BazTech - Yadda

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Synthrusting remagnetization of the Krizna nappe: high resolution palaeo- and rock magnetic study in the Strazovce section, Strazovske vrchy Mts, Central West Carpathians (Slovakia)

Grabowski J., Michalik J., Szaniawski R., Grotek I.

Acta Geologica Polonica

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2009

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Vol. 59, no. 2

137-155

EN

Palaeomagnetic studies of the uppermost Jurassic to lower Cretaceous pelagic carbonates in the Krizna nappe in the Strazovske vrchy Mts (Central West Carpathians, Slovakia) revealed the presence of secondary magnetite-related magnetization of exclusively normal polarity (component B), which was most probably acquired during the thrusting episode in the late Cretaceous. Three formations exposed in the Strazovce section were the subject of investigation: Jasenina Kimmeridgian.Tithonian), Osnica (Lower.Middle Berriasian) and Mraznica (Upper Berriasian.Hauterivian). Component B is ubiquitous throughout the section but is strongest in the Mraznica Formation. This formation contains a lot of superparamagnetic particles and shows rock magnetic characteristics typical of chemically remagnetized carbonates. The remaining two formations, although also remagnetized, bear traces of an older, probably primary magnetization (component C). The fold test for component B is apparently positive; however the inclination in pre-folding coordinates is too steep for any expected palaeoinclination of Jurassic to recent age. Additional tectonic correction must be applied to match the palaeoinclinations with expected values. Although there is some uncertainty in this additional correction, all plausible options suggest that the rocks must have been magnetized when they dipped in the opposite direction to the thrusting direction. This interpretation is concordant with the internal tectonics of the Krizna nappe, consisting of imbricated units of duplex-type structure.

2

Palaeomagnetic age of remagnetizations in Silurian dolomites, Rástla quarry (Central Estonia)

Plado J., Preeden U., Puura V., Pesonen L. J., Kirsimäe K., Pani T., Elbra T.

Geological Quarterly

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2008

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Vol. 52, No 3

213-213

EN

Alternating field and thermal demagnetization of dolomite samples from the Silurian (Llandovery) horizontally-bedded sequence of central Estonia reveal two secondary magnetization components (A and B) both of chemical origin. A low-coercivity (demagnetized at -50 mT) component A (D = 60.7°, I = 7.7°, alfa95 = 16.6°) with high dispersion (k = 14.2), yields a palaeopole at 18.2°N and 139.5°E that points towards the Late Devonian — Mississipian segment of the Baltica APWP (Apparent Polar WanderPath). A high-coercivity component B (D = 13.5°, I = 60.7°, k = 67.0, alfa 95 = 4.7°) carries both normal and reversed polarities. Comparing the palaeopole (71.1°N and 173.3°E) with the European APWP reveals a Cretaceous age. These two remagnetizations are linked to mineral assemblages of magnetite and maghemite (A), and hematite (B) determined from mineralogical (X-ray, SEM and optical microscopy) and rock magnetic (acquisition and thermal demagnetization of a 3-component IRM; Lowrie-test) studies. The results suggest that the first (A) Palaeozoic remagnetization was caused by low-temperature hydrothermal circulation due to the influence of the Caledonian (more likely) or Hercynian Orogeny after the diagenetic dolomitization of carbon ates. Hematite, carrying the component B, and goethite, are the latest ferromagnetic minerals that have precipitated into the existing pore space (hematite) and walls of microscopic fractures (goethite) that opened to allow ac cess for oxygen-rich fluids during the Late Mesozoic.

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New palaeomagnetic data from the Palaeozoic carbonates of the Moravo-Silesian Zone (Czech Republic): evidence for a timing and origin of the late Variscan remagnetization

Grabowski J., Bábek O., Nawrocki J., Tomek Č.

