Rock magnetic characteristics of the Wenchuan-Maoxian fault zone of the Longmen Shan fault and its earthquake faulting characteristics

The Wenchuan-Maoxian Fault Zone (WMFZ), the boundary fault between the Songpan-Ganzi fold zone and the Longmen Shan Fault Zone (LSFZ), did not rupture during the 2008 M-W 7.9 Wenchuan earthquake. Furthermore, there were no records for the existence of the former large magnitude earthquakes had been found in this fault zone. These problems hinder progress in understanding both the earthquake mechanism of the Wenchuan earthquake and the formation and evolution of the LSFZ. Therefore, the faulting environment of the WMFZ is crucial in understanding the earthquake mechanism of the LSFZ. Rock magnetic study of fault rocks can effectively reveal the physical and chemical environment of the faults. Here, we carried out rock magnetic analyses combined with microstructural and geochemical analyses of fault rocks and wall rocks from the northern WMFZ. The highest magnetic susceptibility values of fault gouge and breccia are about 30 and 15 times of that of the wall rocks, respectively. Rock magnetic analysis results indicate that the magnetic minerals in the fault gouge are pyrrhotite, goethite together with some magnetite, while those in the fault breccia are pyrrhotite and goethite. The high temperature of frictional heating and fluids induced thermal decomposition of paramagnetic minerals, forming pyrrhotite and magnetite to a lesser extent, and thus contributing to a higher magnetic susceptibility values of the fault gouge. Higher magnetic susceptibility values of fault breccia results from the newly-formed pyrrhotite by reductive low-temperature hydrothermal fluids with large amounts of sulfur. Goethite reveals the presence of reductive low-temperature hydrothermal fluids after an earthquake. The high magnetic susceptibility values of fault rocks and the newly-formed magnetite indicate that large earthquakes with frictional heating (>500 degrees C) did have occurred along the WMFZ in the past time, and this fault kept in a reductive environment with low-temperature hydrothermal fluids that contain high sulfur during and after the earthquakes.