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High-velocity friction experiments were conducted on two natural fault gouges retrieved from the Yingxiu-Beichuan fault zone (Sichuan, China), which accommodated the 2008 Wenchuan Mw 7.9 earthquake. The experiments simulate large earthquake slip; the rotary shear apparatus enables to gather information as a function of shear displacement (and slip velocity) in a single experiment. The two starting fault gouges are essentially paramagnetic with one containing goethite. The experimentally sheared gouges both show a significant magnetic enhancement and softening with increasing slip distance. Rock magnetic measurements reveal that magnetite was formed during the experiment due to thermochemical reactions of iron adsorbed on clay minerals (smectite, chlorite). Scanning electron microscope observations showed that the new magnetite occurs as spherical and sintered irregular aggregates. This implies a high-temperature origin. The peak temperatures are estimated numerically to range from~260 to~440°C. Also, during the experiment on the goethite-bearing fault gouge, goethite was altered, reduced to magnetite due to the decomposition of trace organic matter present in the starting material. Magnetic susceptibility and magnetization of sheared samples are linearly increasing with the temperature rise induced by frictional heating; coercivities are decreasing. The grain size of the newly formed magnetite increases with heating temperature. This demonstrates that short-duration frictional heating generated by fast seismic slip induces thermal alteration: neoformation of ferrimagnetic minerals leads to magnetic enhancement and softening of slip zones. Thus, rock magnetic methods are a useful tool for diagnosing the earthquake slip and estimating the temperature rise of coseismic frictional heating.