Paper No. 3
Presentation Time: 4:25 PM
TESTING THERMAL VISCOUS REMANENT MAGNETIZATION AS A TOOL TO DATE GEOMORPHIC EVENTS AT THE ICICLE CREEK MORAINES, NORTH CASCADES WASHINGTON
GLOBOKAR, Danika, Earth and Space Sciences, University of Washington, Seattle, WA 98195, CRIDER, Juliet G., Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195-1310, BURMESTER, Russell F., Geology Department, Western Washington University, 516 High Street, Bellingham, WA 98225 and HOUSEN, Bernard, Geology Department, Western Washington University, Bellingham, WA 98225, dmglobokar@gmail.com
When a rock forms, it acquires a natural remanent magnetization (NRM) 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 partial thermal viscous remanent magnetization (pTVRM) which partially overprints the NRM. The strength of the pTVRM is dependent on the exposure time and temperature (Pullaiah et al. 1975). For rocks with single-domain magnetic minerals, given the temperature and duration of heating required to remove the pTVRM, along with estimates of the environmental temperature, we can determine the exposure time required to produce it, thereby dating displacement. This technique has been successfully tested in basalt. Here, we evaluate the potential for pTVRM dating using a suite of well-dated, granodiorite moraines in the Icicle Creek drainage of the North Cascades, Washington. Prior work has shown that the primary magnetic carrier in boulders is single-domain magnetite. The five nested moraines have independently-determined cosmogenic nuclide surface exposure ages ranging 13-112 ka.
Samples were collected from approximately 10 large (>2 m) boulders along the crest of each moraine, with one to three samples per boulder. Prior to thermal demagnetization, samples were treated with liquid nitrogen to minimize contribution from multi-domain magnetite grains. Samples were then heated for 30 minutes in discreet temperature steps of 10oC or smaller from 50oC to 250oC, with the goal of identifying the last acquired pTVRM component and its unblocking temperature (Tu). In pilot results, although only a small fraction of samples displayed a distinct pTVRM magnetic component, their Tu values are consistent with the relative ages of the sampled moraines (higher Tu for older deposits) but higher than theory predicts for the ages of the deposits.
Key issues for translating Tu to age include site average vs. max temperature and its impact on pTVRM acquisition, the recurrence interval of forest fires and lightning strikes, and the confounding influence of weathering and large, multi-domain magnetite grains. We believe that resolution of these issues will give pTVRM dating the potential to be a viable dating method for Quaternary geomorphic events.
