IRM Conferences and Conference Contributions

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This collection includes contributions to conferences organized by the Institute for Rock Magnetism, and contributions to other conferences made by personnel of the Institute for Rock Magnetism.

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Now showing 1 - 8 of 8
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    Magnetic fabric and inclination shallowing studies: depositional and post-depositional processes in hematite- and magnetite-bearing rocks
    (2009-12) Bilardello, Dario; Kodama, Kenneth P.
    Magnetic anisotropy-based inclinations corrections of both hematite and magnetite-bearing sedimentary rocks indicate latitudinal variations of inclination shallowing. Rocks formed at mid latitudes suffer from more shallowing than those formed closer to the equator, consistent with the tan Im= f * tan If relationship, where Im is the measured inclination and If is the field inclination during deposition. Shallowing of the paleomagnetic vectors can be expressed in terms of the flattening factor f, a function of the rock’s magnetic fabric and the individual particle anisotropy, the a factor. Estimation of the f factor enables performing simplified inclination corrections. f factors derived from anisotropy-based inclination corrections were combined with f factors derived from corrections that use models of geomagnetic field secular variation for hematite and magnetite bearing rocks. Magnetite data indicate a smaller range of f factors, leading to smaller ranges of inclination shallowing. Using the reported range of f factors enables more precise estimations of inclination corrections. Hematite data, on the other hand, show a broader range of f factors, which makes estimating inclination shallowing and correcting for it more difficult. However, because hematite has magneto-crystalline anisotropy, the value of a doesn’t have much variation, thus requiring a precise measure of the magnetic fabric only. Hematite fabrics were measured for an inclination shallowing study of red beds from the Maritime Provinces of Canada (Shepody Fm), using a high field anisotropy of isothermal remanence technique (hf-AIR). The technique allows to fully saturate hematite’s remanence without the need to demagnetize the samples between the different positions required to measure the anisotropy tensor, eliminating the risk of thermo-chemical alteration. The technique makes it possible for typical paleomagnetic laboratories to measure the remanence anisotropy of high coercivity hematite. The precise measurement of the fabric allowed interpreting it as a compactional fabric that was reoriented by strain during folding following a flexural-slip model.
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    A simplified Red Bed Inclination Correction: Case Study from the Permian Esterel Group of France
    (2008-12) Bilardello, Dario; Kodama, Kenneth, P.
    Magnetic anisotropy-based inclinations corrections have been performed in the paleomagnetic laboratory at Lehigh University, on both hematite and magnetite-bearing sedimentary rocks. Results of these corrections indicate a latitudinal variation of inclination shallowing with the formations initially located at mid latitudes suffering from more shallowing than those initially closer to the equator, consistent with the tan (Im)= f * tan (If) relationship observed by King (1955) for inclination shallowing, where Im is the measured inclination and If is the field inclination during deposition. Shallowing of the paleomagnetic vectors can be expressed in terms of the flattening factor f, that relates tan (Im) to tan (If). Anisotropy- derived hematite f factors from the Maritime Provinces of Canada and Northwest China were combined with f factors derived from corrections that use models of geomagnetic field secular variation (the EI technique of Tauxe and Kent, 2004) on red bed Formations from North America, Greenland and Europe. The dataset was used to derive a probability density function for f. The mean f value will allow a simplified inclination correction for hematite-bearing red bed formations that are suspected to be affected by inclination shallowing. This approach was tested by correcting the Permian Esterel Group red beds from France: using the distribution mean f factor of 0.64 (±0.11, ±1 standard deviation), the corrected red bed paleopole becomes statistically indistinguishable from the paleopole measured for the Esterel Group volcanic rocks that have not suffered from inclination shallowing. f data was also compiled for magnetite-bearing sedimentary rocks from the Perforada Formation and the Valle Group from Baja California, Mexico, the Pigeon Point Formation of Central California, the Ladd and the Point Loma Formations from Southern California, the Nanaimo Group of British Columbia and the Deer Lake Group of Newfoundland that have been corrected for inclination shallowing, yielding a most probable f factor of 0.67 (±0.06). Based on our results, the maximum amounts of shallowing that can be expected for sedimentary rocks is 12.4° for hematite-bearing rocks, and 11.8° for magnetite-bearing rocks. These values are statistically indistinguishable. Therefore, we combined the datasets and have obtained an f factor of 0.66 (±0.1) that can be used for either hematite or magnetite-bearing sedimentary rocks. A major implication of this result is that a rock's NRM, either acquired by chemical processes soon after deposition or by depositional processes that accurately record the ambient magnetic field, may be susceptible to similar amounts of inclination shallowing, most likely caused by burial compaction.
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    Constraining Nebular Magnetic Fields in the Outer Solar System from CO Chondrites
    (2019-06) Borlina, Caue S; Weiss, Benjamin P; Bryson, James F J; Fu, Roger R; Lima, Eduardo A
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    Scientific Drilling at Darwin Crater and Lake Selina: Long Continental Sedimentary Archives from Tasmania
    (2019-06) Lise-Pronovost, Agathe; Fletcher, Michael-Shawn; Mariani, Michela; Simon, Quentin
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    Evidence for widespread Remagnetizations in South America, case study of the Itararé Group rocks of Brazil
    (2019-06) Bilardello, Dario; Callebert, William, C; Davis, Joshua, R
    Paleomagnetism of South American Jurassic/Cretaceous rocks has been troubled by elongated distributions of poles which has led to contrasting interpretations. Moreover, many discordant paleomagnetic poles from the Carboniferous to the Triassic have also been recognized and systematically explained by a variety of processes, but this portion of the South American apparent polar wonder path (APWP) still remains problematic. We have conducted a paleomagnetic study of the sedimentary Permo-Carboniferous Itararé Group rocks and three intruding mafic sills of likely Cretaceous age within the state of São Paulo, Brazil. The site-mean VGP distributions obtained from the sedimentary rocks define elongations that include the VGPs of the mafic intrusions. We interpret these distributions as remagnetization paths toward the directions characteristic of the sills. Careful analysis of the paleomagnetic data of the Itararé sedimentary rocks enables isolation of a primary VGP distribution that is consistent with the reference Carboniferous pole position. The paleomagnetic directions of the sills are partially overprinted by the present time averaged and current Earth’s magnetic field. Combined rock- and paleomagnetic data suggest that interacting SD grains carry a very recent magnetic overprint that is visco-chemical in origin and cannot be fully erased. The dominant distribution of PSD-MD grains carries the high-temperature component, which is either a primary magnetization coincidentally close to the time averaged dipole field direction, or a secondary thermo-viscous magnetization. Extending our study to other Carboniferous to Triassic South American paleomagnetic records reveals that the majority of these data are elongated, similarly to the Itararé Group rocks. Regardless of the age of the rocks, the elongations systematically intersect at the location of the Late Cretaceous reference pole, and at a long- recognized problematic location (“X”) observed in certain Jurassic and Cretaceous rock formations. We interpret the elongated VGP distributions to reflect remagnetizations from the primary VGP positions toward Jurassic-Cretaceous and “X” pole locations, which occurred as a result of the widespread magmatic events associated with the opening of the South Atlantic. The extent of the remagnetizations is formation-specific and other rock-formations should be carefully re-evaluated.