Geological Quarterly

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2008

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Vol. 52, No 4

321-321

EN

Palaeomagnetic studies were carried out in the Devonian–Early Carboniferous carbonates of the Moravo-Silesian Zone — MSZ (Czech Republic) in order to evaluate the timing and origin of late Variscan magnetic over printing. Sampling localities were spread out along the strike of the MSZ from the SW to NE. Previously published thermal maturity data have demonstrated a significant gradient from SW (burial temperatures 150–200gradeC) to NE of the region (250–300gradeC). A late Variscan remagnetization direction (component A), carried by magnetite, was identified in 6 localities. Three phases of the remagnetization in the MSZ might be distinguished which might be assigned to Early to Late Carboniferous, Late Carboniferous and Early Permian. They are coeval with remagnetization events distinguished in Ardennes. A correlation exists between thermal indices and un blocking temperature spectra of component A. Thermal activation nomograms show that component A might be either a thermoviscous or thermochemical remanent magnetization acquired due to a thermal event (deep burial) of 1–10 My duration and stabilized during subsequent uplift. A more ancient component B, identified in the SW part, previously interpreted as primary, is shown to be a synfolding remagnetisation. Itindicates 70grade clockwise rotations before the acquis tion of the component A .

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Stability of iron oxides and their role in the formation of rock magnetism

Pilchin A.N., Eppelbaum L.V.

Acta Geophysica

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2007

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Vol. 55, no. 2

133-153

EN

Thermodynamic conditions (first of all, temperature) are the main dynamic factors in the transformation process of ferrous to ferric iron (TFFI). TFFI usually takes place within a temperature range of 473-843 K (most active at temperatures above 673 K) and does not require presence of the oxidizing agents above 673 K. Analysis of the chemical composition of different rocks and minerals indicates that only for some sedimentary rocks is the relative content of ferrous iron oxide less than its value in magnetite, and this value is minimal for oceanic sediments. The relative content of ferrous iron oxide in oceanic magmatic rocks exceeds this value in continental magmatic rocks and depends on the rate of rock cooling. An investigation of the role of the titanium oxide content of different rocks on stability of ferrous iron oxide against its transformation to ferric iron oxide shows that a significant correlation (r = 0.79) does exist between the relative content of ferrous iron oxide and ratio of TiO2/Fe2O3. Temperature within the solar nebula at location of the Earth was within the temperature range of the TFFI. During the Earth accretion and its early evolution, ferric iron oxide was unstable and most likely did not exist. The first magnetic minerals containing ferric iron could have appeared only after the Earth’s surface had cooled below ~843 K. The formation of the first Algoma-type banded iron formations could be used as a marker of the Earth’s surface cooling below ~843 K.

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Paleomagnetism and remagnetization of Upper Devonian synorogenic clastic sediments from the Pogorzała Formation (Świebodzice Depression, West Sudetes, Poland

Kądziołko-Hofmokl M., Jeleńska M., Aifa T., Edel J.B., Żelaźniewicz A.

Geologia Sudetica

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1999

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Vol. 32, nr 2

113-126

EN

The Upper Devonian highly polymict conglomerates (site 26) and sandstones with clasts (site 27) of the Pogorzała Formation in the synorogenic Świebodzice Depression, West Sudetes, are hydrothermally altered and show signs of penetration by mineralized fluids. Nearly all the magnetic minerals present (mainly Fe-oxides and pyrrhotite accompanied by Fe-hydroxides) are of secondary origin. Rocks from each site carry multicomponent natural remanence composed of Mesozoic/post-Mesozoic and Palaeozoic components. In the conglomerates (site 26) two Palaeozoic components, labelled P and C, occur in the matrix and pebbles, whereas in the sandstones with large clasts (site 27) only one Palaeozoic component labeled C1 occurs. This means that the results of the conglomerate test for both sites are negative and the studied rocks were remagnetized during several remagnetization episodes. The overprints present in site 26 closely fit the reference data for the Baltica Plate for the Early Permian component (P) and Viséan component (C). The overprint present in site 27 is slightly shifted from the Westphalian (C1) segment of the reference path. The P component is also close to the path of polar wander for Variscan Europe.