AMS measurements on obsidian from the Inyo Domes, CA: a comparison of magnetic and mineral preferred orientation fabrics

Measurements of the anisotropy of magnetic susceptibility (AMS) on lavas can yield information concerning flow directions, deformation regimes, and relative shear rates. Here we report results of measurements made on samples of rhyolitic obsidian collected from Obsidian Dome, CA. Comparisons of AMS measurements with previously determined 3D microlite orientation distributions in these samples serve as a calibration of the AMS technique. Despite the low values of bulk susceptibility (1.06–7.04×10−3 SI) found in the dome relative to typical values in basalts (10−2 SI), the degree of anisotropy is extremely high in all specimens (3.4–10.7). Furthermore, susceptibility tensors exhibit a wide range of shapes varying from prolate to triaxial and flattened ellipsoids (B parameter from −8.6 to +6.4) corresponding to various degrees of microlite dispersion. Our results also indicate that despite the tight clustering of principal susceptibility axes obtained on each site, bulk flow directions inferred from the orientations of kmax axes may be misleading because the orientation of the local extension changes from place to place within the dome. However, it is shown that the AMS of each specimen provides a good measure of the direction of local extension. Consequently, it is possible to interpret spatial variations in flow regime from systematic changes in the orientation of principal susceptibility axes. As the proposed association between the local deformation regime and AMS is strongly dependent on the aspect ratio of the crystals embedded in the flowing lava, some caution must be exerted when interpreting AMS results from other flows, and a direct comparison between different flows may not be granted in many occasions.