importance of the results is that they show that low-angle normal faults can operate in a brittle upper crustal regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00118.x
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  • 3
    Electronic Resource
    Electronic Resource
    Palaeogeographic significance of mid-Silurian palaeomagnetic results from southern Britain—major revision of the apparent polar wander path for eastern Avalonia (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 113 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Early Wenlock (c. 430 Ma) lavas from the East Mendips Inlier, Southern Briain, are characterized by either single-component magnetizations (Type 1) or a more complex system of three magnetization components (Type 2) during stepwise thermal demagnetization. The type of demagnetization behaviour is related to changes in magnetic petrology within the lava pile. The lower and intermediate unblocking temperature components of Type 2 represent magnetic overprints of Tertiary/Recent and Hercynian (mid-Carboniferous) origin respectively. The primary nature of the high unblocking temperature magnetization component, Types 1 and 2 (Dec = 095°C, Inc = -24°, α95= 8.8°), is demonstrated by positive agglomerate tests. Conversely, an overlying Upper Old Red Sandstone sequence did not provide primary magnetizations; the magnetic signature is governed by syn-tectonic Hercynian remagnetizations.The primary Silurian data demonstrate that the East Mendips Inlier experienced considerable clockwise rotation, probably of the order of 80°, prior to partial remagnetization during Hercynian deformation and folding. The Silurian palaeopole (VGP: 13°N, 271°E, dp/dm = 5/9) cannot therefore be used for apparent polar wander path construction. Nevertheless, the palaeolatitude estimate (13°± 5S) suggests that southern Britain (eastern Avalonia) was at comparable mid-Silurian latitudes to Baltica and northern Britain (10-20°S). Both the Iapetus Ocean and Tornquist Sea therefore, which separated southern Britain from Laurentia and Baltica respectively in Ordovician times, were closed by Wenlock time.As Ordovician and Silurian palaeomagnetic data from Baltica and Eastern Avalonia now record a comparable northwards translation from high southerly to near-equatorial latitudes during this time, we postulate pre-Wenlock collision of these blocks to form BALONIA (Baltica + eastern Avalonia). A new apparent polar wander path for eastern Avalonia constructed on the basis of this palaeogeographic model is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb04658.x
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  • 4
    Electronic Resource
    Electronic Resource
    A high southerly palaeolatitude for Southern Britain in Early Ordovician times: palaeomagnetic data from the Treffgarne Volcanic Formation SW Wales (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 108 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A palaeomagnetic pole is reported from late Tremadoc-early Arenig volcanics of the Treffgarne Volcanic Formation, southwest Wales. This pole, located at N56d̀, E306d̀ (dp/dm=9/10), implies that Southern Britain was positioned at c. 60d̀S during Early Ordovician times. These data are considered palaeomagnetically reliable based upon a positive intraformational conglomerate test. Comparison with palaeomagnetic results from the southern Laurentian margin suggests the intervening Iapetus Ocean to have reached a width of at least 5000 km at this time.A partial magnetic overprint of Hercynian age is also identified (pole: S42d̀, E346d̀, dp/dm= 5/9) and is correlated with similar remagnetization features across southern and central Wales. Geological indicators of palaeotemperature in the south/central Welsh Basin most likely reflect a Hercynian thermochemical pulse.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00840.x
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  • 5
    Electronic Resource
    Electronic Resource
    Experimental CRM production in a basaltic rock; evidence for stable, intermediate palaeomagnetic directions (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Remagnetization experiments involving secondary thermoremanent and chemical remanent magnetization (TRM-CRM) have been carried out on samples from a basaltic lava flow. The rock carries a stable NRM residing in deuterically oxidized (class II-III) titanomagnetite grains, and some haematite formed by low-temperature oxidation. Heating to 525d̀C in a controlled field of 52 μT for periods ranging from 1 to 400 hr, resulted in oxidation to haematite of a parent phase close to magnetite in composition. The direction of the resulting CRM is controlled both by the remagnetizing field and the primary remanence (NRM), giving rise to stable high-blocking remanence components with intermediate directions, and precluding identification of the primary NRM component.Control experiments carried out on chemically stable specimens gave a partial TRM (PTRM) parallel to the remagnetizing field, with blocking temperatures in agreement with single-domain theory. After removal of this secondary PTRM, the primary direction in these specimens was regained.The results imply that caution is advisable in analysing the palaeomagnetic directions of any rock which has undergone a thermochemical event.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb00809.x
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  • 6
    Electronic Resource
    Electronic Resource
