Overpressure recorded in pseudotachylyte caused by frictional melting during earthquakes

Earthquakes are among the most catastrophic geological events that last only several to tens of seconds. During earthquakes, many processes may occur including rupturing, frictional sliding, pore fluid pressurization and occasionally frictional melting etc. However, little direct records of these fast processes can be preserved through geological time. During rapid shearing, frictional melt may form that lubricates the rocks and facilitates furthering sliding. The frictional melt layer may quench quickly within seconds to minutes depending on its thickness. After quenching, the product pseudotachylyte is formed that preserves valuable information about the conditions when the frictional melt was generated. To gain further insights into these conditions, we collected several pseudotachylyte-bearing granulite sample from the Bergen Arcs, Western Norway. Here, we present a pseudotachylyte vein that is ca. 1-2 cm thick and free of injection veins along the 2 m visible length of the vein. The pseudotachylyte matrix is made up of fine-grained omphacite (Jd₃₈), sodic plagioclase (Ab₈₃) and kyanite with minor rutile and sulphides. Many dendritic garnets were found within the pseudotachylyte layer showing a gradual grain size reduction towards the wall rock side. This suggests that these garnets were crystallized during rapid quenching. The stability of epidote, kyanite and quartz in the wall rock plagioclase, and omphacite and albitic plagioclase together with quartz in the pseudotachylyte matrix constrains the ambient pressure to be ca. 1.5-1.7 GPa and the temperature to range between 650-750°C. In contrast, the constrained pressure condition of the dendritic garnets in the pseudotachylyte layer reflects a pressure higher than 2 GPa. Based on elastic model, it is not possible to maintain several kbar overpressure within a thin melt layer due to thermal pressurization or melting expansion. A potential explanation is that a GPa level differential stress was acting on the rock volume and that the melt pressure approached the normal stress that was higher than 2 GPa when shear rigidity vanished during frictional melting. This suggests that under high differential stress, the weakening due to frictional melting may potentially lead to dramatic pressure rise within the pseudotachylyte melt.