Fault zone behaviour: microstructural evidence from slow slip shear failure mylonites

The behavior of major plate boundary fault zones in the brittle-ductile transition remains poorly understood despite the significant role the behavior of this zone has in the formation ofhydrothermal ores and nucleation of earthquakes. Recent geophysical evidence of continental faults and subduction zones suggests that aseismic slip occurs on deep extensions of fault zones. These episodic, low frequency events of slow slip are widely linked to the presence of overpressured metamorphic fluids, such as is interpreted from the geophysical signal of the Alpine Fault, New Zealand. A combination of detailed petrographic and mineral chemistry analysis with quantitative microstructural analysis using Electron Backscatter Diffraction is used to investigate indicators of slow slip shear failure. This study shows that Alpine Fault mylonites periodically behaved in a brittle manner, as evidenced by mm to cm scale rupture microstructures such as several generations of quartz dominated veining, microcracking of plastically deformed quartz grains, fracturing combined with recrystallization of garnet, the localization of highly porous shear bands and localized fluid influx as evidenced by only locally developed metamorphic reaction structures. Recognition of such structures can be used as first order indicators for slow slip fault failure.