Paleomagnetism of the Miocene Grotto and Snoqualmie Batholiths, Central Cascades, Washington

Paleomagnetic data have been obtained from granitic rocks of the Miocene Grotto and Snoqualmie batholiths in the Central Cascades, Washington. Alternating field magnetic cleaning demonstrated that a wide range of magnetic stability was present and that many specimens were unsuitable for paleomagnetic analysis. A simple statistic (Qd′), defined as the ratio of remanent magnetic intensity of the entire sample after af cleaning to weak-field magnetic susceptibility, proved effective in discriminating between specimens of marginal stability. Experiments with saturation isothermal remanent magnetism indicated that stable NRM is dominated by magnetic grains with single-domain characteristics, whereas grains with multidomain characteristics dominate the NRM of the unstable samples. After cleaning, a Miocene pole was obtained for the Central Cascades at 208.5°E, 85.5°N (δp = 4.5, δm = 5.4, and k = 360.4). This pole is statistically indistinguishable from other Miocene poles of North America, which suggests that any major tectonic movement postulated for the Central Cascades relative to North America must have occurred before the Miocene.