    On the palaeogeography of Baltica during the Palaeozoic: new palaeomagnetic data from the Scandinavian Caledonides (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 103 (1990), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Based on new palaeomagnetic results from the North Norwegian Caledonides, we propose new apparent polar wander paths for Baltica during the Early–Mid Palaeozoic and discuss their palaeogeographic implications.In Cambrian and Early Ordovician times, Baltica occupied southerly latitudes of the order of 30° to 50°, but was ‘inverted’ with respect to its present orientation. Consequently, the Russian Platform faced Avalonia and Gondwana, the latter continent occupying high southerly latitudes. Closure of the Tornquist Sea was then accompanied by continental scale, anticlockwise rotation of Baltica relative to Avalonia. This rotation probably occurred during mid-Ordovician times, although as yet, the timing of final suturing is poorly constrained by available palaeomagnetic data. At this time Laurentia occupied an equatorial position.Baltica then moved northwards in Late Ordovician and Silurian times, and subsequently collided obliquely with Laurentia to produce the Mid-Silurian to Early Devonian Scandian Orogeny. Oblique convergence, set up sinistral orogen-parallel shear zones, on which major movements ceased by Late Silurian times. After amalgamation, Baltica and Laurentia occupied equatorial to tropical southerly latitudes.Reconstructions for the Siluro-Devonian boundary are now relatively straightforward. Euramerica was assembled by that time, and occupied equatorial (N. Baltica) to high (c. 60°) southerly latitudes (S. Laurentia) prior to northerly movement and the final assembly of Pangea.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1990.tb01768.x
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  • 7
    Electronic Resource
    Electronic Resource
    The British Siluro-Devonian palaeofield, the Great Glen Fault and analytical methods in palaeomagnetism: comments on paper by K. M. Storetvedt et al. (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Late Silurian-early Devonian palaeomagnetic poles throughout the British Isles lie in a coherent group about 1°S and 314°E (A95= 9.6°). The clustering of these poles, which are derived from 11 individual studies of Siluro-Devonian rocks of all the major tectonic elements of Britain, carries two important tectonic implications:〈list xml:id="l1" style="custom"〉1the British sector of the Iapetus Ocean, recognizable from Ordovician poles, had closed by late Silurian times; and2any postulated mega-shear, whether related to Acadian or Hercynian deformation, is below the limit of palaeomagnetic resolution.The collective rejection of all Siluro-Devonian results by Storetvedt et al. (1990a, b) is demonstrated to be unfounded.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb06725.x
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  • 8
    Electronic Resource
    Electronic Resource
    Comments on the paper ''bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments'', by Brooks B. Ellwood
    Amsterdam : Elsevier
    Earth and Planetary Science Letters 71 (1984), S. 349-350 
    Details
    ISSN: 0012-821X
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0012-821X(84)90099-2
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  • 9
    Electronic Resource
    Electronic Resource
    On the origin and the tectonic implications of magnetic overprinting of the Old Red Sandstone, Shetland (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 99 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The magnetic signature of the Old Red Sandstone (ORS) and post-orogenic plutonics on Shetland is influenced by three principal components, A (pole: S51, EOO3), B (pole: S24, E340) and C (pole: S10, E003). As evidenced from negative fold tests, A is secondary, and most likely thermochemical in origin, presumably associated with hydrothermal fluids circulating in faults and crush zones subsequent to Permian-Early Triassic extensional reactivation of older Caledouian fault structures. From the ORS, the B component also can be proven to represent a magnetic overprint. The secondary nature of component B, and the fact that it reasonably can be correlated on both sides of the Walls Boundary Fault, commonly assumed to be the continuation of the Great Glen Fault, argues against recent suggestions of mega-shearing within the Great Glen Fault system. The precise time of acquisition of B is uncertain, but we consider a lower Carboniferous age synchronous with late-post orogenic plutonic activity (334-358 Ma) to be most likely. This implies that the B component carried by the plutonic rocks may represent a primary cooling event. C is exclusively carried out by Middle Devonian andesites and basalts from the Esha Ness Peninsula. It is evidently of post-fold origin, and we relate this earliest magnetic overprinting to Middle-Upper Devonian (Svalbardian) tectonism which affected the North Atlantic domain during this period.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb02056.x
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  • 10
    Electronic Resource
    Electronic Resource
    Timing of Late Neoproterozoic glaciation on Baltica constrained by detrital zircon geochronology in the Hedmark Group, south-east Norway (2005)
    Oxford, UK : Blackwell Science Ltd
    Terra nova 17 (2005), S. 0 
    Details
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Moelv Tillite is the Late Neoproterozoic Varanger glacial deposit recorded in the Hedmark Group, SE Norway. Paired U–Pb and Lu–Hf data collected on detrital zircons in the Rendalen Formation underlying the Moelv Tillite have identified an uncommon 677 ± 15 to 620 ± 14 Ma population, that constrain the deposition of the Moelv Tillite to be younger than 620 ± 14 Ma. The youngest detrital zircons may be derived from granite magmatism related to the 616 ± 3 Ma Egersund dolerite magmatism, situated in the western part of the Sveconorwegian orogen. The Moelv Tillite, which is not overlain by a cap carbonate, possibly correlates with the c. 580 Ma Squantum-Gaskiers glacial deposits of Avalonia. Available palaeomagnetic data for the Late Neoproterozoic suggest that Baltica was located at intermediate to high latitude between 620 and 555 Ma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3121.2005.00609.x
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