Thermoviscous remanent magnetism of Columbia River basalt blocks in the cascade landslide

Abstract

We studied sixteen basalt samples from a landslide in the Columbia River Gorge to determine if they had acquired a thermoviscous remanent magnetism (TVRM) since the slide was emplaced about 800 years ago. All samples were thermally demagnetized at 20 heating steps until 360°C, where a large change in susceptibility was noted. Analysis of the directional changes during demagnetization indicates that the samples contain up to four TVRM components, in addition to an NRM component. The TVRM components with the lowest blocking temperatures are tightly clustered around the present field direction while the NRM directions are consistent with a random distribution, as expected for a landslide deposit. Measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single‐domain magnetite. The temperature at which the first TVRM component was removed ranges from 70°C to 100°C. This result can be compared to nomographs that relate the time and temperature used to demagnetize a TVRM with the time and temperature at which the TVRM was acquired. Our results are more consistent with the nomograph of Pullaiah et al . [1975] than with the nomograph of Middleton & Schmidt [1982].