The origin of high magnetic remanence in fault pseudotachylites:: Theoretical considerations and implication for coseismic electrical currents

Several examples of fault-related pseudotachylites display a significantly higher initial magnetic susceptibility than their granitic host rock (10: 1 to 20: 1). These higher values are attributed to the presence of fine magnetic particles formed during melt quenching. The hysteresis properties of the particles indicate a single domain (SD) to pseudo single domain (PSD) magnetic grain size. The Curie temperature (T-c) of the magnetic particles is close to 580 degrees C. The natural remanent magnetization (NRM) of these pseudotachylites is also significantly higher than that of the host rock (up to 300: 1). Such anomalously high remanence cannot be explained by a magnetization acquired in the Earth's magnetic field, regardless of pseudotachylite age. Ground lightning and other strong electric pulses can cause anomalously high NRM intensities. A ground lightning explanation seems unlikely to explain the systematically high NRM intensities, particularly in the case of recently exposed samples that have been collected from active quarries. Alternatively, high NRM intensities could be explained by earthquake lightning (EQL), a seismic phenomenon occasionally reported in connection with large magnitude earthquakes (M > 6.0). The coseismic electrical properties of the pseudotachylite vein-host rock system are characterized by (1) a core of molten material (high conductivity), (2) vapor-rich margins of thermally and mechanically fractured host rocks (low conductivity) and (3) moderately fractured to undeformed host rock (normal conductivity). Such a core conductor bordered by insulating margins is potentially responsible for the propagation of EQL pulses. The coseismic thermal history of pseudotachylite veins has been modeled in 2-D using conductive heat transfer equations. It shows that EQL can be recorded only during a brief time interval (less than 1 min) for a given vein thickness and host-rock temperatures. If the vein is too thick or if the host rock is too hot, the pseudotachylite remains above Tc after the electric pulse has lapsed. (c) 2005 Elsevier B.V All rights reserved.