THE MAGNETIC PROPERTIES OF FAULT PSEUDOTACHYLITES: ORIGIN AND IMPLICATIONS FOR FRICTIONAL MELTING PROCESSES

Several examples of fault-related pseudotachylites (California, Italian Alps, Japan) 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 inclusions formed during melt quenching. The hysteresis properties of the inclusions indicate a SD to PSD magnetic grain size. The Curie temperature (Tc) of the magnetic inclusions is close to 580*C. The magnetic remanence of these pseudotachylites (NRM) is also significantly higher than that of the host rock (up to 300:1). Such anomalously high remanence cannot be explained by a magnetization resulting from the Earth magnetic field, regardless of the pseudotachylite age. Ground lightening and other strong electric pulses can cause anomalously high NRMs. A ground lightening explanation seems unlikely to explain the systematically high NRMs particularly in the case of recently exposed samples that have been collected from active quarries. Alternatively, high NRMs could be explained by earthquake lightening (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 Jaeger conductive heat transfer equations. It shows that EQL can be recorded only during a brief time interval (in the order of 1 minute) 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.