Quantitative comparison of microfabric and magnetic fabric in black shales from the Appalachian plateau (western Pennsylvania, U.S.A.)

Abstract

Anisotropy of magnetic susceptibility (AMS) has been shown to be a good proxy for crystallographic preferred orientation (texture). However, it is not clear in detail how different factors, e.g. modal composition and preferred orientation, define the total AMS in a rock. Black shale samples from a drill core on the Appalachian Plateau in western Pennsylvania were analyzed with respect to their microfabric as determined by texture and microstructure, and AMS. Low- and high-field AMS, which was measured at room temperature, reveals that the AMS of the samples is dominated by the para- and diamagnetic phases. Synchrotron diffraction was applied to determine the texture of all relevant mineral phases incorporated in the samples. Muscovite and chlorite pole figures show single maxima perpendicular to the foliation, reflecting dominant flattening strain. From these textures and the modal compositions, AMS models were calculated using the intrinsic magnetic anisotropy of the single crystals. The modeled and measured AMS are comparable in terms of their principal directions and shapes of the anisotropy ellipsoid for the dominantly paramagnetic samples, with the maximum susceptibility axes oriented subhorizontal to the NE-SW. Both, textures and AMS, indicate that the samples have undergone largely bedding compaction with a weak tectonic overprint linked to the Alleghenian orogeny. Our results suggest that the microfabric and the magnetic fabrics on the plateau, 200 km away from the Allegheny Front, detect the tectonic shortening. The AMS modeling demonstrates how the contribution of the intrinsic AMS of each mineral controls the AMS of the whole rock.