Testing thermal viscous remanent magnetization (TVRM) as a tool to date geomorphic events
Globokar, Danika Monas
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When a rock forms, it acquires a thermal remanent magnetization (TRM) aligned with Earth's magnetic field. If the rock becomes misaligned with the magnetic field (by e.g. rockfall or glacial plucking and deposition), it may acquire a thermal viscous remanent magnetization (TVRM) which partially overprints the TRM. The strength of the TVRM is dependent on the exposure time and temperature (Neel, 1949). Given the temperature and duration of heating required to remove the TVRM, along with estimates of the environmental temperature, one can determine the exposure time required to produce it, thereby dating displacement. I evaluate the potential for TVRM dating using a suite of cosmogenically-dated, granodiorite moraines in the Icicle Creek drainage of the North Cascades, Washington, with ages ranging 13-112 ka. About 40% of boulders and 25% of samples contained both a TVRM and TRM component. A subset of these were identified as "qualifying samples", whose TVRM components were in the direction of magnetic north. This is a critical distinction to make, as it indicates that the TVRM was more likely acquired since moraine emplacement. The temperature at which a TVRM is removed from a sample is the unblocking temperature (Tu), or turning point temperature. I used nomographs published by Pullaiah et al. (1975) and Middleton and Schmidt (1982) to translate Tu to a displacement age and compared output ages from both methods. The Middleton and Schmidt equation yielded moraine ages within about an order of magnitude of cosmogenic ages, while the equation of Pullaiah et al. yielded ages that differed by multiple orders of magnitude. This difference suggests that pseudo-single-domain magnetite is the remanence carrier in the moraine boulders. Error inherent in the dating method includes mis-identification of the turning point due to a diffuse TVRM/TRM relationship, correcting for oven temperature gradients, and relying on assumptions for field acquisition conditions, all of which have the potential to introduce large variation into an age. At present, TVRM is a useful relative dating method to confirm geomorphic interpretations, and may provide approximate age constrains where no other methods are applicable.