Postseismic fault healing in the dry lower crust facilitated by ultrafast metamorphism

Fault healing (i.e., strength recovery) is a crucial process impacting the magnitude and timescale of fault failure within the earthquake cycle. Detailed knowledge of the processes and their timescale during fault healing occurring after coseismic faulting in the lower crust remain elusive. Earthquakes are frequently recorded in silicate rocks as pseudotachylytes (quenched coseismic-derived frictional melts). We examine the pristinely preserved microstructures of a lower-crustal pseudotachylyte vein formed in the minutes after a lower-crustal earthquake. Garnet-clinopyroxene geothermometry, major and trace element mapping, electron backscatter diffraction, and transmission electron microscopy combined with a 1D cooling model demonstrate that these microstructures crystallised rapidly within an hour following the earthquake, from >1200 °C to the ambient temperature of ~700 °C. This pseudotachylyte vein captured a geologically instantaneous temporal sequence of phase nucleation and growth, resulting from ultrafast fault healing during the incipient postseismic transition. This transition is marked by the rapid growth and sintering of orthopyroxene and garnet from comminuted grains. Preservation of primary vein-scale garnet compositional zoning further demonstrates that garnet crystallisation from the frictional melt was faster than chemical homogenisation in melt. These microstructures provide an exceptional record of the processes occurring immediately after an earthquake because of the dry ambient conditions.