Widespread remagnetization of Helvetic limestones

Abstract

The rock and paleomagnetism of shallow water Mesozoic limestones from the Helvetic nappes, the autochthonous cover sediments of the Aiguilles Rouges and Aar massifs, and the Chaine Subalpine of Switzerland and NE France have been studied. Fold tests indicate that the magnetization after alternating field (af) and thermal cleaning postdates Neogene folding; no reversed directions were observed. Acquisition of viscous remanent magnetization (VRM) in the laboratory shows that the natural remanent magnetization (NRM) intensity can be accounted for by viscous build‐up during the Brunhes epoch. The alternating magnetic field required to destroy the VRM produced in various times (t) is proportional to log t. The behavior of the magnetization at low temperatures indicates that the VRM is probably held by very fine grains whose room temperature state is close to superparamagnetism. The blocking temperatures of the NRM are mainly below 350°C. Thermomagnetic experiments, coercivity spectra, rotational hysteresis, and optical examination suggest that pyrrhotite, occurring as minute inclusions within pyrite grains, is responsible for the VRM.