Magnetic vs. structural fabrics, Whipple detachment structure, Whipple Wash, California

Abstract

Abstract Orientations of ellipsoids of anisotropy of magnetic susceptibility (AMS) are compared with those of mesoscopic structural features for the breccia and upper mylonite zones for the Whipple detachment zone in Whipple Wash, Whipple Mountains, eastern California. The anisotropy of the breccia, which ranges up to 5%, is much less than that of the mylonitic gneiss, which can exceed 100%. Slight differences in trend are evident between the AMS ellipsoid elongations and those of mineral lineations. Maximum axes (χ 1 ) of the AMS ellipsoids, for both breccia and mylonite, trend generally SSW-NNE with shallow plunge, whereas mineral ‘stretching’ lineations in the mylonite gneiss have trends near NE-SW with shallow plunge. Measured fold axes are close to the χ 1 axes, SSW-NNE. χ 1 axes in the breccia show more variation, E-W and SSW-NNE, at different structural levels. Mesoscopic mineral lineations in the breccia were not discernible. For the planar elements, AMS ellipsoid flattening is generally more closely parallel to foliation surfaces in the mylonite than in the breccia. This is evident from overlapping minimum susceptibility axes (χ 3 ) and foliation pole clusters. χ 3 axes are more scattered than are poles of foliation. The scatter in χ 3 is possibly the superimposed susceptibility effect of several structural surfaces of different ages: joints, small faults, transposed metamorphic foliations, and mylonitic foliation. Especially interesting are the discovery of good χ 1 definitions in the breccia, the apparent lack of correlation of χ 1 axes with the very prominent ‘stretching’ lineations in the mylonite, and the similarity of the χ 1 axial trends with those of the fold/fracture pole patterns.