Magnetic fabrics of the Morcles Nappe complex

Abstract

Anisotropy of magnetic susceptibility (AMS) measured with high fields, at ambient and low temperatures (77 K), has been used to separate ferromagnetic ( sensu lato), paramagnetic and diamagnetic sub-fabrics for deformed limestones located in a nappe structure from southwest Switzerland. The magnetic fabrics are dominantly controlled by paramagnetic and diamagnetic minerals, and ferromagnetic minerals contribute significantly only at two of the eight study locations. Separation of the magnetic sub-fabrics is possible in the root zone and overturned limb of the Morcles Nappe, which contains a high ratio of diamagnetic matrix calcite to paramagnetic secondary phases. Each of the sub-fabrics displays a relationship with structural cleavage and the crystallographic preferred orientation (CPO) of calcite in the root zone and overturned limb. The maximum axis of the paramagnetic AMS ellipsoid ( k max) lies close to the pole to cleavage, with the intermediate ( k int) and minimum ( k min) axes near the cleavage plane. Conversely, the diamagnetic k min lies close to the pole to cleavage with k max and k int axes near the cleavage plane. Slight offsets in the poles to cleavage and in the principal AMS directions are observed. They are interpreted to be the consequence of a second phase deformation event, affecting only the CPO but not the macroscopic structural elements. The diamagnetic sub-fabric reflects the CPO of calcite, produced through recrystallization during nappe deformation. Iron-rich calcite appears to control the paramagnetic sub-fabric. Susceptibility ellipsoids approach oblate shapes for the diamagnetic sub-fabric, with the most extreme shapes reaching susceptibility differences (Δ k = k max − k min) close to that of single crystal calcite, whereas the paramagnetic sub-fabric is represented mainly by prolate shape ellipsoids. The low-field AMS ellipsoids show mixed oblate and prolate shapes, without consistency within or between sites. It is evident that, when the magnetic sub-fabrics of the root zone and overturned limb are separated, their individual shapes and distribution of principal directions mirror the developed CPO.