Metallic iron formed by melting: A new mechanism for magnetic highs in pseudotachylyte

Previous studies of rock magnetism in fault rocks imply frictional heating temperatures from similar to 300 degrees C to similar to 700 degrees C, which are far below the temperatures needed to form pseudotachylyte. Here, heating experiments were performed at elevated temperatures (as high as 1750 degrees C) on cataclasites from the Wenchuan Earthquake Fault Scientific Drilling borehole 2 (WFSD-2) cores, Longmen Shan thrust belt, China. Based on microstructural, geochemical, and rock magnetic analyses, the main conclusions are as follows. The melting occurred at 1100 degrees C. The newly formed magnetite generated by the thermal decomposition of paramagnetic minerals contributed to the high magnetic susceptibility values of samples below 1100 degrees C. Above 1300 degrees C, many circular metallic iron spherulites were formed by the reducing action of Fe-bearing minerals at elevated temperatures. As the temperature increased, metallic iron content and magnetic susceptibility increased, indicating that the newly formed metallic iron was responsible for the high magnetic susceptibility values. Therefore, in addition to the newly formed magnetite, the metallic iron is another factor contributing to the magnetic highs of pseudotachylytes. The frictional melting temperature reached 1300 degrees C during ancient earthquakes in the Longmen Shan thrust belt, indicating that metallic iron might be responsible for the strong magnetic highs in pseudotachylyte.