Microstructure and petrology of a Calabrian garnet-bearing pseudotachylyte - A link to lower-crustal seismicity

A pseudotachylyte vein enclosed in tilted Hercynian lower crust of the Calabrian Serre Massif provides information on the source of a fossil seismic event. The pseudotachylyte contains euhedral garnet that formed by direct crystallization from a frictional melt and not during a later metamorphic overprint. Pseudotachylyte formation started with grain-size reduction caused by ductile deformation and brittle deformation. Subsequent frictional melting affected almost all phases including quartz but excluding monazite. After rapid cooling, chilled margins composed of glass and iron sulphide droplets formed. The central part of the vein solidified as biotite, and plagioclase crystallized directly from the frictional melt. Garnet occurs in three different grainsize classes; there are significant differences between very small garnets in the chilled margins and larger garnets in the central part of the pseudotachylyte vein. This proves that garnet rapidly crystallized from the pseudotachylyte melt after the seismic event. Garnet composition is used to define the depth of garnet growth, implying that the seismogenic zone was located at a depth of about 21-23 km. The melt temperature was between approximately 1515 and 2040 °C, representing the melting points of β-quartz and monazite, respectively. This study supports the interpretation from seismic data that lower-crustal seismicity occurs and affirms, by direct observation, that the classical jelly sandwich model of the lithosphere is not always appropriate for crustal sections. In contrast to well-known regions with dry and very strong lower crusts (e.g. the Caledonides), the studied part of the Hercynian lower crust contains significant amounts of biotite, which release aqueous fluids during frictional melting. In addition, fluid inclusions in quartz fragments within the pseudotachylyte indicate that fluids probably played an important role during fault-zone evolution at the time of frictional melting.