Inverse magnetic fabric in deformed limestones of the Morcles Nappe, Switzerland

Anisotropy of magnetic susceptibility (AMS) was measured at 15 sites in deformed Helvetic limestones of the Morcles nappe in western Switzerland. Inverse magnetic fabrics, characterized by axes of maximum susceptibility subperpendicular to bedding or cleavage, were observed at four sites in the root zone of the nappe. Acquisition and thermal demagnetization of isothermal remanent magnetization show that the ferromagnetic mineralogy is dominated by pyrrhotite, with various proportions of magnetite, hematite and goethite. The matrix minerals were studied in thin section with a staining technique to distinguish different carbonates, by X‐ray texture goniometry and by microprobe analysis. Observations of AMS at room and liquid nitrogen temperatures, and of the temperature dependence of bulk susceptibility, lead to the conclusion that the inverse fabrics result from a paramagnetic component due to Fe2+ ions in the calcite matrix.