High-T quenching microstructures in cauliflower garnet capture instantaneous post-seismic processes in a lower-crustal seismogenic fault

Pristine microstructures preserved in a pseudotachylyte (coseismic-derived quenched frictional melt) are recordkeepers of the time-lapse processes associated with the dynamic rupture propagation and the earthquake slip. Unlocking these near-instantaneous processes from the microstructures in a pseudotachylyte provides insights into the chemo-mechanical processes operating during, and immediately following an earthquake.

Inclusion-rich, mutually intergrown garnet aggregates with a morphology akin to a cauliflower are a common product phase in lower-crustal pseudotachylytes. Pseudotachylyte veins in gabbroic rocks from Lofoten (Norway) formed at ambient temperatures of 650–700 °C and display pristine quenching microstructures in the matrix such as plagioclase microlites, dendritic clinopyroxene, cauliflower garnet aggregates, survivor lithoclasts of plagioclase, olivine, and orthopyroxene, and coronas of cauliflower garnet at the interphase boundary between orthopyroxene and the pseudotachylyte matrix. There is a lack of hydrous phases such as amphibole or biotite. Quantitative compositional maps across an entire vein show an irregular or patchy intercrystalline major element distribution in cauliflower garnet. Pyrope and spessartine contents are higher in garnet coronas around survivor lithoclasts comprised of orthopyroxene and olivine, while grossular is highest near plagioclase survivor lithoclasts and near the contact with the plagioclase-rich wall-rock. In some instances, trace elements maps show sharp compositional zoning within single garnet grains. In addition, electron backscattered diffraction data indicate that the garnet corona grains in contact with orthopyroxene along the pseudotachylyte vein boundary show evidence of crystal-lattice distortion through dislocation glide. Collectively, these microstructures indicate that the anhydrous pseudotachylyte melt quenched extremely quickly. The quenching of garnet proceeded faster than the chemical homogenisation in the frictional melt, freezing in and preserving local compositional variations without any later recrystallization at amphibolite-facies ambient conditions.

Using garnet-clinopyroxene geothermometry on cauliflower garnet cores and rims in contact with clinopyroxene inclusions and microlites, respectively, and using a cooling model for the pseudotachylyte vein, we estimate that garnet quenched from the frictional melt starting at ~1100–900 ºC, as recorded in garnet cores, and ceased growing upon reaching the ambient temperature of ~650–700 ºC in <1 hour. In the short duration of mineral growth, garnet captured the incipient distribution of major and minor elements from the frictional melt and was able to record the post-seismic stress relaxation that localized in the form of solid-state creep along the pseudotachylyte vein margin during quenching. High-T microstructures are preserved because dislocations in garnet are immobile at ambient lower-crustal temperatures, and anhydrous conditions inhibit recrystallisation, diffusion, and viscous deformation.